diff options
| author | Vlastimil Babka <vbabka@suse.cz> | 2021-02-26 17:22:00 +0100 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2021-03-03 17:44:32 +0100 |
| commit | cc916628eae7a8494b9a3de329d7baf12fd964c6 (patch) | |
| tree | 400b7bf8a01d08f826bd09d6762c2dacec2e3f0a /mm | |
| parent | 776ce24f7bbf8b6fa4c47f7a194af8990ff2cbd2 (diff) | |
mm, thp: make do_huge_pmd_wp_page() lock page for testing mapcount
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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Diffstat (limited to 'mm')
| -rw-r--r-- | mm/huge_memory.c | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 05ca01ef97f7..14cd0ef33b62 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1022,6 +1022,19 @@ int do_huge_pmd_wp_page(struct fault_env *fe, pmd_t orig_pmd) * We can only reuse the page if nobody else maps the huge page or it's * part. */ + if (!trylock_page(page)) { + get_page(page); + spin_unlock(fe->ptl); + lock_page(page); + spin_lock(fe->ptl); + if (unlikely(!pmd_same(*fe->pmd, orig_pmd))) { + unlock_page(page); + put_page(page); + goto out_unlock; + } + put_page(page); + } + if (page_trans_huge_mapcount(page, NULL) == 1) { pmd_t entry; entry = pmd_mkyoung(orig_pmd); @@ -1029,8 +1042,10 @@ int do_huge_pmd_wp_page(struct fault_env *fe, pmd_t orig_pmd) if (pmdp_set_access_flags(vma, haddr, fe->pmd, entry, 1)) update_mmu_cache_pmd(vma, fe->address, fe->pmd); ret |= VM_FAULT_WRITE; + unlock_page(page); goto out_unlock; } + unlock_page(page); get_page(page); spin_unlock(fe->ptl); alloc: |