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Re: [PATCH v4] Introduce v3 namespaced file capabilities

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  Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-13 17:50 +0200
    Re: [PATCH v4] Introduce v3 namespaced file capabilities Tycho Andersen <tycho@docker.com> - 2017-06-13 19:20 +0200
      Re: [PATCH v4] Introduce v3 namespaced file capabilities James Bottomley <James.Bottomley@HansenPartnership.com> - 2017-06-13 19:50 +0200
        Re: [PATCH v4] Introduce v3 namespaced file capabilities Tycho Andersen <tycho@docker.com> - 2017-06-13 22:50 +0200
          Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-13 22:50 +0200
            Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-13 23:00 +0200
            Re: [PATCH v4] Introduce v3 namespaced file capabilities Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-06-13 23:00 +0200
              Re: [PATCH v4] Introduce v3 namespaced file capabilities Tycho Andersen <tycho@docker.com> - 2017-06-13 23:20 +0200
            Re: [PATCH v4] Introduce v3 namespaced file capabilities Tycho Andersen <tycho@docker.com> - 2017-06-13 23:00 +0200
      Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-13 19:50 +0200
        Re: [PATCH v4] Introduce v3 namespaced file capabilities Tycho Andersen <tycho@docker.com> - 2017-06-13 23:00 +0200
    Re: [PATCH v4] Introduce v3 namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-13 19:20 +0200
      Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-13 20:20 +0200
        Re: [PATCH v4] Introduce v3 namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-14 02:00 +0200
          Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-14 14:50 +0200
            Re: [PATCH v4] Introduce v3 namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-15 05:10 +0200
              Re: [PATCH v4] Introduce v3 namespaced file capabilities Christian Brauner <christian.brauner@mailbox.org> - 2017-06-16 11:10 +0200
              Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-17 00:30 +0200
              Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-17 23:00 +0200
                Re: [PATCH v4] Introduce v3 namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-19 00:20 +0200
                  Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-19 03:20 +0200
                    Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-19 15:10 +0200
                    Re: [PATCH v4] Introduce v3 namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-20 08:30 +0200
                  Re: [PATCH v4] Introduce v3 namespaced file capabilities ebiederm@xmission.com (Eric W. Biederman) - 2017-06-19 23:50 +0200
                    Re: [PATCH v4] Introduce v3 namespaced file capabilities Amir Goldstein <amir73il@gmail.com> - 2017-06-20 07:50 +0200
                      Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-20 14:30 +0200
                        Re: [PATCH v4] Introduce v3 namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-20 19:40 +0200
                          Re: [PATCH v4] Introduce v3 namespaced file capabilities Amir Goldstein <amir73il@gmail.com> - 2017-06-20 22:00 +0200
                      Re: [PATCH v4] Introduce v3 namespaced file capabilities Vivek Goyal <vgoyal@redhat.com> - 2017-06-20 22:00 +0200
    Re: [PATCH v4] Introduce v3 namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-14 01:50 +0200
      Re: [PATCH v4] Introduce v3 namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-14 02:00 +0200

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#1668697

On 06/18/2017 06:14 PM, Serge E. Hallyn wrote:
> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>> On 06/14/2017 11:05 PM, Serge E. Hallyn wrote:
>>> On Wed, Jun 14, 2017 at 08:27:40AM -0400, Stefan Berger wrote:
>>>> On 06/13/2017 07:55 PM, Serge E. Hallyn wrote:
>>>>> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>>>>>>   If all extended
>>>>>> attributes were to support this model, maybe the 'uid' could be
>>>>>> associated with the 'name' of the xattr rather than its 'value' (not
>>>>>> sure whether that's possible).
>>>>> Right, I missed that in your original email when I saw it this morning.
>>>>> It's not what my patch does, but it's an interesting idea.  Do you have
>>>>> a patch to that effect?  We might even be able to generalize that to
>>>> No, I don't have a patch. It may not be possible to implement it.
>>>> The xattr_handler's  take the name of the xattr as input to get().
>>> That may be ok though.  Assume the host created a container with
>>> 100000 as the uid for root, which created a container with 130000 as
>>> uid for root.  If root in the nested container tries to read the
>>> xattr, the kernel can check for security.foo[130000] first, then
>>> security.foo[100000], then security.foo.  Or, it can do a listxattr
>>> and look for those.  Am I overlooking one?
>>>
>>>> So one could try to encode the mapped uid in the name. However, that
>>> I thought that's exactly what you were suggesting in your original
>>> email?  "security.capability[uid=2000]"
>>>
>>>> could lead to problems with stale xattrs in a shared filesystem over
>>>> time unless one could limit the number of xattrs with the same
>>>> prefix, e.g., security.capability*. So I doubt that it would work.
>>> Hm.  Yeah.  But really how many setups are there like that?  I.e. if
>>> you launch a regular docker or lxd container, the image doesn't do a
>>> bind mount of a shared image, it layers something above it or does a
>>> copy.  What setups do you know of where multiple containers in different
>>> user namespaces mount the same filesystem shared and writeable?
>> I think I have something now that accomodates userns access to
>> security.capability:
>>
>> https://github.com/stefanberger/linux/commits/xattr_for_userns
> Thanks!
>
>> Encoding of uid is in the attribute name now as follows:
>> security.foo@uid=<uid>
>>
>> 1) The 'plain' security.capability is only r/w accessible from the
>> host (init_user_ns).
>> 2) When userns reads/writes 'security.capability' it will read/write
>> security.capability@uid=<uid> instead, with uid being the uid of
>> root , e.g. 1000.
>> 3) When listing xattrs for userns the host's security.capability is
>> filtered out to avoid read failures iof 'security.capability' if
>> security.capability@uid=<uid> is read but not there. (see 1) and 2))
>> 4) security.capability* may all be read from anywhere
>> 5) security.capability@uid=<uid> may be read or written directly
>> from a userns if <uid> matches the uid of root (current_uid())
> This looks very close to what we want.  One exception - we do want
> to support root in a user namespace being able to write
> security.capability@uid=<x> where <x> is a valid uid mapped in its
> namespace.  In that case the name should be rewritten to be
> security.capability@uid=<y> where y is the unmapped kuid.val.

I'll try to write a patch on top of the existing one.

Can  you adapt your test cases. I haven't tried them, but having them 
would be important.

>
> Eric,
>
> so far my patch hasn't yet hit Linus' tree.  Given that, would you
> mind taking a look and seeing what you think of this approach?  If
> we may decide to go this route, we probably should stop my patch
> from hitting Linus' tree before we have to continue supporting it.
>
> Stefan,
>
> so do you think the general framework could be re-used by IMA?  If
> we can move the capability-specific code in fs/xattr.c into
> an LSM hook in a way that IMA can also use, then this is a definite
> win.

I am fairly sure that this would be easily possible and some of the if 
statements with string comparisons would likely only have to be extended 
with another comparison.

