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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 23 Feb 2022 07:38:54 -0800
Message-ID: <CAJuCfpGd2VCa6xm0QRMP8GGk_Jg-nCKTgr9=JGjdwuPyOoX07w@mail.gmail.com>
Subject: Re: [PATCH v4 3/3] mm: fix use-after-free when anon vma name is used
 after vma is freed
To:     Michal Hocko <mhocko@suse.com>
Cc:     akpm@linux-foundation.org, ccross@google.com,
        sumit.semwal@linaro.org, dave.hansen@intel.com,
        keescook@chromium.org, willy@infradead.org,
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On Wed, Feb 23, 2022 at 7:29 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 23-02-22 06:10:54, Suren Baghdasaryan wrote:
> > On Wed, Feb 23, 2022 at 12:55 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Tue 22-02-22 07:43:40, Suren Baghdasaryan wrote:
> > > > On Tue, Feb 22, 2022 at 12:06 AM Michal Hocko <mhocko@suse.com> wrote:
> > > > >
> > > > > On Mon 21-02-22 21:40:25, Suren Baghdasaryan wrote:
> > > > > > When adjacent vmas are being merged it can result in the vma that was
> > > > > > originally passed to madvise_update_vma being destroyed.  In the current
> > > > > > implementation, the name parameter passed to madvise_update_vma points
> > > > > > directly to vma->anon_name->name and it is used after the call to
> > > > > > vma_merge.  In the cases when vma_merge merges the original vma and
> > > > > > destroys it, this will result in use-after-free bug as shown below:
> > > > > >
> > > > > > madvise_vma_behavior << passes vma->anon_name->name as name param
> > > > > >   madvise_update_vma(name)
> > > > > >     vma_merge
> > > > > >       __vma_adjust
> > > > > >         vm_area_free <-- frees the vma
> > > > > >     replace_vma_anon_name(name) <-- UAF
> > > > >
> > > > > This seems to be stale because bare const char pointer is not passed in
> > > > > the call chain. In fact I am not even sure there is any actual UAF here
> > > > > after the rework.
> > > > > Could you be more specific in describing the scenario?
> > > >
> > > > Yes, sorry, I need to update the part of the description talking about
> > > > passing vma->anon_name->name directly.
> > > > I think UAF is still there, it's just harder to reproduce (admittedly
> > > > I could not reproduce it with the previous reproducer). The scenario
> > > > would be when a vma with vma->anon_name->kref == 1 is being merged
> > > > with another one and freed in the process:
> > > >
> > > > madvise_vma_behavior
> > > >    anon_name = vma_anon_name(vma) <-- does not increase refcount
> > > >    madvise_update_vma(anon_name)
> > > >      *prev = vma_merge <-- returns another vma
> > > >        __vma_adjust
> > > >          vm_area_free(vma)
> > > >            free_vma_anon_name
> > > >              anon_vma_name_put
> > > >                vma_anon_name_free <-- frees the vma->anon_name
> > > >      vma = *prev <-- original vma was freed
> > >
> > > How come this is not a UAF in the first place?
> >
> > Sorry, I got you confused. The original vma that was passed as a
> > parameter to vma_merge(vma) was freed and vma_merge() returns the area
> > it was merged with:
> >
> >     *prev = vma_merge(vma_a) <-- vma_a is merged with adjacent vma_b,
> > vma_a is freed and vma_b is returned
> >     vma = *prev <-- "vma" now points to vma_b
>
> OK, I see the problem now. Essentially if we have two adjacent vmas
> which have the same name but wrapped by two anon_vma_name instances
> and we merge them together then the first one can be freed along with
> its anon_vma_name which is still used by replace_anon_vma_name.
>
> Thanks for the patience.

Thanks for reviewing! I posted the next version of the patchset at
https://lore.kernel.org/linux-mm/20220223153613.835563-1-surenb@google.com/

> --
> Michal Hocko
> SUSE Labs