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Re: [PATCH RFC] Introduce new security.nscapability xattr

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  Re: [PATCH RFC] Introduce new security.nscapability xattr Jann Horn <jann@thejh.net> - 2016-01-20 13:50 +0100
    Re: [PATCH RFC] Introduce new security.nscapability xattr "Serge E. Hallyn" <serge@hallyn.com> - 2016-01-27 17:10 +0100
      Re: [PATCH RFC] Introduce new security.nscapability xattr Jann Horn <jann@thejh.net> - 2016-01-27 18:30 +0100
        Re: [PATCH RFC] Introduce new security.nscapability xattr Andy Lutomirski <luto@amacapital.net> - 2016-01-28 01:40 +0100
          Re: [PATCH RFC] Introduce new security.nscapability xattr "Serge E. Hallyn" <serge.hallyn@ubuntu.com> - 2016-01-29 08:40 +0100

#1313197 — Re: [PATCH RFC] Introduce new security.nscapability xattr

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On Fri, Dec 04, 2015 at 02:21:16PM -0600, Serge E. Hallyn wrote:
> Quoting Eric W. Biederman (ebiederm@xmission.com):
> > "Serge E. Hallyn" <serge.hallyn@ubuntu.com> writes:
> > 
> > > A common way for daemons to run with minimal privilege is to start as root,
> > > perhaps setuid-root, choose a desired capability set, set PR_SET_KEEPCAPS,
> > > then change uid to non-root.  A simpler way to achieve this is to set file
> > > capabilities on a not-setuid-root binary.  However, when installing a package
> > > inside a (user-namespaced) container, packages cannot be installed with file
> > > capabilities.  For this reason, containers must install ping setuid-root.
> > 
> > Don't ping sockets avoid that specific problem?
> > 
> > I expect the general case still holds.
> > 
> > > To achieve this, we would need for containers to be able to request file
> > > capabilities be added to a file without causing these to be honored in the
> > > initial user namespace.
> > >
> > > To this end, the patch below introduces a new capability xattr format.  The
> > > main enhancement over the existing security.capability xattr is that we
> > > tag capability sets with a uid - the uid of the root user in the namespace
> > > where the capabilities are set.  The capabilities will be ignored in any
> > > other namespace.  The special case of uid == -1 (which must only ever be
> > > able to be set by kuid 0) means use the capabilities in all
> > > namespaces.
> 
> really since security.capability xattrs are currently honored in all
> namespaces this isn't really necessary.  Until and unless Seth's set
> changes that.
> 
> > 
> > A quick comment on this.
> > 
> > We currently allow capabilities that have been gained to be valid in all
> > descendent user namespaces.
> > 
> > Applying this principle to the on-disk capabilities would make it so
> > that uid 0 would mean capabilities in all namespaces.
> > 
> > It might be worth it to introduce a fixed sized array with a length
> > parameter of perhaps 32 entries which is a path of root uids as seen by
> > the initial user namespace.  That way the entire construction of the
> > user namespace could be verified.  AKA verify the current user namespace
> > and the parent and the parents parent.  Up to the user namespace the
> > current filesystem is mounted in. We would look at how much space
> > allows an xattr to be stored without causing filesystems a challenge
> > to properly size such an array.
> > 
> > Given that uids are fundamentally flat that might not be particularly
> > useful.  If we add an alternative way of identifying user namespaces
> > say a privileged operation that set a uuid, then the complete path would
> > be more interesting.
> > 
> > > An alternative format would use a pair of uids to indicate a range of rootids.
> > > This would allow root in a user namespace with uids 100000-165536 mapped to
> > > set the xattr once on a file, then launch nested containers wherein the file
> > > could be used with privilege.  That's not what this patch does, but would be
> > > a trivial change if people think it would be worthwhile.
> > >
> > > This patch does not actually address the real problem, which is setting the
> > > xattrs from inside containers.  For that, I think the best solution is to
> > > add a pair of new system calls, setfcap and getfcap. Userspace would for
> > > instance call fsetfcap(fd, cap_user_header_t, cap_user_data_t), to which
> > > the kernel would, if not in init_user_ns, react by writing an appropriate
> > > security.nscapability xattr.
> > 
> > That feels hard to maintain, but you may be correct that we have a small
> > enough userspace that it would not be a problem.
> > 
> > Eric
> > 
> > 
> > > The libcap2 library's cap_set_file/cap_get_file could be switched over
> > > transparently to use this to hide its use from all callers.
> > >
> > > Comments appreciated.
> > >
> > > Note - In this patch, file capabilities only work for containers which have
> > > a root uid defined.  We may want to allow -1 uids to work in all
> > > namespaces.  There certainly would be uses for this, but I'm a bit unsettled
> > > about the implications of allowing a program privilege in a container where
> > > there is no uid with privilege.  This needs more thought.
> 
> So for actually enabling (user-namespaced) containers to use these, there
> are a few possibilities that come to mine.
> 
> 1. A new setfcap (/getfcap) syscall.  Uses mapped uid 0 from
> current_user_ns() to write a value in the security.nscapability xattr.
> Userspace doesn't need to worry at all about namespace issues.

