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Date:   Fri, 25 Feb 2022 17:04:58 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Suren Baghdasaryan <surenb@google.com>, shy828301@gmail.com,
        rientjes@google.com, willy@infradead.org, hannes@cmpxchg.org,
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        david@redhat.com, jannh@google.com, shakeelb@google.com,
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        linux-kernel@vger.kernel.org, kernel-team@android.com,
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        Liam Howlett <liam.howlett@oracle.com>
Subject: Re: [PATCH 1/1] mm: fix use-after-free bug when mm->mmap is reused
 after being freed
Message-Id: <20220225170458.2fc661d3088def39b2ed3e41@linux-foundation.org>
In-Reply-To: <YhisviDOip+axLDe@dhcp22.suse.cz>
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        <20220224201859.a38299b6c9d52cb51e6738ea@linux-foundation.org>
        <YhisviDOip+axLDe@dhcp22.suse.cz>
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On Fri, 25 Feb 2022 11:17:34 +0100 Michal Hocko <mhocko@suse.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@google.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.