Regards,
    Stefan

>
> -serge
>

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#1669049

On 06/18/2017 09:13 PM, Stefan Berger wrote:
> On 06/18/2017 06:14 PM, Serge E. Hallyn wrote:
>> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>>> On 06/14/2017 11:05 PM, Serge E. Hallyn wrote:
>>>> On Wed, Jun 14, 2017 at 08:27:40AM -0400, Stefan Berger wrote:
>>>>> On 06/13/2017 07:55 PM, Serge E. Hallyn wrote:
>>>>>> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>>>>>>>   If all extended
>>>>>>> attributes were to support this model, maybe the 'uid' could be
>>>>>>> associated with the 'name' of the xattr rather than its 'value' 
>>>>>>> (not
>>>>>>> sure whether that's possible).
>>>>>> Right, I missed that in your original email when I saw it this 
>>>>>> morning.
>>>>>> It's not what my patch does, but it's an interesting idea.  Do 
>>>>>> you have
>>>>>> a patch to that effect?  We might even be able to generalize that to
>>>>> No, I don't have a patch. It may not be possible to implement it.
>>>>> The xattr_handler's  take the name of the xattr as input to get().
>>>> That may be ok though.  Assume the host created a container with
>>>> 100000 as the uid for root, which created a container with 130000 as
>>>> uid for root.  If root in the nested container tries to read the
>>>> xattr, the kernel can check for security.foo[130000] first, then
>>>> security.foo[100000], then security.foo.  Or, it can do a listxattr
>>>> and look for those.  Am I overlooking one?
>>>>
>>>>> So one could try to encode the mapped uid in the name. However, that
>>>> I thought that's exactly what you were suggesting in your original
>>>> email?  "security.capability[uid=2000]"
>>>>
>>>>> could lead to problems with stale xattrs in a shared filesystem over
>>>>> time unless one could limit the number of xattrs with the same
>>>>> prefix, e.g., security.capability*. So I doubt that it would work.
>>>> Hm.  Yeah.  But really how many setups are there like that? I.e. if
>>>> you launch a regular docker or lxd container, the image doesn't do a
>>>> bind mount of a shared image, it layers something above it or does a
>>>> copy.  What setups do you know of where multiple containers in 
>>>> different
>>>> user namespaces mount the same filesystem shared and writeable?
>>> I think I have something now that accomodates userns access to
>>> security.capability:
>>>
>>> https://github.com/stefanberger/linux/commits/xattr_for_userns
>> Thanks!
>>
>>> Encoding of uid is in the attribute name now as follows:
>>> security.foo@uid=<uid>
>>>
>>> 1) The 'plain' security.capability is only r/w accessible from the
>>> host (init_user_ns).
>>> 2) When userns reads/writes 'security.capability' it will read/write
>>> security.capability@uid=<uid> instead, with uid being the uid of
>>> root , e.g. 1000.
>>> 3) When listing xattrs for userns the host's security.capability is
>>> filtered out to avoid read failures iof 'security.capability' if
>>> security.capability@uid=<uid> is read but not there. (see 1) and 2))
>>> 4) security.capability* may all be read from anywhere
>>> 5) security.capability@uid=<uid> may be read or written directly
>>> from a userns if <uid> matches the uid of root (current_uid())
>> This looks very close to what we want.  One exception - we do want
>> to support root in a user namespace being able to write
>> security.capability@uid=<x> where <x> is a valid uid mapped in its
>> namespace.  In that case the name should be rewritten to be
>> security.capability@uid=<y> where y is the unmapped kuid.val.
>
> I'll try to write a patch on top of the existing one.

Did that now in a 2nd patch (that also fixes a few problems of the 1st). 
In a user ns mapped to 1000 root can write security.capability@uid=123, 
which then ends up writing to security.capability@uid=1123. The reading 
also works with @uid=123. When listing xattrs only those get shown that 
actually have valid mappings.

https://github.com/stefanberger/linux/commits/xattr_for_userns

    Stefan

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#1670323

On Sun, Jun 18, 2017 at 09:13:28PM -0400, Stefan Berger wrote:
> Can  you adapt your test cases. I haven't tried them, but having
> them would be important.

branch nsfscaps of github.com/hallyn/ltp now has a patch on top
which makes it work with your capabilities.  Tests are passing.

thanks,
-serge

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#1669970

"Serge E. Hallyn" <serge@hallyn.com> writes:

> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>> On 06/14/2017 11:05 PM, Serge E. Hallyn wrote:
>> >On Wed, Jun 14, 2017 at 08:27:40AM -0400, Stefan Berger wrote:
>> >>On 06/13/2017 07:55 PM, Serge E. Hallyn wrote:
>> >>>Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>> >>>>  If all extended
>> >>>>attributes were to support this model, maybe the 'uid' could be
>> >>>>associated with the 'name' of the xattr rather than its 'value' (not
>> >>>>sure whether that's possible).
>> >>>Right, I missed that in your original email when I saw it this morning.
>> >>>It's not what my patch does, but it's an interesting idea.  Do you have
>> >>>a patch to that effect?  We might even be able to generalize that to
>> >>No, I don't have a patch. It may not be possible to implement it.
>> >>The xattr_handler's  take the name of the xattr as input to get().
>> >That may be ok though.  Assume the host created a container with
>> >100000 as the uid for root, which created a container with 130000 as
>> >uid for root.  If root in the nested container tries to read the
>> >xattr, the kernel can check for security.foo[130000] first, then
>> >security.foo[100000], then security.foo.  Or, it can do a listxattr
>> >and look for those.  Am I overlooking one?
>> >
>> >>So one could try to encode the mapped uid in the name. However, that
>> >I thought that's exactly what you were suggesting in your original
>> >email?  "security.capability[uid=2000]"
>> >
>> >>could lead to problems with stale xattrs in a shared filesystem over
>> >>time unless one could limit the number of xattrs with the same
>> >>prefix, e.g., security.capability*. So I doubt that it would work.
>> >Hm.  Yeah.  But really how many setups are there like that?  I.e. if
>> >you launch a regular docker or lxd container, the image doesn't do a
>> >bind mount of a shared image, it layers something above it or does a
>> >copy.  What setups do you know of where multiple containers in different
>> >user namespaces mount the same filesystem shared and writeable?
>> 
>> I think I have something now that accomodates userns access to
>> security.capability:
>> 
>> https://github.com/stefanberger/linux/commits/xattr_for_userns
>
> Thanks!
>
>> Encoding of uid is in the attribute name now as follows:
>> security.foo@uid=<uid>
>> 
>> 1) The 'plain' security.capability is only r/w accessible from the
>> host (init_user_ns).
>> 2) When userns reads/writes 'security.capability' it will read/write
>> security.capability@uid=<uid> instead, with uid being the uid of
>> root , e.g. 1000.
>> 3) When listing xattrs for userns the host's security.capability is
>> filtered out to avoid read failures iof 'security.capability' if
>> security.capability@uid=<uid> is read but not there. (see 1) and 2))
>> 4) security.capability* may all be read from anywhere
>> 5) security.capability@uid=<uid> may be read or written directly
>> from a userns if <uid> matches the uid of root (current_uid())
>
> This looks very close to what we want.  One exception - we do want
> to support root in a user namespace being able to write
> security.capability@uid=<x> where <x> is a valid uid mapped in its
> namespace.  In that case the name should be rewritten to be
> security.capability@uid=<y> where y is the unmapped kuid.val.
>
> Eric,
>
> so far my patch hasn't yet hit Linus' tree.  Given that, would you
> mind taking a look and seeing what you think of this approach?  If
> we may decide to go this route, we probably should stop my patch
> from hitting Linus' tree before we have to continue supporting it.

Agreed.  I will take a look.  I also want to see how all of this works
in the context of stackable filesystems.  As that is the one case that
looked like it could be a problem case in your current patchset.