Your patch has an "ncaps" field and supports giving (possibly different)
privileges to different namespace root users.
I think that with a setfcap() syscall as you describe, it would be hard
to add more than one security.nscapability entry to a file without
poking holes into stuff:

If setfcap() requires inode write access, every namespace root for whom
a security.nscapability entry can be added can overwrite the file, so
namespace roots might be able to execute code as each other.

If setfcap() doesn't require inode write access, that isn't a big
security issue, but it allows unprivileged users to waste disk space
in a writable-mounted filesystem on which they have no write access
anywhere. I'm not sure whether that is a sufficiently big problem for
anyone to care.


Another issue with this would be that it makes restoring sub-namespace
capabilities from a backup somewhat ugly, at least if you're not in the
init ns: You'd have to parse the capabilities attribute, then, for every
attribute you want to restore whose rootid doesn't refer to your
namespace, clone(CLONE_NEWUSER), map the target uid to 0 from the parent
process, seteuid(0), do the setfcap() and exit.

It would require less synchronization with my ptrace hardening patch
that checks whether uids are mapped (if that lands, I don't think it has
yet?) because that would allow you to first setuid, then clone(), map
the uid to 0 inside the namespace and setfcap(), but it'd still require
special-case code in backup software.


> 2. Just expect userspace to write a xattr;  kernel checks that no values
> are changed for any other namespaces.  This could be a lot of parsing and
> verifying in the kernel.

This option would allow the first problem I described with option 1 (if
it is a problem?) to be worked around by letting a helper in the outer
namespace add capabilities for lower namespaces - but it wouldn't be
pretty.

I think it would also allow tar, which already supports xattrs, to
deal with namespace capabilities without needing any additional code.
I think that would be nice to have.


> 3. Switch the xattr scheme - instead of one security.nscapability xattr
> with multiple entries, use security.nscapability.$(rootid).  Now the
> kernel only needs to verify that the $rootid is valid for the writing
> task, and we don't need a new syscall.  OTOH userspace needs to know
> what it's doing.  Of course we can still hide that behind libcap2's helpers.

This doesn't sound so good - how would the namespace know its rootid?
If it is just one level below init_ns, it can grab it from
/proc/self/uid_map, but for deeper levels that won't work.


> Any opinions on which way seems best?  1 does seem cleanest (and supports
> use of seccomp if we want to forbit its use by some containers), but
> involves a new pair of syscalls.  2 seems to me to be right out, but
> others might disagree...

