Messages in this thread | | | Date | Fri, 26 Feb 2021 14:09:52 -0500 | From | Sasha Levin <> | Subject | Re: [PATCH 4.9 STABLE] mm, thp: make do_huge_pmd_wp_page() lock page for testing mapcount |
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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@suse.de> >Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Queued up, thanks!
-- Thanks, Sasha
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