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Date:   Thu, 15 Oct 2020 00:08:58 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andrii Nakryiko <andrii@kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        kernel-team@fb.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] fs: fix NULL dereference due to data race in
 prepend_path()
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On Wed, Oct 14, 2020 at 02:49:18PM -0700, Linus Torvalds wrote:
> On Wed, Oct 14, 2020 at 2:40 PM Andrii Nakryiko <andrii@kernel.org> wrote:
> >
> > Fix data race in prepend_path() with re-reading mnt->mnt_ns twice without
> > holding the lock. is_mounted() does check for NULL, but is_anon_ns(mnt->mnt_ns)
> > might re-read the pointer again which could be NULL already, if in between
> > reads one of kern_unmount()/kern_unmount_array()/umount_tree() sets mnt->mnt_ns
> > to NULL.
> 
> This seems like the obviously correct fix, so I think I'll just apply
> it directly.
> 
> Al? Holler if you have any issues with this..

See upthread.  If you've already grabbed it, I'll just push a followup cleanup.