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[PATCH 5.0 096/137] mm/page_alloc.c: avoid potential NULL pointer dereference

[ Upstream commit 8139ad043d632c0e9e12d760068a7a8e91659aa1 ]

ac.preferred_zoneref->zone passed to alloc_flags_nofragment() can be NULL.
'zone' pointer unconditionally derefernced in alloc_flags_nofragment().
Bail out on NULL zone to avoid potential crash.  Currently we don't see
any crashes only because alloc_flags_nofragment() has another bug which
allows compiler to optimize away all accesses to 'zone'.

Link: http://lkml.kernel.org/r/20190423120806.3503-1-aryabinin@xxxxxxxxxxxxx
Fixes: 6bb154504f8b ("mm, page_alloc: spread allocations across zones before introducing fragmentation")
Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
 mm/page_alloc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index eedb57f9b40b5..d59be95ba45cf 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3385,6 +3385,9 @@ alloc_flags_nofragment(struct zone *zone, gfp_t gfp_mask)
 		alloc_flags |= ALLOC_KSWAPD;
+	if (!zone)
+		return alloc_flags;
 	if (zone_idx(zone) != ZONE_NORMAL)
 		goto out;