Eric

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#1670300

On Tue, Jun 20, 2017 at 12:34 AM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> "Serge E. Hallyn" <serge@hallyn.com> writes:
>
>> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>>> On 06/14/2017 11:05 PM, Serge E. Hallyn wrote:
>>> >On Wed, Jun 14, 2017 at 08:27:40AM -0400, Stefan Berger wrote:
>>> >>On 06/13/2017 07:55 PM, Serge E. Hallyn wrote:
>>> >>>Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>>> >>>>  If all extended
>>> >>>>attributes were to support this model, maybe the 'uid' could be
>>> >>>>associated with the 'name' of the xattr rather than its 'value' (not
>>> >>>>sure whether that's possible).
>>> >>>Right, I missed that in your original email when I saw it this morning.
>>> >>>It's not what my patch does, but it's an interesting idea.  Do you have
>>> >>>a patch to that effect?  We might even be able to generalize that to
>>> >>No, I don't have a patch. It may not be possible to implement it.
>>> >>The xattr_handler's  take the name of the xattr as input to get().
>>> >That may be ok though.  Assume the host created a container with
>>> >100000 as the uid for root, which created a container with 130000 as
>>> >uid for root.  If root in the nested container tries to read the
>>> >xattr, the kernel can check for security.foo[130000] first, then
>>> >security.foo[100000], then security.foo.  Or, it can do a listxattr
>>> >and look for those.  Am I overlooking one?
>>> >
>>> >>So one could try to encode the mapped uid in the name. However, that
>>> >I thought that's exactly what you were suggesting in your original
>>> >email?  "security.capability[uid=2000]"
>>> >
>>> >>could lead to problems with stale xattrs in a shared filesystem over
>>> >>time unless one could limit the number of xattrs with the same
>>> >>prefix, e.g., security.capability*. So I doubt that it would work.
>>> >Hm.  Yeah.  But really how many setups are there like that?  I.e. if
>>> >you launch a regular docker or lxd container, the image doesn't do a
>>> >bind mount of a shared image, it layers something above it or does a
>>> >copy.  What setups do you know of where multiple containers in different
>>> >user namespaces mount the same filesystem shared and writeable?
>>>
>>> I think I have something now that accomodates userns access to
>>> security.capability:
>>>
>>> https://github.com/stefanberger/linux/commits/xattr_for_userns
>>
>> Thanks!
>>
>>> Encoding of uid is in the attribute name now as follows:
>>> security.foo@uid=<uid>
>>>
>>> 1) The 'plain' security.capability is only r/w accessible from the
>>> host (init_user_ns).
>>> 2) When userns reads/writes 'security.capability' it will read/write
>>> security.capability@uid=<uid> instead, with uid being the uid of
>>> root , e.g. 1000.
>>> 3) When listing xattrs for userns the host's security.capability is
>>> filtered out to avoid read failures iof 'security.capability' if
>>> security.capability@uid=<uid> is read but not there. (see 1) and 2))
>>> 4) security.capability* may all be read from anywhere
>>> 5) security.capability@uid=<uid> may be read or written directly
>>> from a userns if <uid> matches the uid of root (current_uid())
>>
>> This looks very close to what we want.  One exception - we do want
>> to support root in a user namespace being able to write
>> security.capability@uid=<x> where <x> is a valid uid mapped in its
>> namespace.  In that case the name should be rewritten to be
>> security.capability@uid=<y> where y is the unmapped kuid.val.
>>
>> Eric,
>>
>> so far my patch hasn't yet hit Linus' tree.  Given that, would you
>> mind taking a look and seeing what you think of this approach?  If
>> we may decide to go this route, we probably should stop my patch
>> from hitting Linus' tree before we have to continue supporting it.
>
> Agreed.  I will take a look.  I also want to see how all of this works
> in the context of stackable filesystems.  As that is the one case that
> looked like it could be a problem case in your current patchset.
>

Apropos stackable filesystems [cc some overlayfs folks], is there any
way that parts of this work could be generalized towards ns aware
trusted@uid.* xattr?

With overlayfs, files are written to underlying fs with mounter's
credentials. How this affects v3 security capabilities and how exactly
security xattrs are handled in overtlayfs I'm not sure. Vivek?

But, if we had an infrastructure to store trusted@<rootid> xattr, then
unprivileged overlayfs mount would become a very reachable goal.
Much closer goal then loop mounting...

Amir.

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#1670725

On 06/20/2017 01:42 AM, Amir Goldstein wrote:
> On Tue, Jun 20, 2017 at 12:34 AM, Eric W. Biederman
> <ebiederm@xmission.com> wrote:
>> "Serge E. Hallyn" <serge@hallyn.com> writes:
>>
>>> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>>>> On 06/14/2017 11:05 PM, Serge E. Hallyn wrote:
>>>>> On Wed, Jun 14, 2017 at 08:27:40AM -0400, Stefan Berger wrote:
>>>>>> On 06/13/2017 07:55 PM, Serge E. Hallyn wrote:
>>>>>>> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>>>>>>>>   If all extended
>>>>>>>> attributes were to support this model, maybe the 'uid' could be
>>>>>>>> associated with the 'name' of the xattr rather than its 'value' (not
>>>>>>>> sure whether that's possible).
>>>>>>> Right, I missed that in your original email when I saw it this morning.
>>>>>>> It's not what my patch does, but it's an interesting idea.  Do you have
>>>>>>> a patch to that effect?  We might even be able to generalize that to
>>>>>> No, I don't have a patch. It may not be possible to implement it.
>>>>>> The xattr_handler's  take the name of the xattr as input to get().
>>>>> That may be ok though.  Assume the host created a container with
>>>>> 100000 as the uid for root, which created a container with 130000 as
>>>>> uid for root.  If root in the nested container tries to read the
>>>>> xattr, the kernel can check for security.foo[130000] first, then
>>>>> security.foo[100000], then security.foo.  Or, it can do a listxattr
>>>>> and look for those.  Am I overlooking one?
>>>>>
>>>>>> So one could try to encode the mapped uid in the name. However, that
>>>>> I thought that's exactly what you were suggesting in your original
>>>>> email?  "security.capability[uid=2000]"
>>>>>
>>>>>> could lead to problems with stale xattrs in a shared filesystem over
>>>>>> time unless one could limit the number of xattrs with the same
>>>>>> prefix, e.g., security.capability*. So I doubt that it would work.
>>>>> Hm.  Yeah.  But really how many setups are there like that?  I.e. if
>>>>> you launch a regular docker or lxd container, the image doesn't do a
>>>>> bind mount of a shared image, it layers something above it or does a
>>>>> copy.  What setups do you know of where multiple containers in different
>>>>> user namespaces mount the same filesystem shared and writeable?
>>>> I think I have something now that accomodates userns access to
>>>> security.capability:
>>>>
>>>> https://github.com/stefanberger/linux/commits/xattr_for_userns
>>> Thanks!
>>>
>>>> Encoding of uid is in the attribute name now as follows:
>>>> security.foo@uid=<uid>
>>>>
>>>> 1) The 'plain' security.capability is only r/w accessible from the
>>>> host (init_user_ns).
>>>> 2) When userns reads/writes 'security.capability' it will read/write
>>>> security.capability@uid=<uid> instead, with uid being the uid of
>>>> root , e.g. 1000.
>>>> 3) When listing xattrs for userns the host's security.capability is
>>>> filtered out to avoid read failures iof 'security.capability' if
>>>> security.capability@uid=<uid> is read but not there. (see 1) and 2))
>>>> 4) security.capability* may all be read from anywhere
>>>> 5) security.capability@uid=<uid> may be read or written directly
>>>> from a userns if <uid> matches the uid of root (current_uid())
>>> This looks very close to what we want.  One exception - we do want
>>> to support root in a user namespace being able to write
>>> security.capability@uid=<x> where <x> is a valid uid mapped in its
>>> namespace.  In that case the name should be rewritten to be
>>> security.capability@uid=<y> where y is the unmapped kuid.val.
>>>
>>> Eric,
>>>
>>> so far my patch hasn't yet hit Linus' tree.  Given that, would you
>>> mind taking a look and seeing what you think of this approach?  If
>>> we may decide to go this route, we probably should stop my patch
>>> from hitting Linus' tree before we have to continue supporting it.
>> Agreed.  I will take a look.  I also want to see how all of this works
>> in the context of stackable filesystems.  As that is the one case that
>> looked like it could be a problem case in your current patchset.
>>
> Apropos stackable filesystems [cc some overlayfs folks], is there any
> way that parts of this work could be generalized towards ns aware
> trusted@uid.* xattr?

I am at least removing all string comparison with xattr names from the 
core code and move the enabled xattr names into a list. For the 
security.* extended attribute names we would enumerated the enabled ones 
in that list, only security.capability for now. I am not sure how the 
trusted.* space works.

