Commit 97dd6b05 authored Feb 26, 2021 by Vlastimil Babka Committed by Bharath Mar 23, 2022

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>
Issue: FP3SEC-44
(cherry picked from commit cc916628)
Change-Id: Ibfe3129de6b4ee6c73191853194d6d2de7463ad2
(cherry picked from commit 2cd62a731c2db771e08e5da75c6386e65ccd00ad)

parent 900e0bbe

mm/huge_memory.c

+15 −0

Original line number	Original line	Diff line number	Diff line
	@@ -1024,6 +1024,19 @@ 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		* We can only reuse the page if nobody else maps the huge page or it's
	* part.		* 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) {		if (page_trans_huge_mapcount(page, NULL) == 1) {
	pmd_t entry;		pmd_t entry;
	entry = pmd_mkyoung(orig_pmd);		entry = pmd_mkyoung(orig_pmd);
	@@ -1031,8 +1044,10 @@ 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))		if (pmdp_set_access_flags(vma, haddr, fe->pmd, entry, 1))
	update_mmu_cache_pmd(vma, fe->address, fe->pmd);		update_mmu_cache_pmd(vma, fe->address, fe->pmd);
	ret \|= VM_FAULT_WRITE;		ret \|= VM_FAULT_WRITE;
			unlock_page(page);
	goto out_unlock;		goto out_unlock;
	}		}
			unlock_page(page);
	get_page(page);		get_page(page);
	spin_unlock(fe->ptl);		spin_unlock(fe->ptl);
	alloc:		alloc: