[CVE-2017-1000405 xenial] mm, thp: Do not make page table dirty unconditionally in touch_p[mu]d()

[Thadeu Lima de Souza Cascardo]

From: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>

Currently, we unconditionally make page table dirty in touch_pmd().
It may result in false-positive can_follow_write_pmd().

We may avoid the situation, if we would only make the page table entry
dirty if caller asks for write access -- FOLL_WRITE.

The patch also changes touch_pud() in the same way.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Hugh Dickins <hughd at google.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(backported from commit a8f97366452ed491d13cf1e44241bc0b5740b1f0)
[cascardo: dropped touch_pud parts]
[cascardo: fix in-place of where touch_pmd would be called]
CVE-2017-1000405
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---

Reproducer has been tested. It "exploits" without the patch. With the patch, it
fails.

touch_pmd was introduced later as a helper. Now, we have only one call site as
follow_devmap_pmd was not on xenial or trusty yet. So, fixing it in place is
just as fine.

Cascardo.

---
 mm/huge_memory.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 6c6f5ccfcda1..8f3769ec8575 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1304,17 +1304,11 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 	VM_BUG_ON_PAGE(!PageHead(page), page);
 	if (flags & FOLL_TOUCH) {
 		pmd_t _pmd;
-		/*
-		 * We should set the dirty bit only for FOLL_WRITE but
-		 * for now the dirty bit in the pmd is meaningless.
-		 * And if the dirty bit will become meaningful and
-		 * we'll only set it with FOLL_WRITE, an atomic
-		 * set_bit will be required on the pmd to set the
-		 * young bit, instead of the current set_pmd_at.
-		 */
-		_pmd = pmd_mkyoung(pmd_mkdirty(*pmd));
+		_pmd = pmd_mkyoung(*pmd);
+		if (flags & FOLL_WRITE)
+			_pmd = pmd_mkdirty(_pmd);
 		if (pmdp_set_access_flags(vma, addr & HPAGE_PMD_MASK,
-					  pmd, _pmd,  1))
+					  pmd, _pmd, flags & FOLL_WRITE))
 			update_mmu_cache_pmd(vma, addr, pmd);
 	}
 	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
-- 
2.14.1