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Re: mm: shmem_zero_setup skip security check and lockdep conflict with XFS

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  Re: mm: shmem_zero_setup skip security check and lockdep conflict  with XFS Daniel Wagner <wagi@monom.org> - 2015-06-15 08:10 +0200
    Re: mm: shmem_zero_setup skip security check and lockdep conflict  with XFS Hugh Dickins <hughd@google.com> - 2015-06-16 22:30 +0200
    Re: mm: shmem_zero_setup skip security check and lockdep conflict  with XFS Hugh Dickins <hughd@google.com> - 2015-06-18 02:30 +0200

#1165038 — Re: mm: shmem_zero_setup skip security check and lockdep conflict with XFS

On 06/14/2015 06:48 PM, Hugh Dickins wrote:
> It appears that, at some point last year, XFS made directory handling
> changes which bring it into lockdep conflict with shmem_zero_setup():
> it is surprising that mmap() can clone an inode while holding mmap_sem,
> but that has been so for many years.
> 
> Since those few lockdep traces that I've seen all implicated selinux,
> I'm hoping that we can use the __shmem_file_setup(,,,S_PRIVATE) which
> v3.13's commit c7277090927a ("security: shmem: implement kernel private
> shmem inodes") introduced to avoid LSM checks on kernel-internal inodes:
> the mmap("/dev/zero") cloned inode is indeed a kernel-internal detail.
> 
> This also covers the !CONFIG_SHMEM use of ramfs to support /dev/zero
> (and MAP_SHARED|MAP_ANONYMOUS).  I thought there were also drivers
> which cloned inode in mmap(), but if so, I cannot locate them now.
> 
> Reported-and-tested-by: Prarit Bhargava <prarit@redhat.com>
> Reported-by: Daniel Wagner <wagi@monom.org>

Reported-and-tested-by: Daniel Wagner <wagi@monom.org>

Sorry for the long delay. It took me a while to figure out my original
setup. I could verify that this patch made the lockdep message go away
on 4.0-rc6 and also on 4.1-rc8.

For the record: SELinux needs to be enabled triggering it.

cheers,
daniel
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#1166374

On Mon, 15 Jun 2015, Daniel Wagner wrote:
> On 06/14/2015 06:48 PM, Hugh Dickins wrote:
> > It appears that, at some point last year, XFS made directory handling
> > changes which bring it into lockdep conflict with shmem_zero_setup():
> > it is surprising that mmap() can clone an inode while holding mmap_sem,
> > but that has been so for many years.
> > 
> > Since those few lockdep traces that I've seen all implicated selinux,
> > I'm hoping that we can use the __shmem_file_setup(,,,S_PRIVATE) which
> > v3.13's commit c7277090927a ("security: shmem: implement kernel private
> > shmem inodes") introduced to avoid LSM checks on kernel-internal inodes:
> > the mmap("/dev/zero") cloned inode is indeed a kernel-internal detail.
> > 
> > This also covers the !CONFIG_SHMEM use of ramfs to support /dev/zero
> > (and MAP_SHARED|MAP_ANONYMOUS).  I thought there were also drivers
> > which cloned inode in mmap(), but if so, I cannot locate them now.
> > 
> > Reported-and-tested-by: Prarit Bhargava <prarit@redhat.com>
> > Reported-by: Daniel Wagner <wagi@monom.org>
> 
> Reported-and-tested-by: Daniel Wagner <wagi@monom.org>

Great, thank you Daniel: we look more convincing now :)

> 
> Sorry for the long delay. It took me a while to figure out my original
> setup. I could verify that this patch made the lockdep message go away
> on 4.0-rc6 and also on 4.1-rc8.

Thank you for taking the trouble.

> 
> For the record: SELinux needs to be enabled triggering it.

Right, selinux was in all the stacktraces we saw, and I was banking
on that security "recursion" being what actually upset lockdep; but
couldn't be sure until you tried it out.

We didn't make -rc8, and I won't be at all surprised if Linus feels
that a year(?)-old lockdep warning is not worth disturbing v4.1
final for, but it should get into v4.2 (thank you, Andrew).

Hugh
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#1167506

 Wed, Jun 17, 2015 at 12:45 PM, Morten Stevens
<mstevens@fedoraproject.org> wrote:
> 2015-06-15 8:09 GMT+02:00 Daniel Wagner <wagi@monom.org>:
>> On 06/14/2015 06:48 PM, Hugh Dickins wrote:
>>> It appears that, at some point last year, XFS made directory handling
>>> changes which bring it into lockdep conflict with shmem_zero_setup():
>>> it is surprising that mmap() can clone an inode while holding mmap_sem,
>>> but that has been so for many years.
>>>
>>> Since those few lockdep traces that I've seen all implicated selinux,
>>> I'm hoping that we can use the __shmem_file_setup(,,,S_PRIVATE) which
>>> v3.13's commit c7277090927a ("security: shmem: implement kernel private
>>> shmem inodes") introduced to avoid LSM checks on kernel-internal inodes:
>>> the mmap("/dev/zero") cloned inode is indeed a kernel-internal detail.
>>>
>>> This also covers the !CONFIG_SHMEM use of ramfs to support /dev/zero
>>> (and MAP_SHARED|MAP_ANONYMOUS).  I thought there were also drivers
>>> which cloned inode in mmap(), but if so, I cannot locate them now.
>>>
>>> Reported-and-tested-by: Prarit Bhargava <prarit@redhat.com>
>>> Reported-by: Daniel Wagner <wagi@monom.org>
>>
>> Reported-and-tested-by: Daniel Wagner <wagi@monom.org>
>>
>> Sorry for the long delay. It took me a while to figure out my original
>> setup. I could verify that this patch made the lockdep message go away
>> on 4.0-rc6 and also on 4.1-rc8.
>
> Yes, it's also fixed for me after applying this patch to 4.1-rc8.

Thank you - Hugh

>
> Best regards,
>
> Morten
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