     Stefan

>
> With overlayfs, files are written to underlying fs with mounter's
> credentials. How this affects v3 security capabilities and how exactly
> security xattrs are handled in overtlayfs I'm not sure. Vivek?
>
> But, if we had an infrastructure to store trusted@<rootid> xattr, then
> unprivileged overlayfs mount would become a very reachable goal.
> Much closer goal then loop mounting...
>
> Amir.
>

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#1670989

On 06/20/2017 08:19 AM, Stefan Berger wrote:
> On 06/20/2017 01:42 AM, Amir Goldstein wrote:
>> On Tue, Jun 20, 2017 at 12:34 AM, Eric W. Biederman
>> <ebiederm@xmission.com> wrote:
>>> "Serge E. Hallyn" <serge@hallyn.com> writes:
>>>
>>>> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>>>>> On 06/14/2017 11:05 PM, Serge E. Hallyn wrote:
>>>>>> On Wed, Jun 14, 2017 at 08:27:40AM -0400, Stefan Berger wrote:
>>>>>>> On 06/13/2017 07:55 PM, Serge E. Hallyn wrote:
>>>>>>>> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
>>>>>>>>>   If all extended
>>>>>>>>> attributes were to support this model, maybe the 'uid' could be
>>>>>>>>> associated with the 'name' of the xattr rather than its 
>>>>>>>>> 'value' (not
>>>>>>>>> sure whether that's possible).
>>>>>>>> Right, I missed that in your original email when I saw it this 
>>>>>>>> morning.
>>>>>>>> It's not what my patch does, but it's an interesting idea.  Do 
>>>>>>>> you have
>>>>>>>> a patch to that effect?  We might even be able to generalize 
>>>>>>>> that to
>>>>>>> No, I don't have a patch. It may not be possible to implement it.
>>>>>>> The xattr_handler's  take the name of the xattr as input to get().
>>>>>> That may be ok though.  Assume the host created a container with
>>>>>> 100000 as the uid for root, which created a container with 130000 as
>>>>>> uid for root.  If root in the nested container tries to read the
>>>>>> xattr, the kernel can check for security.foo[130000] first, then
>>>>>> security.foo[100000], then security.foo.  Or, it can do a listxattr
>>>>>> and look for those.  Am I overlooking one?
>>>>>>
>>>>>>> So one could try to encode the mapped uid in the name. However, 
>>>>>>> that
>>>>>> I thought that's exactly what you were suggesting in your original
>>>>>> email?  "security.capability[uid=2000]"
>>>>>>
>>>>>>> could lead to problems with stale xattrs in a shared filesystem 
>>>>>>> over
>>>>>>> time unless one could limit the number of xattrs with the same
>>>>>>> prefix, e.g., security.capability*. So I doubt that it would work.
>>>>>> Hm.  Yeah.  But really how many setups are there like that?  I.e. if
>>>>>> you launch a regular docker or lxd container, the image doesn't do a
>>>>>> bind mount of a shared image, it layers something above it or does a
>>>>>> copy.  What setups do you know of where multiple containers in 
>>>>>> different
>>>>>> user namespaces mount the same filesystem shared and writeable?
>>>>> I think I have something now that accomodates userns access to
>>>>> security.capability:
>>>>>
>>>>> https://github.com/stefanberger/linux/commits/xattr_for_userns
>>>> Thanks!
>>>>
>>>>> Encoding of uid is in the attribute name now as follows:
>>>>> security.foo@uid=<uid>
>>>>>
>>>>> 1) The 'plain' security.capability is only r/w accessible from the
>>>>> host (init_user_ns).
>>>>> 2) When userns reads/writes 'security.capability' it will read/write
>>>>> security.capability@uid=<uid> instead, with uid being the uid of
>>>>> root , e.g. 1000.
>>>>> 3) When listing xattrs for userns the host's security.capability is
>>>>> filtered out to avoid read failures iof 'security.capability' if
>>>>> security.capability@uid=<uid> is read but not there. (see 1) and 2))
>>>>> 4) security.capability* may all be read from anywhere
>>>>> 5) security.capability@uid=<uid> may be read or written directly
>>>>> from a userns if <uid> matches the uid of root (current_uid())
>>>> This looks very close to what we want.  One exception - we do want
>>>> to support root in a user namespace being able to write
>>>> security.capability@uid=<x> where <x> is a valid uid mapped in its
>>>> namespace.  In that case the name should be rewritten to be
>>>> security.capability@uid=<y> where y is the unmapped kuid.val.
>>>>
>>>> Eric,
>>>>
>>>> so far my patch hasn't yet hit Linus' tree.  Given that, would you
>>>> mind taking a look and seeing what you think of this approach?  If
>>>> we may decide to go this route, we probably should stop my patch
>>>> from hitting Linus' tree before we have to continue supporting it.
>>> Agreed.  I will take a look.  I also want to see how all of this works
>>> in the context of stackable filesystems.  As that is the one case that
>>> looked like it could be a problem case in your current patchset.
>>>
>> Apropos stackable filesystems [cc some overlayfs folks], is there any
>> way that parts of this work could be generalized towards ns aware
>> trusted@uid.* xattr?
>
> I am at least removing all string comparison with xattr names from the 
> core code and move the enabled xattr names into a list. For the 
> security.* extended attribute names we would enumerated the enabled 
> ones in that list, only security.capability for now. I am not sure how 
> the trusted.* space works.

I extended 'the infrastructure' now to support prefix matching for 
trusted.* and probably others as well. It's fairly easy to do that but 
would not write the code like that for exact string matching to support 
security.capability. The patch lets me write trusted.foo@uid=100 from 
within the userns if uid=100 exists, rejects it otherwise. It may be 
written out as trusted.foo@uid=1100 for root mapping to uid 1000. I can 
list this entry on the host. For some reason trusted.* is not listed at 
all inside the userns. So something else needs to be enabled as well. 
For now it looks like this:


https://github.com/stefanberger/linux/commit/8ae131e731c9e1def92a2100697632ea35e007d0

Regards,
     Stefan

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#1671055

On Tue, Jun 20, 2017 at 8:33 PM, Stefan Berger
<stefanb@linux.vnet.ibm.com> wrote:
> On 06/20/2017 08:19 AM, Stefan Berger wrote:
>>
>> On 06/20/2017 01:42 AM, Amir Goldstein wrote:

>>>>
>>> Apropos stackable filesystems [cc some overlayfs folks], is there any
>>> way that parts of this work could be generalized towards ns aware
>>> trusted@uid.* xattr?
>>
>>
>> I am at least removing all string comparison with xattr names from the
>> core code and move the enabled xattr names into a list. For the security.*
>> extended attribute names we would enumerated the enabled ones in that list,
>> only security.capability for now. I am not sure how the trusted.* space
>> works.
>
>
> I extended 'the infrastructure' now to support prefix matching for trusted.*
> and probably others as well. It's fairly easy to do that but would not write
> the code like that for exact string matching to support security.capability.
> The patch lets me write trusted.foo@uid=100 from within the userns if
> uid=100 exists, rejects it otherwise. It may be written out as
> trusted.foo@uid=1100 for root mapping to uid 1000. I can list this entry on
> the host. For some reason trusted.* is not listed at all inside the userns.
> So something else needs to be enabled as well. For now it looks like this:
>
>
> https://github.com/stefanberger/linux/commit/8ae131e731c9e1def92a2100697632ea35e007d0
>

That looks useful!
I hope someone who knows his way around trusted xattr can say what's missing.

Thanks,
Amir.

