Re: [PATCH 4.9 STABLE] mm, thp: make do_huge_pmd_wp_page() lock page for testing mapcount

[Sasha Levin]

On Fri, Feb 26, 2021 at 05:22:00PM +0100, Vlastimil Babka wrote:
> Jann reported [1] a race between __split_huge_pmd_locked() and
> page_trans_huge_map_swapcount() which can result in a page to be reused
> instead of COWed. This was later assigned CVE-2020-29368.
>
> This was fixed by commit c444eb564fb1 ("mm: thp: make the THP mapcount atomic
> against __split_huge_pmd_locked()") by doing the split under the page lock,
> while all users of page_trans_huge_map_swapcount() were already also under page
> lock. The fix was backported also to 4.9 stable series.
>
> When testing the backport on a 4.12 based kernel, Nicolai noticed the POC from
> [1] still reproduces after backporting c444eb564fb1 and identified a missing
> page lock in do_huge_pmd_wp_page() around the call to
> page_trans_huge_mapcount(). The page lock was only added in ba3c4ce6def4 ("mm,
> THP, swap: make reuse_swap_page() works for THP swapped out") in 4.14. The
> commit also wrapped page_trans_huge_mapcount() into
> page_trans_huge_map_swapcount() for the purposes of COW decisions.
>
> I have verified that 4.9.y indeed also reproduces with the POC. Backporting
> ba3c4ce6def4 alone however is not possible as it's part of a larger effort of
> optimizing THP swapping, which would be risky to backport fully.
>
> Therefore this 4.9-stable-only patch just wraps page_trans_huge_mapcount()
> in page_trans_huge_mapcount() under page lock the same way as ba3c4ce6def4
> does, but without the page_trans_huge_map_swapcount() part. Other callers
> of page_trans_huge_mapcount() are all under page lock already. I have verified
> the POC no longer reproduces afterwards.
>
> [1] https://bugs.chromium.org/p/project-zero/issues/detail?id=2045
>
> Reported-by: Nicolai Stange <nstange@xxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Queued up, thanks!

--
Thanks,
Sasha