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>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 05ca01ef97f7f757e8a726822f7542260e62ebb9..14cd0ef33b628d430af5117838f2f14fe4ef1d46 100644 (file)
--- 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: