[CVE-2014-3122][Lucid][Precise/lts-backport-quantal] mm: try_to_unmap_cluster() should lock_page() before mlocking

Luis Henriques luis.henriques at canonical.com
Thu May 8 09:28:02 UTC 2014

From: Vlastimil Babka <vbabka at suse.cz>

A BUG_ON(!PageLocked) was triggered in mlock_vma_page() by Sasha Levin
fuzzing with trinity.  The call site try_to_unmap_cluster() does not lock
the pages other than its check_page parameter (which is already locked).

The BUG_ON in mlock_vma_page() is not documented and its purpose is
somewhat unclear, but apparently it serializes against page migration,
which could otherwise fail to transfer the PG_mlocked flag.  This would
not be fatal, as the page would be eventually encountered again, but
NR_MLOCK accounting would become distorted nevertheless.  This patch adds
a comment to the BUG_ON in mlock_vma_page() and munlock_vma_page() to that

The call site try_to_unmap_cluster() is fixed so that for page !=
check_page, trylock_page() is attempted (to avoid possible deadlocks as we
already have check_page locked) and mlock_vma_page() is performed only
upon success.  If the page lock cannot be obtained, the page is left
without PG_mlocked, which is again not a problem in the whole unevictable
memory design.

Signed-off-by: Vlastimil Babka <vbabka at suse.cz>
Signed-off-by: Bob Liu <bob.liu at oracle.com>
Reported-by: Sasha Levin <sasha.levin at oracle.com>
Cc: Wanpeng Li <liwanp at linux.vnet.ibm.com>
Cc: Michel Lespinasse <walken at google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro at jp.fujitsu.com>
Acked-by: Rik van Riel <riel at redhat.com>
Cc: David Rientjes <rientjes at google.com>
Cc: Mel Gorman <mgorman at suse.de>
Cc: Hugh Dickins <hughd at google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim at lge.com>
Cc: <stable at vger.kernel.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(back ported from commit 57e68e9cd65b4b8eb4045a1e0d0746458502554c)
BugLink: http://bugs.launchpad.net/bugs/1316268
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 mm/mlock.c |  2 ++
 mm/rmap.c  | 14 ++++++++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index 32832726..019586f 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -78,6 +78,7 @@ void __clear_page_mlock(struct page *page)
 void mlock_vma_page(struct page *page)
+	/* Serialize with page migration */
 	if (!TestSetPageMlocked(page)) {
@@ -105,6 +106,7 @@ void mlock_vma_page(struct page *page)
 void munlock_vma_page(struct page *page)
+	/* For try_to_munlock() and to serialize with page migration */
 	if (TestClearPageMlocked(page)) {
diff --git a/mm/rmap.c b/mm/rmap.c
index 2b03b31..e5b4407 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1441,9 +1441,19 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
 		BUG_ON(!page || PageAnon(page));
 		if (locked_vma) {
-			mlock_vma_page(page);   /* no-op if already mlocked */
-			if (page == check_page)
+			if (page == check_page) {
+				/* we know we have check_page locked */
+				mlock_vma_page(page);
 				ret = SWAP_MLOCK;
+			} else if (trylock_page(page)) {
+				/*
+				 * If we can lock the page, perform mlock.
+				 * Otherwise leave the page alone, it will be
+				 * eventually encountered again later.
+				 */
+				mlock_vma_page(page);
+				unlock_page(page);
+			}
 			continue;	/* don't unmap */

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