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#1319114

On Wed, Jan 20, 2016 at 01:48:16PM +0100, Jann Horn wrote:
> On Fri, Dec 04, 2015 at 02:21:16PM -0600, Serge E. Hallyn wrote:
> > Quoting Eric W. Biederman (ebiederm@xmission.com):
> > > "Serge E. Hallyn" <serge.hallyn@ubuntu.com> writes:
> > > 
> > > > A common way for daemons to run with minimal privilege is to start as root,
> > > > perhaps setuid-root, choose a desired capability set, set PR_SET_KEEPCAPS,
> > > > then change uid to non-root.  A simpler way to achieve this is to set file
> > > > capabilities on a not-setuid-root binary.  However, when installing a package
> > > > inside a (user-namespaced) container, packages cannot be installed with file
> > > > capabilities.  For this reason, containers must install ping setuid-root.
> > > 
> > > Don't ping sockets avoid that specific problem?
> > > 
> > > I expect the general case still holds.
> > > 
> > > > To achieve this, we would need for containers to be able to request file
> > > > capabilities be added to a file without causing these to be honored in the
> > > > initial user namespace.
> > > >
> > > > To this end, the patch below introduces a new capability xattr format.  The
> > > > main enhancement over the existing security.capability xattr is that we
> > > > tag capability sets with a uid - the uid of the root user in the namespace
> > > > where the capabilities are set.  The capabilities will be ignored in any
> > > > other namespace.  The special case of uid == -1 (which must only ever be
> > > > able to be set by kuid 0) means use the capabilities in all
> > > > namespaces.
> > 
> > really since security.capability xattrs are currently honored in all
> > namespaces this isn't really necessary.  Until and unless Seth's set
> > changes that.
> > 
> > > 
> > > A quick comment on this.
> > > 
> > > We currently allow capabilities that have been gained to be valid in all
> > > descendent user namespaces.
> > > 
> > > Applying this principle to the on-disk capabilities would make it so
> > > that uid 0 would mean capabilities in all namespaces.
> > > 
> > > It might be worth it to introduce a fixed sized array with a length
> > > parameter of perhaps 32 entries which is a path of root uids as seen by
> > > the initial user namespace.  That way the entire construction of the
> > > user namespace could be verified.  AKA verify the current user namespace
> > > and the parent and the parents parent.  Up to the user namespace the
> > > current filesystem is mounted in. We would look at how much space
> > > allows an xattr to be stored without causing filesystems a challenge
> > > to properly size such an array.
> > > 
> > > Given that uids are fundamentally flat that might not be particularly
> > > useful.  If we add an alternative way of identifying user namespaces
> > > say a privileged operation that set a uuid, then the complete path would
> > > be more interesting.
> > > 
> > > > An alternative format would use a pair of uids to indicate a range of rootids.
> > > > This would allow root in a user namespace with uids 100000-165536 mapped to
> > > > set the xattr once on a file, then launch nested containers wherein the file
> > > > could be used with privilege.  That's not what this patch does, but would be
> > > > a trivial change if people think it would be worthwhile.
> > > >
> > > > This patch does not actually address the real problem, which is setting the
> > > > xattrs from inside containers.  For that, I think the best solution is to
> > > > add a pair of new system calls, setfcap and getfcap. Userspace would for
> > > > instance call fsetfcap(fd, cap_user_header_t, cap_user_data_t), to which
> > > > the kernel would, if not in init_user_ns, react by writing an appropriate
> > > > security.nscapability xattr.
> > > 
> > > That feels hard to maintain, but you may be correct that we have a small
> > > enough userspace that it would not be a problem.
> > > 
> > > Eric
> > > 
> > > 
> > > > The libcap2 library's cap_set_file/cap_get_file could be switched over
> > > > transparently to use this to hide its use from all callers.
> > > >
> > > > Comments appreciated.
> > > >
> > > > Note - In this patch, file capabilities only work for containers which have
> > > > a root uid defined.  We may want to allow -1 uids to work in all
> > > > namespaces.  There certainly would be uses for this, but I'm a bit unsettled
> > > > about the implications of allowing a program privilege in a container where
> > > > there is no uid with privilege.  This needs more thought.
> > 
> > So for actually enabling (user-namespaced) containers to use these, there
> > are a few possibilities that come to mine.
> > 
> > 1. A new setfcap (/getfcap) syscall.  Uses mapped uid 0 from
> > current_user_ns() to write a value in the security.nscapability xattr.
> > Userspace doesn't need to worry at all about namespace issues.
> 
> Your patch has an "ncaps" field and supports giving (possibly different)
> privileges to different namespace root users.
> I think that with a setfcap() syscall as you describe, it would be hard
> to add more than one security.nscapability entry to a file without
> poking holes into stuff:
> 
> If setfcap() requires inode write access, every namespace root for whom
> a security.nscapability entry can be added can overwrite the file, so
> namespace roots might be able to execute code as each other.
> 
> If setfcap() doesn't require inode write access, that isn't a big
> security issue, but it allows unprivileged users to waste disk space
> in a writable-mounted filesystem on which they have no write access
> anywhere. I'm not sure whether that is a sufficiently big problem for
> anyone to care.
> 
> 
> Another issue with this would be that it makes restoring sub-namespace
> capabilities from a backup somewhat ugly, at least if you're not in the
> init ns: You'd have to parse the capabilities attribute, then, for every
> attribute you want to restore whose rootid doesn't refer to your
> namespace, clone(CLONE_NEWUSER), map the target uid to 0 from the parent
> process, seteuid(0), do the setfcap() and exit.
> 
> It would require less synchronization with my ptrace hardening patch
> that checks whether uids are mapped (if that lands, I don't think it has
> yet?) because that would allow you to first setuid, then clone(), map
> the uid to 0 inside the namespace and setfcap(), but it'd still require
> special-case code in backup software.
> 
> 
> > 2. Just expect userspace to write a xattr;  kernel checks that no values
> > are changed for any other namespaces.  This could be a lot of parsing and
> > verifying in the kernel.
> 
> This option would allow the first problem I described with option 1 (if
> it is a problem?) to be worked around by letting a helper in the outer
> namespace add capabilities for lower namespaces - but it wouldn't be

Note that regardless, the fscaps would still be stored as an xattr.
The setfcap/getfacp syscalls would be for convenience, but a suitably
privileged task in the init_user_ns could just write the xattr.

You've made a good point here and in your prev email: when a namespaced
file cap exists for say uid 1000, perhaps it should apply for every
descendent ns of uid 1000.  In this case maybe it makes sense to drop
the nscaps field and actually only allow one namespace fscap.  It must
be set by the owner of the file (i.e., uid 1000, or uid 0 if a
host-root-owned file) or someone privileged over it (i.e. uid 100000
who is root in his ns and i_sb->s_user_ns->owner for the file).

So if the host has /bin/ping with cap_net_raw=ep, any user in any
ns which can reach and xecute it can run the file with those privileges
(in their own ns).  If uid 1000 creates a user_ns with root 100000,
that container can create a $rootfs/bin/ping with cap_net_raw=ep
with uid tag 100000.  If that container creates a sub-container owned
by uid 150000, that container can also run $rootfs/bin/ping with
cap_net_raw=ep, but uid 1001 on the host does not.  Now we don't have
the risk of wasting disk space, and restoring a cap-endowed file in
a sub-userns is trivial.

In this case we can probably also do away with the setfcap() syscall,
as userspace only needs to look for the "0 x y" /proc/self/uid_map
entry and enter x in the xattr.

Does this seem reasonable and somewhat risk-averse?

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#1319168

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On Wed, Jan 27, 2016 at 10:08:15AM -0600, Serge E. Hallyn wrote:
> On Wed, Jan 20, 2016 at 01:48:16PM +0100, Jann Horn wrote:
> > On Fri, Dec 04, 2015 at 02:21:16PM -0600, Serge E. Hallyn wrote:
> > > Quoting Eric W. Biederman (ebiederm@xmission.com):
> > > > "Serge E. Hallyn" <serge.hallyn@ubuntu.com> writes:
> > > > 
> > > > > A common way for daemons to run with minimal privilege is to start as root,
> > > > > perhaps setuid-root, choose a desired capability set, set PR_SET_KEEPCAPS,
> > > > > then change uid to non-root.  A simpler way to achieve this is to set file
> > > > > capabilities on a not-setuid-root binary.  However, when installing a package
> > > > > inside a (user-namespaced) container, packages cannot be installed with file
> > > > > capabilities.  For this reason, containers must install ping setuid-root.
> > > > 
> > > > Don't ping sockets avoid that specific problem?
> > > > 
> > > > I expect the general case still holds.
> > > > 
> > > > > To achieve this, we would need for containers to be able to request file
> > > > > capabilities be added to a file without causing these to be honored in the
> > > > > initial user namespace.
> > > > >
> > > > > To this end, the patch below introduces a new capability xattr format.  The
> > > > > main enhancement over the existing security.capability xattr is that we
> > > > > tag capability sets with a uid - the uid of the root user in the namespace
> > > > > where the capabilities are set.  The capabilities will be ignored in any
> > > > > other namespace.  The special case of uid == -1 (which must only ever be
> > > > > able to be set by kuid 0) means use the capabilities in all
> > > > > namespaces.
> > > 
> > > really since security.capability xattrs are currently honored in all
> > > namespaces this isn't really necessary.  Until and unless Seth's set
> > > changes that.
> > > 
> > > > 
> > > > A quick comment on this.
> > > > 
> > > > We currently allow capabilities that have been gained to be valid in all
> > > > descendent user namespaces.
> > > > 
> > > > Applying this principle to the on-disk capabilities would make it so
> > > > that uid 0 would mean capabilities in all namespaces.
> > > > 
> > > > It might be worth it to introduce a fixed sized array with a length
> > > > parameter of perhaps 32 entries which is a path of root uids as seen by
> > > > the initial user namespace.  That way the entire construction of the
> > > > user namespace could be verified.  AKA verify the current user namespace
> > > > and the parent and the parents parent.  Up to the user namespace the
> > > > current filesystem is mounted in. We would look at how much space
> > > > allows an xattr to be stored without causing filesystems a challenge
> > > > to properly size such an array.
> > > > 
> > > > Given that uids are fundamentally flat that might not be particularly
> > > > useful.  If we add an alternative way of identifying user namespaces
> > > > say a privileged operation that set a uuid, then the complete path would
> > > > be more interesting.
> > > > 
> > > > > An alternative format would use a pair of uids to indicate a range of rootids.
> > > > > This would allow root in a user namespace with uids 100000-165536 mapped to
> > > > > set the xattr once on a file, then launch nested containers wherein the file
> > > > > could be used with privilege.  That's not what this patch does, but would be
> > > > > a trivial change if people think it would be worthwhile.
> > > > >
> > > > > This patch does not actually address the real problem, which is setting the
> > > > > xattrs from inside containers.  For that, I think the best solution is to
> > > > > add a pair of new system calls, setfcap and getfcap. Userspace would for
> > > > > instance call fsetfcap(fd, cap_user_header_t, cap_user_data_t), to which
> > > > > the kernel would, if not in init_user_ns, react by writing an appropriate
> > > > > security.nscapability xattr.
> > > > 
> > > > That feels hard to maintain, but you may be correct that we have a small
> > > > enough userspace that it would not be a problem.
> > > > 
> > > > Eric
> > > > 
> > > > 
> > > > > The libcap2 library's cap_set_file/cap_get_file could be switched over
> > > > > transparently to use this to hide its use from all callers.
> > > > >
> > > > > Comments appreciated.
> > > > >
> > > > > Note - In this patch, file capabilities only work for containers which have
> > > > > a root uid defined.  We may want to allow -1 uids to work in all
> > > > > namespaces.  There certainly would be uses for this, but I'm a bit unsettled
> > > > > about the implications of allowing a program privilege in a container where
> > > > > there is no uid with privilege.  This needs more thought.
> > > 
> > > So for actually enabling (user-namespaced) containers to use these, there
> > > are a few possibilities that come to mine.
> > > 
> > > 1. A new setfcap (/getfcap) syscall.  Uses mapped uid 0 from
> > > current_user_ns() to write a value in the security.nscapability xattr.
> > > Userspace doesn't need to worry at all about namespace issues.
> > 
> > Your patch has an "ncaps" field and supports giving (possibly different)
> > privileges to different namespace root users.
> > I think that with a setfcap() syscall as you describe, it would be hard
> > to add more than one security.nscapability entry to a file without
> > poking holes into stuff:
> > 
> > If setfcap() requires inode write access, every namespace root for whom
> > a security.nscapability entry can be added can overwrite the file, so
> > namespace roots might be able to execute code as each other.
> > 
> > If setfcap() doesn't require inode write access, that isn't a big
> > security issue, but it allows unprivileged users to waste disk space
> > in a writable-mounted filesystem on which they have no write access
> > anywhere. I'm not sure whether that is a sufficiently big problem for
> > anyone to care.
> > 
> > 
> > Another issue with this would be that it makes restoring sub-namespace
> > capabilities from a backup somewhat ugly, at least if you're not in the
> > init ns: You'd have to parse the capabilities attribute, then, for every
> > attribute you want to restore whose rootid doesn't refer to your
> > namespace, clone(CLONE_NEWUSER), map the target uid to 0 from the parent
> > process, seteuid(0), do the setfcap() and exit.
> > 
> > It would require less synchronization with my ptrace hardening patch
> > that checks whether uids are mapped (if that lands, I don't think it has
> > yet?) because that would allow you to first setuid, then clone(), map
> > the uid to 0 inside the namespace and setfcap(), but it'd still require
> > special-case code in backup software.
> > 
> > 
> > > 2. Just expect userspace to write a xattr;  kernel checks that no values
> > > are changed for any other namespaces.  This could be a lot of parsing and
> > > verifying in the kernel.
> > 
> > This option would allow the first problem I described with option 1 (if
> > it is a problem?) to be worked around by letting a helper in the outer
> > namespace add capabilities for lower namespaces - but it wouldn't be
> 
> Note that regardless, the fscaps would still be stored as an xattr.
> The setfcap/getfacp syscalls would be for convenience, but a suitably
> privileged task in the init_user_ns could just write the xattr.

