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Date:   Wed, 23 Feb 2022 16:29:24 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
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Subject: Re: [PATCH v4 3/3] mm: fix use-after-free when anon vma name is used
 after vma is freed
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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.
-- 
Michal Hocko
SUSE Labs