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

From: Hugh Dickins
Date: Tue Jun 16 2015 - 16:27:26 EST

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@xxxxxxxxxx>
> > Reported-by: Daniel Wagner <wagi@xxxxxxxxx>
> Reported-and-tested-by: Daniel Wagner <wagi@xxxxxxxxx>

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).

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