Yes - I was thinking of a privileged task in an intermediate namespace,
which could then be allowed to backup and restore capability xattrs for
all mapped uids.
I'm not sure whether that's a realistic/common scenario. Maybe if someone
uses an LXC VPS inside which he uses LXC to further isolate various
services.


> You've made a good point here and in your prev email: when a namespaced
> file cap exists for say uid 1000, perhaps it should apply for every
> descendent ns of uid 1000.  In this case maybe it makes sense to drop
> the nscaps field and actually only allow one namespace fscap.  It must
> be set by the owner of the file (i.e., uid 1000, or uid 0 if a
> host-root-owned file) or someone privileged over it (i.e. uid 100000
> who is root in his ns and i_sb->s_user_ns->owner for the file).
> 
> So if the host has /bin/ping with cap_net_raw=ep, any user in any
> ns which can reach and xecute it can run the file with those privileges
> (in their own ns).  If uid 1000 creates a user_ns with root 100000,
> that container can create a $rootfs/bin/ping with cap_net_raw=ep
> with uid tag 100000.  If that container creates a sub-container owned
> by uid 150000, that container can also run $rootfs/bin/ping with
> cap_net_raw=ep, but uid 1001 on the host does not.  Now we don't have
> the risk of wasting disk space, and restoring a cap-endowed file in
> a sub-userns is trivial.
> 
> In this case we can probably also do away with the setfcap() syscall,
> as userspace only needs to look for the "0 x y" /proc/self/uid_map
> entry and enter x in the xattr.

