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Date:   Wed, 23 Feb 2022 09:55:54 +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 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?

>      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