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#1671060

On Tue, Jun 20, 2017 at 08:42:45AM +0300, Amir Goldstein wrote:
> On Tue, Jun 20, 2017 at 12:34 AM, Eric W. Biederman
> <ebiederm@xmission.com> wrote:
> > "Serge E. Hallyn" <serge@hallyn.com> writes:
> >
> >> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
> >>> On 06/14/2017 11:05 PM, Serge E. Hallyn wrote:
> >>> >On Wed, Jun 14, 2017 at 08:27:40AM -0400, Stefan Berger wrote:
> >>> >>On 06/13/2017 07:55 PM, Serge E. Hallyn wrote:
> >>> >>>Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
> >>> >>>>  If all extended
> >>> >>>>attributes were to support this model, maybe the 'uid' could be
> >>> >>>>associated with the 'name' of the xattr rather than its 'value' (not
> >>> >>>>sure whether that's possible).
> >>> >>>Right, I missed that in your original email when I saw it this morning.
> >>> >>>It's not what my patch does, but it's an interesting idea.  Do you have
> >>> >>>a patch to that effect?  We might even be able to generalize that to
> >>> >>No, I don't have a patch. It may not be possible to implement it.
> >>> >>The xattr_handler's  take the name of the xattr as input to get().
> >>> >That may be ok though.  Assume the host created a container with
> >>> >100000 as the uid for root, which created a container with 130000 as
> >>> >uid for root.  If root in the nested container tries to read the
> >>> >xattr, the kernel can check for security.foo[130000] first, then
> >>> >security.foo[100000], then security.foo.  Or, it can do a listxattr
> >>> >and look for those.  Am I overlooking one?
> >>> >
> >>> >>So one could try to encode the mapped uid in the name. However, that
> >>> >I thought that's exactly what you were suggesting in your original
> >>> >email?  "security.capability[uid=2000]"
> >>> >
> >>> >>could lead to problems with stale xattrs in a shared filesystem over
> >>> >>time unless one could limit the number of xattrs with the same
> >>> >>prefix, e.g., security.capability*. So I doubt that it would work.
> >>> >Hm.  Yeah.  But really how many setups are there like that?  I.e. if
> >>> >you launch a regular docker or lxd container, the image doesn't do a
> >>> >bind mount of a shared image, it layers something above it or does a
> >>> >copy.  What setups do you know of where multiple containers in different
> >>> >user namespaces mount the same filesystem shared and writeable?
> >>>
> >>> I think I have something now that accomodates userns access to
> >>> security.capability:
> >>>
> >>> https://github.com/stefanberger/linux/commits/xattr_for_userns
> >>
> >> Thanks!
> >>
> >>> Encoding of uid is in the attribute name now as follows:
> >>> security.foo@uid=<uid>
> >>>
> >>> 1) The 'plain' security.capability is only r/w accessible from the
> >>> host (init_user_ns).
> >>> 2) When userns reads/writes 'security.capability' it will read/write
> >>> security.capability@uid=<uid> instead, with uid being the uid of
> >>> root , e.g. 1000.
> >>> 3) When listing xattrs for userns the host's security.capability is
> >>> filtered out to avoid read failures iof 'security.capability' if
> >>> security.capability@uid=<uid> is read but not there. (see 1) and 2))
> >>> 4) security.capability* may all be read from anywhere
> >>> 5) security.capability@uid=<uid> may be read or written directly
> >>> from a userns if <uid> matches the uid of root (current_uid())
> >>
> >> This looks very close to what we want.  One exception - we do want
> >> to support root in a user namespace being able to write
> >> security.capability@uid=<x> where <x> is a valid uid mapped in its
> >> namespace.  In that case the name should be rewritten to be
> >> security.capability@uid=<y> where y is the unmapped kuid.val.
> >>
> >> Eric,
> >>
> >> so far my patch hasn't yet hit Linus' tree.  Given that, would you
> >> mind taking a look and seeing what you think of this approach?  If
> >> we may decide to go this route, we probably should stop my patch
> >> from hitting Linus' tree before we have to continue supporting it.
> >
> > Agreed.  I will take a look.  I also want to see how all of this works
> > in the context of stackable filesystems.  As that is the one case that
> > looked like it could be a problem case in your current patchset.
> >
> 
> Apropos stackable filesystems [cc some overlayfs folks], is there any
> way that parts of this work could be generalized towards ns aware
> trusted@uid.* xattr?
> 
> With overlayfs, files are written to underlying fs with mounter's
> credentials.

We do switch to mounter's credential for privileged operations but
for a newly created file final selinux label is created as if task
created that file.

>How this affects v3 security capabilities and how exactly
> security xattrs are handled in overtlayfs I'm not sure. Vivek?

Given we switch to mounter's creds for operations on underlying
filesystem (setxattr, getxattr), I thought that it probably
will call xattr_userns_name() in nested manner. Once using tasks's
creds and second time using mounter's creds. So that probably
should have made it security.foo@uid@uid. I tried patches quickly
but setcap/getcap inside containers work. So may be it is
due to the fact that mounting was done from init_user_ns and
following line of code will avoid adding @uid in that case.

+       /* no name changes for init_user_ns or uid == 0 */
+       if (current_user_ns() == &init_user_ns || uid.val == 0)
+               goto out_copy;
+

I have not looked deeper. Still curious how getxattr() path works
when we switch to mounter's creds. In that case underlying file
system will get the impression that mounter task is trying to
do getxattr() in security.capability set by container task. I
am assuming we allow that?

I need to spend more time understanding this.

Vivek

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#1665303

Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
> On 05/08/2017 02:11 PM, Serge E. Hallyn wrote:
> >Root in a non-initial user ns cannot be trusted to write a traditional
> >security.capability xattr.  If it were allowed to do so, then any
> >unprivileged user on the host could map his own uid to root in a private
> >namespace, write the xattr, and execute the file with privilege on the
> >host.
> >
> >However supporting file capabilities in a user namespace is very
> >desirable.  Not doing so means that any programs designed to run with
> >limited privilege must continue to support other methods of gaining and
> >dropping privilege.  For instance a program installer must detect
> >whether file capabilities can be assigned, and assign them if so but set
> >setuid-root otherwise.  The program in turn must know how to drop
> >partial capabilities, and do so only if setuid-root.
> 
> Hi Serge,
> 
> 
>   I have been looking at patch below primarily to learn how we could
> apply a similar technique to security.ima and security.evm for a
> namespaced IMA. From the paragraphs above I thought that you solved
> the problem of a shared filesystem where one now can write different
> security.capability xattrs by effectively supporting for example
> security.capability[uid=1000] and security.capability[uid=2000]

Interesting idea.  Worth considering.

> written into the filesystem. Each would then become visible as
> security.capability if the userns mapping is set appropriately.
> However, this doesn't seem to be how it is implemented. There seems

Indeed, when I was considering supporting multiple simulatenous
xattrs, I did it as something like:

struct vfs_ns_cap_data {
	struct {
		__le32 permitted;
		__le32 inheritable;
	} data[VFS_CAP_U32];
	__le32 rootid;
};

struct vfs_ns_cap {
	__le32 magic_etc;
	__le32 n_entries;
	struct ns_cap_data data[0];
}; // followed by n_entries of struct ns_cap_data

You're instead suggesting encoding the rootuid in the name,
which is interesting.

> to be only a single such entry with uid appended to it and, if it
> was a shared filesystem, the first one to set this attribute blocks
> everyone else from writing the xattr. Is that how it works? Would

Approximately - indeed there is only a single xattr.  But it can be
overwritten, so long as the writer has CAP_SETFCAP over the user_ns
which mounted the filesystem.

