linux-kernel - Re: [PATCH v4 3/3] mm: fix use-after-free when anon vma name is used after vma is freed

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Date:   Tue, 22 Feb 2022 07:43:40 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     akpm@...ux-foundation.org, ccross@...gle.com,
        sumit.semwal@...aro.org, dave.hansen@...el.com,
        keescook@...omium.org, willy@...radead.org,
        kirill.shutemov@...ux.intel.com, vbabka@...e.cz,
        hannes@...xchg.org, ebiederm@...ssion.com, brauner@...nel.org,
        legion@...nel.org, ran.xiaokai@....com.cn, sashal@...nel.org,
        chris.hyser@...cle.com, dave@...olabs.net, pcc@...gle.com,
        caoxiaofeng@...ong.com, david@...hat.com, gorcunov@...il.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com,
        syzbot+aa7b3d4b35f9dc46a366@...kaller.appspotmail.com
Subject: Re: [PATCH v4 3/3] mm: fix use-after-free when anon vma name is used
 after vma is freed

On Tue, Feb 22, 2022 at 12:06 AM Michal Hocko <mhocko@...e.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
     replace_vma_anon_name(vma, >>anon_name<<) <-- UAF

Does this make sense or did I miss something?
Thanks,
Suren.

>
> > Fix this by raising the name refcount and stabilizing it.
> >
> > Fixes: 9a10064f5625 ("mm: add a field to store names for private anonymous memory")
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > Reported-by: syzbot+aa7b3d4b35f9dc46a366@...kaller.appspotmail.com
> > ---
> > changes in v3:
> > - Reapplied the fix after code refactoring, per Michal Hocko
> >
> >  mm/madvise.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index a395884aeecb..00e8105430e9 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -140,6 +140,8 @@ static int replace_vma_anon_name(struct vm_area_struct *vma,
> >  /*
> >   * Update the vm_flags on region of a vma, splitting it or merging it as
> >   * necessary.  Must be called with mmap_sem held for writing;
> > + * Caller should ensure anon_name stability by raising its refcount even when
> > + * anon_name belongs to a valid vma because this function might free that vma.
> >   */
> >  static int madvise_update_vma(struct vm_area_struct *vma,
> >                             struct vm_area_struct **prev, unsigned long start,
> > @@ -1021,8 +1023,10 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
> >       }
> >
> >       anon_name = vma_anon_name(vma);
> > +     anon_vma_name_get(anon_name);
> >       error = madvise_update_vma(vma, prev, start, end, new_flags,
> >                                  anon_name);
> > +     anon_vma_name_put(anon_name);
> >
> >  out:
> >       /*
> > --
> > 2.35.1.473.g83b2b277ed-goog
>
> --
> Michal Hocko
> SUSE Labs