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

From: Daniel Wagner <wagi@monom.org>
To: Hugh Dickins <hughd@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Prarit Bhargava <prarit@redhat.com>,
	Morten Stevens <mstevens@fedoraproject.org>,
	Dave Chinner <david@fromorbit.com>,
	Eric Paris <eparis@redhat.com>,
	Eric Sandeen <esandeen@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: mm: shmem_zero_setup skip security check and lockdep conflict with XFS
Date: Mon, 15 Jun 2015 08:09:16 +0200	[thread overview]
Message-ID: <557E6C0C.3050802@monom.org> (raw)
In-Reply-To: <alpine.LSU.2.11.1506140944380.11018@eggly.anvils>

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