[PATCH 1/2] mm/page_alloc: avoid potential NULL pointer dereference

[Andrey Ryabinin]

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'.

Fixes: 6bb154504f8b ("mm, page_alloc: spread allocations across zones before 
introducing fragmentation")
Signed-off-by: Andrey Ryabinin <aryabi...@virtuozzo.com>
---
 mm/page_alloc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 933bd42899e8..2b2c7065102f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3461,6 +3461,9 @@ alloc_flags_nofragment(struct zone *zone, gfp_t gfp_mask)
                alloc_flags |= ALLOC_KSWAPD;
 
 #ifdef CONFIG_ZONE_DMA32
+       if (!zone)
+               return alloc_flags;
+
        if (zone_idx(zone) != ZONE_NORMAL)
                goto out;
 
-- 
2.21.0