That won't work in a namespace >1 levels below the init ns though,
right? Because /proc/self/uid_map only shows the uids in the current
user ns and its parent, not the kuids.

$ id
uid=1000[...]
$ unshare -Ur unshare -Ur cat /proc/self/uid_map
         0          0          1

This could of course be worked around by adding a "map this uid up
to a kuid" ioctl for user namespaces to ns_file_operations or so,
but I'm not sure how I feel about that.

Can we let the kernel map the uid in the file attribute up/down
on attribute access? If it's just one binary 32-bit field at the
start of the attribute value, the code should be fairly
straightforward.

> Does this seem reasonable and somewhat risk-averse?

I think it sounds good from a security perspective.

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#1320169

On Wed, Jan 27, 2016 at 9:22 AM, Jann Horn <jann@thejh.net> wrote:
> I think it sounds good from a security perspective.

I'm a bit late to the game, but I have a question: why should this be
keyed to the *root* uid of the namespace in particular?  Certainly if
user foo trusts the cap bits on some file, then user foo might trust
those caps to be exerted over any namespace that user foo owns, since
user foo owns the namespace.

But another option would be to include a list of uids and gids such
that the cap bits on the file are trusted by any namespace that maps
only uids and gids in the list.  After all, the existence of a
namespace with root user foo that also maps bar and baz along with a
file with caps set means that, if baz can get to the file and
permissions are set appropriately, then baz now owns bar (via any
number of fs-related capabilities).  So maybe bar and baz should have
to be listed as well.

But maybe this doesn't matter.

In any event, at the end of the day, the right answer to all of this
is to stop using setuid and stop using cap bits too and start using
privileged daemons or other things that don't use the eternally
fragile grant-privilege-on-execve mechanisms.

--Andy

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#1321529

On Wed, Jan 27, 2016 at 04:36:02PM -0800, Andy Lutomirski wrote:
> On Wed, Jan 27, 2016 at 9:22 AM, Jann Horn <jann@thejh.net> wrote:
> > I think it sounds good from a security perspective.
> 
> I'm a bit late to the game, but I have a question: why should this be
> keyed to the *root* uid of the namespace in particular?  Certainly if
> user foo trusts the cap bits on some file, then user foo might trust
> those caps to be exerted over any namespace that user foo owns, since
> user foo owns the namespace.

...  Tying it to a kuid which represents the userns->owner of any
namespace in which the capability will be honored might be fine
with me.  Is that what you mean?  So if uid 1000 creates a userns
mapping uids 100000-200000, and 100000 in that container puts X=pe
on /bin/foo, uid 101000 in that container runs /bin/foo with privilege
X.  Uid 101000 in someone else's container does not.

Although, if I create two containers and provide them different
uidmaps, it may well be because I want them segragated and want
to minimize the changes of one container breaking out into the
other.  This risks breaking that.

> But another option would be to include a list of uids and gids such
> that the cap bits on the file are trusted by any namespace that maps
> only uids and gids in the list.  After all, the existence of a
> namespace with root user foo that also maps bar and baz along with a
> file with caps set means that, if baz can get to the file and
> permissions are set appropriately, then baz now owns bar (via any
> number of fs-related capabilities).  So maybe bar and baz should have
> to be listed as well.
> 
> But maybe this doesn't matter.
> 
> In any event, at the end of the day, the right answer to all of this
> is to stop using setuid and stop using cap bits too and start using
> privileged daemons or other things that don't use the eternally
> fragile grant-privilege-on-execve mechanisms.

Heh, that's why I wrote a p9auth driver a few years ago, but it
was too early for such a thing.

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