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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 23 Feb 2022 06:10:54 -0800
Message-ID: <CAJuCfpE+dpCmA6sRQJBanRP2uJiQHGdoGGjvkhL_GqxRdY9aCg@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,
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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

>
> >      replace_vma_anon_name(vma, >>anon_name<<) <-- UAF
> >
> > Does this make sense or did I miss something?
>
> Sorry for being dense but I still do not see it. If *prev has been freed
> then we already have a different UAF. Admittedly, I am not really fluent
> at vma_merge code path so I am not really sure your chain above is
> really possible. I will try to double check later.
> --
> Michal Hocko
> SUSE Labs