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, 24 Feb 2022 21:50:54 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, mhocko@...nel.org,
        mhocko@...e.com, shy828301@...il.com, rientjes@...gle.com,
        hannes@...xchg.org, guro@...com, riel@...riel.com,
        minchan@...nel.org, kirill@...temov.name, aarcange@...hat.com,
        brauner@...nel.org, christian@...uner.io, hch@...radead.org,
        oleg@...hat.com, david@...hat.com, jannh@...gle.com,
        shakeelb@...gle.com, luto@...nel.org, christian.brauner@...ntu.com,
        fweimer@...hat.com, jengelh@...i.de, timmurray@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com,
        syzbot+2ccf63a4bd07cf39cab0@...kaller.appspotmail.com,
        Liam Howlett <liam.howlett@...cle.com>
Subject: Re: [PATCH 1/1] mm: fix use-after-free bug when mm->mmap is reused
 after being freed

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.

>
> But maybe I misunderstood the bug that's being solved here.