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

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Message-ID: <20220310155454.g6lt54yxel3ixnp3@revolver>
Date:   Thu, 10 Mar 2022 15:55:02 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Suren Baghdasaryan <surenb@...gle.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

* 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 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