> that work differently with an overlay filesystem ? I think a similar
> model could also work for IMA, but maybe you have some thoughts. The
> only thing I would be concerned about is blocking the parent
> container's root user from setting an xattr.
> 
> Regards,
>    Stefan
> 
> 
> >
> >This patch introduces v3 of the security.capability xattr.  It builds a
> >vfs_ns_cap_data struct by appending a uid_t rootid to struct
> >vfs_cap_data.  This is the absolute uid_t (that is, the uid_t in user
> >namespace which mounted the filesystem, usually init_user_ns) of the
> >root id in whose namespaces the file capabilities may take effect.
> >
> >When a task asks to write a v2 security.capability xattr, if it is
> >privileged with respect to the userns which mounted the filesystem, then
> >nothing should change.  Otherwise, the kernel will transparently rewrite
> >the xattr as a v3 with the appropriate rootid.  This is done during the
> >execution of setxattr() to catch user-space-initiated capability writes.
> >Subsequently, any task executing the file which has the noted kuid as
> >its root uid, or which is in a descendent user_ns of such a user_ns,
> >will run the file with capabilities.
> >
> >Similarly when asking to read file capabilities, a v3 capability will
> >be presented as v2 if it applies to the caller's namespace.
> >
> >If a task writes a v3 security.capability, then it can provide a uid for
> >the xattr so long as the uid is valid in its own user namespace, and it
> >is privileged with CAP_SETFCAP over its namespace.  The kernel will
> >translate that rootid to an absolute uid, and write that to disk.  After
> >this, a task in the writer's namespace will not be able to use those
> >capabilities (unless rootid was 0), but a task in a namespace where the
> >given uid is root will.
> >
> >Only a single security.capability xattr may exist at a time for a given
> >file.  A task may overwrite an existing xattr so long as it is
> >privileged over the inode.  Note this is a departure from previous
> >semantics, which required privilege to remove a security.capability
> >xattr.  This check can be re-added if deemed useful.
> >
> >This allows a simple setxattr to work, allows tar/untar to work, and
> >allows us to tar in one namespace and untar in another while preserving
> >the capability, without risking leaking privilege into a parent
> >namespace.
> >
> >Example using tar:
> >
> >  $ cp /bin/sleep sleepx
> >  $ mkdir b1 b2
> >  $ lxc-usernsexec -m b:0:100000:1 -m b:1:$(id -u):1 -- chown 0:0 b1
> >  $ lxc-usernsexec -m b:0:100001:1 -m b:1:$(id -u):1 -- chown 0:0 b2
> >  $ lxc-usernsexec -m b:0:100000:1000 -- tar --xattrs-include=security.capability --xattrs -cf b1/sleepx.tar sleepx
> >  $ lxc-usernsexec -m b:0:100001:1000 -- tar --xattrs-include=security.capability --xattrs -C b2 -xf b1/sleepx.tar
> >  $ lxc-usernsexec -m b:0:100001:1000 -- getcap b2/sleepx
> >    b2/sleepx = cap_sys_admin+ep
> >  # /opt/ltp/testcases/bin/getv3xattr b2/sleepx
> >    v3 xattr, rootid is 100001
> >
> >A patch to linux-test-project adding a new set of tests for this
> >functionality is in the nsfscaps branch at github.com/hallyn/ltp
> >
> >Changelog:
> >    Nov 02 2016: fix invalid check at refuse_fcap_overwrite()
> >    Nov 07 2016: convert rootid from and to fs user_ns
> >    (From ebiederm: mar 28 2017)
> >      commoncap.c: fix typos - s/v4/v3
> >      get_vfs_caps_from_disk: clarify the fs_ns root access check
> >      nsfscaps: change the code split for cap_inode_setxattr()
> >    Apr 09 2017:
> >        don't return v3 cap for caps owned by current root.
> >       return a v2 cap for a true v2 cap in non-init ns
> >    Apr 18 2017:
> >       . Change the flow of fscap writing to support s_user_ns writing.
> >       . Remove refuse_fcap_overwrite().  The value of the previous
> >         xattr doesn't matter.
> >    Apr 24 2017:
> >       . incorporate Eric's incremental diff
> >       . move cap_convert_nscap to setxattr and simplify its usage
> >    May 8, 2017:
> >       . fix leaking dentry refcount in cap_inode_getsecurity
> >
> >Signed-off-by: Serge Hallyn <serge@hallyn.com>
> >---
> >  fs/xattr.c                      |   6 +
> >  include/linux/capability.h      |   2 +
> >  include/linux/security.h        |   2 +
> >  include/uapi/linux/capability.h |  22 +++-
> >  security/commoncap.c            | 270 +++++++++++++++++++++++++++++++++++++---
> >  5 files changed, 280 insertions(+), 22 deletions(-)
> >
> >diff --git a/fs/xattr.c b/fs/xattr.c
> >index 7e3317c..0a9dea4 100644
> >--- a/fs/xattr.c
> >+++ b/fs/xattr.c
> >@@ -444,6 +444,12 @@ setxattr(struct dentry *d, const char __user *name, const void __user *value,
> >  		if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
> >  		    (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
> >  			posix_acl_fix_xattr_from_user(kvalue, size);
> >+		else if (strcmp(kname, XATTR_NAME_CAPS) == 0) {
> >+			error = cap_convert_nscap(d, &kvalue, size);
> >+			if (error < 0)
> >+				goto out;
> >+			size = error;
> >+		}
> >  	}
> >
> >  	error = vfs_setxattr(d, kname, kvalue, size, flags);
> >diff --git a/include/linux/capability.h b/include/linux/capability.h
> >index 6ffb67e..b52e278 100644
> >--- a/include/linux/capability.h
> >+++ b/include/linux/capability.h
> >@@ -248,4 +248,6 @@ extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns);
> >  /* audit system wants to get cap info from files as well */
> >  extern int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps);
> >
> >+extern int cap_convert_nscap(struct dentry *dentry, void **ivalue, size_t size);
> >+
> >  #endif /* !_LINUX_CAPABILITY_H */
> >diff --git a/include/linux/security.h b/include/linux/security.h
> >index 96899fa..bd49cc1 100644
> >--- a/include/linux/security.h
> >+++ b/include/linux/security.h
> >@@ -86,6 +86,8 @@ extern int cap_inode_setxattr(struct dentry *dentry, const char *name,
> >  extern int cap_inode_removexattr(struct dentry *dentry, const char *name);
> >  extern int cap_inode_need_killpriv(struct dentry *dentry);
> >  extern int cap_inode_killpriv(struct dentry *dentry);
> >+extern int cap_inode_getsecurity(struct inode *inode, const char *name,
> >+				 void **buffer, bool alloc);
> >  extern int cap_mmap_addr(unsigned long addr);
> >  extern int cap_mmap_file(struct file *file, unsigned long reqprot,
> >  			 unsigned long prot, unsigned long flags);
> >diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> >index 49bc062..fd4f87d 100644
> >--- a/include/uapi/linux/capability.h
> >+++ b/include/uapi/linux/capability.h
> >@@ -60,9 +60,13 @@ typedef struct __user_cap_data_struct {
> >  #define VFS_CAP_U32_2           2
> >  #define XATTR_CAPS_SZ_2         (sizeof(__le32)*(1 + 2*VFS_CAP_U32_2))
> >
> >-#define XATTR_CAPS_SZ           XATTR_CAPS_SZ_2
> >-#define VFS_CAP_U32             VFS_CAP_U32_2
> >-#define VFS_CAP_REVISION	VFS_CAP_REVISION_2
> >+#define VFS_CAP_REVISION_3	0x03000000
> >+#define VFS_CAP_U32_3           2
> >+#define XATTR_CAPS_SZ_3         (sizeof(__le32)*(2 + 2*VFS_CAP_U32_3))
> >+
> >+#define XATTR_CAPS_SZ           XATTR_CAPS_SZ_3
> >+#define VFS_CAP_U32             VFS_CAP_U32_3
> >+#define VFS_CAP_REVISION	VFS_CAP_REVISION_3
> >
> >  struct vfs_cap_data {
> >  	__le32 magic_etc;            /* Little endian */
> >@@ -72,6 +76,18 @@ struct vfs_cap_data {
> >  	} data[VFS_CAP_U32];
> >  };
> >
> >+/*
> >+ * same as vfs_cap_data but with a rootid at the end
> >+ */
> >+struct vfs_ns_cap_data {
> >+	__le32 magic_etc;
> >+	struct {
> >+		__le32 permitted;    /* Little endian */
> >+		__le32 inheritable;  /* Little endian */
> >+	} data[VFS_CAP_U32];
> >+	__le32 rootid;
> >+};
> >+
> >  #ifndef __KERNEL__
> >
> >  /*
> >diff --git a/security/commoncap.c b/security/commoncap.c
> >index 78b3783..c28d126 100644
> >--- a/security/commoncap.c
> >+++ b/security/commoncap.c
> >@@ -332,6 +332,209 @@ int cap_inode_killpriv(struct dentry *dentry)
> >  	return error;
> >  }
> >
> >+static bool rootid_owns_currentns(kuid_t kroot)
> >+{
> >+	struct user_namespace *ns;
> >+
> >+	if (!uid_valid(kroot))
> >+		return false;
> >+
> >+	for (ns = current_user_ns(); ; ns = ns->parent) {
> >+		if (from_kuid(ns, kroot) == 0)
> >+			return true;
> >+		if (ns == &init_user_ns)
> >+			break;
> >+	}
> >+
> >+	return false;
> >+}
> >+
> >+static __u32 sansflags(__u32 m)
> >+{
> >+	return m & ~VFS_CAP_FLAGS_EFFECTIVE;
> >+}
> >+
> >+static bool is_v2header(size_t size, __le32 magic)
> >+{
> >+	__u32 m = le32_to_cpu(magic);
> >+	if (size != XATTR_CAPS_SZ_2)
> >+		return false;
> >+	return sansflags(m) == VFS_CAP_REVISION_2;
> >+}
> >+
> >+static bool is_v3header(size_t size, __le32 magic)
> >+{
> >+	__u32 m = le32_to_cpu(magic);
> >+
> >+	if (size != XATTR_CAPS_SZ_3)
> >+		return false;
> >+	return sansflags(m) == VFS_CAP_REVISION_3;
> >+}
> >+
> >+/*
> >+ * getsecurity: We are called for security.* before any attempt to read the
> >+ * xattr from the inode itself.
> >+ *
> >+ * This gives us a chance to read the on-disk value and convert it.  If we
> >+ * return -EOPNOTSUPP, then vfs_getxattr() will call the i_op handler.
> >+ *
> >+ * Note we are not called by vfs_getxattr_alloc(), but that is only called
> >+ * by the integrity subsystem, which really wants the unconverted values -
> >+ * so that's good.
> >+ */
> >+int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
> >+			  bool alloc)
> >+{
> >+	int size, ret;
> >+	kuid_t kroot;
> >+	uid_t root, mappedroot;
> >+	char *tmpbuf = NULL;
