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 17:04:58 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, shy828301@...il.com,
        rientjes@...gle.com, willy@...radead.org, 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 Fri, 25 Feb 2022 11:17:34 +0100 Michal Hocko <mhocko@...e.com> wrote:

> On Thu 24-02-22 20:18:59, 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.
> 
> But this is a regression introduced in this release cycle so the patch
> should be merged before Maple tree patches, no?

Yes, I'll be sending this one-liner upstream very soon and we'll then
undo it in the maple-tree patchset.