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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 17 Feb 2022 11:54:00 -0800
Message-ID: <CAJuCfpFOy_oRp=WWqtzE7bJR7p51FzzBPtbKhWeTGOKC-n41Cg@mail.gmail.com>
Subject: Re: [PATCH v3 1/1] mm: fix use-after-free when anon vma name is used
 after vma is freed
To:     Michal Hocko <mhocko@suse.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Colin Cross <ccross@google.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Matthew Wilcox <willy@infradead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Christian Brauner <brauner@kernel.org>, legion@kernel.org,
        ran.xiaokai@zte.com.cn, sashal@kernel.org,
        Chris Hyser <chris.hyser@oracle.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Peter Collingbourne <pcc@google.com>, caoxiaofeng@yulong.com,
        David Hildenbrand <david@redhat.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>,
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On Wed, Feb 16, 2022 at 12:06 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 15-02-22 15:02:54, Suren Baghdasaryan wrote:
> > On Tue, Feb 15, 2022 at 12:05 PM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > One thing I was considering is to check agains ref counte overflo (a
> > > deep process chain with many vmas could grow really high. ref_count
> > > interface doesn't provide any easy way to check for overflows as far as
> > > I could see from a quick glance so I gave up there but the logic would
> > > be really straightforward. We just create a new anon_vma_name with the same
> > > content and use it when duplicating if the usage grow really
> > > (arbitrarily) high.
> >
> > I went over proposed changes. I see a couple small required fixes
> > (resetting the name to NULL seems to be missing and I think
> > dup_vma_anon_name needs some tweaking) but overall quite
> > straight-forward.
>
> OK, great that this makes sense to you. As I've said I didn't really go
> into details, not even dared to boot that to test. So it will very
> likely need some more work but I do not expect this to grow much.
>
> > I'll post a separate patch to do this refactoring.
> > The original patch is fixing the UAF issue, so I don't want to mix it
> > with refactoring. Please let me know if you see an issue with
> > separating it that way.
>
> Well, I am not sure TBH. Look at diffstats. Your fix
> 2 files changed, 63 insertions(+), 17 deletions(-)
> the refactoring which should fix this and potentially others that might
> be still lurking there (because mixing shared pointers and their internal
> objects just begs for problems) is
> 7 files changed, 63 insertions(+), 86 deletions(-)
>
> more files touched for sure but the net result is much more clear and a
> much more code removed.
> The overflow logic would make it bigger but I guess the existing scheme
> needs it as well.

Ok, I'll see how to slice it after it's complete and tested.
Thanks for the input!

>
> I would also claim that both approaches are really painful to review
> because the existing model spreads into several areas and it is not
> really clear you caught them all just by staring into the diff so both
> will be rather painful to backport to older kernels. Fortunately this
> would be only 5.17.
> --
> Michal Hocko
> SUSE Labs