> >+	struct vfs_cap_data *cap;
> >+	struct vfs_ns_cap_data *nscap;
> >+	struct dentry *dentry;
> >+	struct user_namespace *fs_ns;
> >+
> >+	if (strcmp(name, "capability") != 0)
> >+		return -EOPNOTSUPP;
> >+
> >+	dentry = d_find_alias(inode);
> >+	if (!dentry)
> >+		return -EINVAL;
> >+
> >+	size = sizeof(struct vfs_ns_cap_data);
> >+	ret = (int) vfs_getxattr_alloc(dentry, XATTR_NAME_CAPS,
> >+				 &tmpbuf, size, GFP_NOFS);
> >+	dput(dentry);
> >+
> >+	if (ret < 0)
> >+		return ret;
> >+
> >+	fs_ns = inode->i_sb->s_user_ns;
> >+	cap = (struct vfs_cap_data *) tmpbuf;
> >+	if (is_v2header((size_t) ret, cap->magic_etc)) {
> >+		/* If this is sizeof(vfs_cap_data) then we're ok with the
> >+		 * on-disk value, so return that.  */
> >+		if (alloc)
> >+			*buffer = tmpbuf;
> >+		else
> >+			kfree(tmpbuf);
> >+		return ret;
> >+	} else if (!is_v3header((size_t) ret, cap->magic_etc)) {
> >+		kfree(tmpbuf);
> >+		return -EINVAL;
> >+	}
> >+
> >+	nscap = (struct vfs_ns_cap_data *) tmpbuf;
> >+	root = le32_to_cpu(nscap->rootid);
> >+	kroot = make_kuid(fs_ns, root);
> >+
> >+	/* If the root kuid maps to a valid uid in current ns, then return
> >+	 * this as a nscap. */
> >+	mappedroot = from_kuid(current_user_ns(), kroot);
> >+	if (mappedroot != (uid_t)-1 && mappedroot != (uid_t)0) {
> >+		if (alloc) {
> >+			*buffer = tmpbuf;
> >+			nscap->rootid = cpu_to_le32(mappedroot);
> >+		} else
> >+			kfree(tmpbuf);
> >+		return size;
> >+	}
> >+
> >+	if (!rootid_owns_currentns(kroot)) {
> >+		kfree(tmpbuf);
> >+		return -EOPNOTSUPP;
> >+	}
> >+
> >+	/* This comes from a parent namespace.  Return as a v2 capability */
> >+	size = sizeof(struct vfs_cap_data);
> >+	if (alloc) {
> >+		*buffer = kmalloc(size, GFP_ATOMIC);
> >+		if (*buffer) {
> >+			struct vfs_cap_data *cap = *buffer;
> >+			__le32 nsmagic, magic;
> >+			magic = VFS_CAP_REVISION_2;
> >+			nsmagic = le32_to_cpu(nscap->magic_etc);
> >+			if (nsmagic & VFS_CAP_FLAGS_EFFECTIVE)
> >+				magic |= VFS_CAP_FLAGS_EFFECTIVE;
> >+			memcpy(&cap->data, &nscap->data, sizeof(__le32) * 2 * VFS_CAP_U32);
> >+			cap->magic_etc = cpu_to_le32(magic);
> >+		}
> >+	}
> >+	kfree(tmpbuf);
> >+	return size;
> >+}
> >+
> >+static kuid_t rootid_from_xattr(const void *value, size_t size,
> >+				struct user_namespace *task_ns)
> >+{
> >+	const struct vfs_ns_cap_data *nscap = value;
> >+	uid_t rootid = 0;
> >+
> >+	if (size == XATTR_CAPS_SZ_3)
> >+		rootid = le32_to_cpu(nscap->rootid);
> >+
> >+	return make_kuid(task_ns, rootid);
> >+}
> >+
> >+static bool validheader(size_t size, __le32 magic)
> >+{
> >+	return is_v2header(size, magic) || is_v3header(size, magic);
> >+}
> >+
> >+/*
> >+ * User requested a write of security.capability.  If needed, update the
> >+ * xattr to change from v2 to v3, or to fixup the v3 rootid.
> >+ *
> >+ * If all is ok, we return the new size, on error return < 0.
> >+ */
> >+int cap_convert_nscap(struct dentry *dentry, void **ivalue, size_t size)
> >+{
> >+	struct vfs_ns_cap_data *nscap;
> >+	uid_t nsrootid;
> >+	const struct vfs_cap_data *cap = *ivalue;
> >+	__u32 magic, nsmagic;
> >+	struct inode *inode = d_backing_inode(dentry);
> >+	struct user_namespace *task_ns = current_user_ns(),
> >+		*fs_ns = inode->i_sb->s_user_ns;
> >+	kuid_t rootid;
> >+	size_t newsize;
> >+
> >+	if (!*ivalue)
> >+		return -EINVAL;
> >+	if (!validheader(size, cap->magic_etc))
> >+		return -EINVAL;
> >+	if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
> >+		return -EPERM;
> >+	if (size == XATTR_CAPS_SZ_2)
> >+		if (ns_capable(inode->i_sb->s_user_ns, CAP_SETFCAP))
> >+			/* user is privileged, just write the v2 */
> >+			return size;
> >+
> >+	rootid = rootid_from_xattr(*ivalue, size, task_ns);
> >+	if (!uid_valid(rootid))
> >+		return -EINVAL;
> >+
> >+	nsrootid = from_kuid(fs_ns, rootid);
> >+	if (nsrootid == -1)
> >+		return -EINVAL;
> >+
> >+	newsize = sizeof(struct vfs_ns_cap_data);
> >+	nscap = kmalloc(newsize, GFP_ATOMIC);
> >+	if (!nscap)
> >+		return -ENOMEM;
> >+	nscap->rootid = cpu_to_le32(nsrootid);
> >+	nsmagic = VFS_CAP_REVISION_3;
> >+	magic = le32_to_cpu(cap->magic_etc);
> >+	if (magic & VFS_CAP_FLAGS_EFFECTIVE)
> >+		nsmagic |= VFS_CAP_FLAGS_EFFECTIVE;
> >+	nscap->magic_etc = cpu_to_le32(nsmagic);
> >+	memcpy(&nscap->data, &cap->data, sizeof(__le32) * 2 * VFS_CAP_U32);
> >+
> >+	kvfree(*ivalue);
> >+	*ivalue = nscap;
> >+	return newsize;
> >+}
> >+
> >  /*
> >   * Calculate the new process capability sets from the capability sets attached
> >   * to a file.
> >@@ -385,7 +588,10 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data
> >  	__u32 magic_etc;
> >  	unsigned tocopy, i;
> >  	int size;
> >-	struct vfs_cap_data caps;
> >+	struct vfs_ns_cap_data data, *nscaps = &data;
> >+	struct vfs_cap_data *caps = (struct vfs_cap_data *) &data;
> >+	kuid_t rootkuid;
> >+	struct user_namespace *fs_ns = inode->i_sb->s_user_ns;
> >
> >  	memset(cpu_caps, 0, sizeof(struct cpu_vfs_cap_data));
> >
> >@@ -393,18 +599,20 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data
> >  		return -ENODATA;
> >
> >  	size = __vfs_getxattr((struct dentry *)dentry, inode,
> >-			      XATTR_NAME_CAPS, &caps, XATTR_CAPS_SZ);
> >+			      XATTR_NAME_CAPS, &data, XATTR_CAPS_SZ);
> >  	if (size == -ENODATA || size == -EOPNOTSUPP)
> >  		/* no data, that's ok */
> >  		return -ENODATA;
> >+
> >  	if (size < 0)
> >  		return size;
> >
> >  	if (size < sizeof(magic_etc))
> >  		return -EINVAL;
> >
> >-	cpu_caps->magic_etc = magic_etc = le32_to_cpu(caps.magic_etc);
> >+	cpu_caps->magic_etc = magic_etc = le32_to_cpu(caps->magic_etc);
> >
> >+	rootkuid = make_kuid(fs_ns, 0);
> >  	switch (magic_etc & VFS_CAP_REVISION_MASK) {
> >  	case VFS_CAP_REVISION_1:
> >  		if (size != XATTR_CAPS_SZ_1)
> >@@ -416,15 +624,27 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data
> >  			return -EINVAL;
> >  		tocopy = VFS_CAP_U32_2;
> >  		break;
> >+	case VFS_CAP_REVISION_3:
> >+		if (size != XATTR_CAPS_SZ_3)
> >+			return -EINVAL;
> >+		tocopy = VFS_CAP_U32_3;
> >+		rootkuid = make_kuid(fs_ns, le32_to_cpu(nscaps->rootid));
> >+		break;
> >+
> >  	default:
> >  		return -EINVAL;
> >  	}
> >+	/* Limit the caps to the mounter of the filesystem
> >+	 * or the more limited uid specified in the xattr.
> >+	 */
> >+	if (!rootid_owns_currentns(rootkuid))
> >+		return -ENODATA;
> >
> >  	CAP_FOR_EACH_U32(i) {
> >  		if (i >= tocopy)
> >  			break;
> >-		cpu_caps->permitted.cap[i] = le32_to_cpu(caps.data[i].permitted);
> >-		cpu_caps->inheritable.cap[i] = le32_to_cpu(caps.data[i].inheritable);
> >+		cpu_caps->permitted.cap[i] = le32_to_cpu(caps->data[i].permitted);
> >+		cpu_caps->inheritable.cap[i] = le32_to_cpu(caps->data[i].inheritable);
> >  	}
> >
> >  	cpu_caps->permitted.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK;
> >@@ -462,8 +682,8 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_c
> >  	rc = get_vfs_caps_from_disk(bprm->file->f_path.dentry, &vcaps);
> >  	if (rc < 0) {
> >  		if (rc == -EINVAL)
> >-			printk(KERN_NOTICE "%s: get_vfs_caps_from_disk returned %d for %s\n",
> >-				__func__, rc, bprm->filename);
> >+			printk(KERN_NOTICE "Invalid argument reading file caps for %s\n",
> >+					bprm->filename);
> >  		else if (rc == -ENODATA)
> >  			rc = 0;
> >  		goto out;
> >@@ -660,15 +880,19 @@ int cap_bprm_secureexec(struct linux_binprm *bprm)
> >  int cap_inode_setxattr(struct dentry *dentry, const char *name,
> >  		       const void *value, size_t size, int flags)
> >  {
> >-	if (!strcmp(name, XATTR_NAME_CAPS)) {
> >-		if (!capable(CAP_SETFCAP))
> >-			return -EPERM;
> >+	/* Ignore non-security xattrs */
> >+	if (strncmp(name, XATTR_SECURITY_PREFIX,
> >+			sizeof(XATTR_SECURITY_PREFIX) - 1) != 0)
> >+		return 0;
> >+
> >+	/*
> >+	 * For XATTR_NAME_CAPS the check will be done in
> >+	 * cap_convert_nscap(), called by setxattr()
> >+	 */
> >+	if (strcmp(name, XATTR_NAME_CAPS) == 0)
> >  		return 0;
> >-	}
> >
> >-	if (!strncmp(name, XATTR_SECURITY_PREFIX,
> >-		     sizeof(XATTR_SECURITY_PREFIX) - 1) &&
> >-	    !capable(CAP_SYS_ADMIN))
> >+	if (!capable(CAP_SYS_ADMIN))
> >  		return -EPERM;
> >  	return 0;
> >  }
> >@@ -686,15 +910,22 @@ int cap_inode_setxattr(struct dentry *dentry, const char *name,
> >   */
> >  int cap_inode_removexattr(struct dentry *dentry, const char *name)
> >  {
> >-	if (!strcmp(name, XATTR_NAME_CAPS)) {
> >-		if (!capable(CAP_SETFCAP))
> >+	/* Ignore non-security xattrs */
> >+	if (strncmp(name, XATTR_SECURITY_PREFIX,
> >+			sizeof(XATTR_SECURITY_PREFIX) - 1) != 0)
> >+		return 0;
> >+
> >+	if (strcmp(name, XATTR_NAME_CAPS) == 0) {
> >+		/* security.capability gets namespaced */
> >+		struct inode *inode = d_backing_inode(dentry);
> >+		if (!inode)
> >+			return -EINVAL;
> >+		if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
> >  			return -EPERM;
> >  		return 0;
> >  	}
> >
> >-	if (!strncmp(name, XATTR_SECURITY_PREFIX,
> >-		     sizeof(XATTR_SECURITY_PREFIX) - 1) &&
> >-	    !capable(CAP_SYS_ADMIN))
> >+	if (!capable(CAP_SYS_ADMIN))
> >  		return -EPERM;
> >  	return 0;
> >  }
> >@@ -1082,6 +1313,7 @@ struct security_hook_list capability_hooks[] = {
> >  	LSM_HOOK_INIT(bprm_secureexec, cap_bprm_secureexec),
> >  	LSM_HOOK_INIT(inode_need_killpriv, cap_inode_need_killpriv),
> >  	LSM_HOOK_INIT(inode_killpriv, cap_inode_killpriv),
> >+	LSM_HOOK_INIT(inode_getsecurity, cap_inode_getsecurity),
> >  	LSM_HOOK_INIT(mmap_addr, cap_mmap_addr),
> >  	LSM_HOOK_INIT(mmap_file, cap_mmap_file),
> >  	LSM_HOOK_INIT(task_fix_setuid, cap_task_fix_setuid),
> 

