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:   Fri, 25 Feb 2022 04:23:14 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, 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 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.

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