linux-kernel - Re: [PATCH 1/1] mm: fix use-after-free bug when mm->mmap is reused after being freed

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Date:   Thu, 10 Mar 2022 08:28:33 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
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        <syzbot+2ccf63a4bd07cf39cab0@...kaller.appspotmail.com>
Subject: Re: [PATCH 1/1] mm: fix use-after-free bug when mm->mmap is reused
 after being freed

On Thu, Mar 10, 2022 at 7:55 AM Liam Howlett <liam.howlett@...cle.com> wrote:
>
> * Suren Baghdasaryan <surenb@...gle.com> [220225 00:51]:
> > On Thu, Feb 24, 2022 at 8:23 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Thu, Feb 24, 2022 at 08:18:59PM -0800, Andrew Morton wrote:
> > > > On Tue, 15 Feb 2022 12:19:22 -0800 Suren Baghdasaryan <surenb@...gle.com> wrote:
> > > >
> > > > > After exit_mmap frees all vmas in the mm, mm->mmap needs to be reset,
> > > > > otherwise it points to a vma that was freed and when reused leads to
> > > > > a use-after-free bug.
> > > > >
> > > > > ...
> > > > >
> > > > > --- a/mm/mmap.c
> > > > > +++ b/mm/mmap.c
> > > > > @@ -3186,6 +3186,7 @@ void exit_mmap(struct mm_struct *mm)
> > > > >             vma = remove_vma(vma);
> > > > >             cond_resched();
> > > > >     }
> > > > > +   mm->mmap = NULL;
> > > > >     mmap_write_unlock(mm);
> > > > >     vm_unacct_memory(nr_accounted);
> > > > >  }
> > > >
> > > > After the Maple tree patches, mm_struct.mmap doesn't exist.  So I'll
> > > > revert this fix as part of merging the maple-tree parts of linux-next.
> > > > I'll be sending this fix to Linus this week.
> > > >
> > > > All of which means that the thusly-resolved Maple tree patches might
> > > > reintroduce this use-after-free bug.
> > >
> > > I don't think so?  The problem is that VMAs are (currently) part of
> > > two data structures -- the rbtree and the linked list.  remove_vma()
> > > only removes VMAs from the rbtree; it doesn't set mm->mmap to NULL.
> > >
> > > With maple tree, the linked list goes away.  remove_vma() removes VMAs
> > > from the maple tree.  So anyone looking to iterate over all VMAs has to
> > > go and look in the maple tree for them ... and there's nothing there.
> >
> > Yes, I think you are right. With maple trees we don't need this fix.
>
>
> Yes, this is correct.  The maple tree removes the entire linked list...
> but since the mm is unstable in the exit_mmap(), I had added the
> destruction of the maple tree there.  Maybe this is the wrong place to
> be destroying the tree tracking the VMAs (althought this patch partially
> destroys the VMA tracking linked list), but it brought my attention to
> the race that this patch solves and the process_mrelease() function.
> Couldn't this be avoided by using mmget_not_zero() instead of mmgrab()
> in process_mrelease()?

That's what we were doing before [1]. That unfortunately has a problem
of process_mrelease possibly calling the last mmput and being blocked
on IO completion in exit_aio. The race between exit_mmap and
process_mrelease is solved by using mmap_lock.
I think by destroying the maple tree in exit_mmap before the
mmap_write_unlock call, you keep things working and functionality
intact. Is there any reason this can't be done?

[1] ba535c1caf3ee78a ("mm/oom_kill: allow process_mrelease to run
under mmap_lock protection")

> That would ensure we aren't stepping on an
> exit_mmap() and potentially the locking change in exit_mmap() wouldn't
> be needed either?  Logically, I view this as process_mrelease() having
> issue with the fact that the mmaps are no longer stable in tear down
> regardless of the data structure that is used.
>
> Thanks,
> Liam
>
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