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#1665308

Quoting Serge E. Hallyn (serge@hallyn.com):
> Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
> > On 05/08/2017 02:11 PM, Serge E. Hallyn wrote:
> > >Root in a non-initial user ns cannot be trusted to write a traditional
> > >security.capability xattr.  If it were allowed to do so, then any
> > >unprivileged user on the host could map his own uid to root in a private
> > >namespace, write the xattr, and execute the file with privilege on the
> > >host.
> > >
> > >However supporting file capabilities in a user namespace is very
> > >desirable.  Not doing so means that any programs designed to run with
> > >limited privilege must continue to support other methods of gaining and
> > >dropping privilege.  For instance a program installer must detect
> > >whether file capabilities can be assigned, and assign them if so but set
> > >setuid-root otherwise.  The program in turn must know how to drop
> > >partial capabilities, and do so only if setuid-root.
> > 
> > Hi Serge,
> > 
> > 
> >   I have been looking at patch below primarily to learn how we could
> > apply a similar technique to security.ima and security.evm for a
> > namespaced IMA. From the paragraphs above I thought that you solved
> > the problem of a shared filesystem where one now can write different
> > security.capability xattrs by effectively supporting for example
> > security.capability[uid=1000] and security.capability[uid=2000]
> 
> Interesting idea.  Worth considering.
> 
> > written into the filesystem. Each would then become visible as
> > security.capability if the userns mapping is set appropriately.
> > However, this doesn't seem to be how it is implemented. There seems
> 
> Indeed, when I was considering supporting multiple simulatenous
> xattrs, I did it as something like:
> 
> struct vfs_ns_cap_data {
> 	struct {
> 		__le32 permitted;
> 		__le32 inheritable;
> 	} data[VFS_CAP_U32];
> 	__le32 rootid;
> };
> 
> struct vfs_ns_cap {
> 	__le32 magic_etc;
> 	__le32 n_entries;
> 	struct ns_cap_data data[0];
> }; // followed by n_entries of struct ns_cap_data
> 
> You're instead suggesting encoding the rootuid in the name,
> which is interesting.
> 
> > to be only a single such entry with uid appended to it and, if it
> > was a shared filesystem, the first one to set this attribute blocks
> > everyone else from writing the xattr. Is that how it works? Would
> 
> Approximately - indeed there is only a single xattr.  But it can be
> overwritten, so long as the writer has CAP_SETFCAP over the user_ns
> which mounted the filesystem.

Hang on.  I've mis-spoken.  That's the requirement for writing a
v2 xattr.  To write a v3 xattr you only need to be privileged
(with CAP_SETFCAP) against the inode.

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