mm/page_alloc.c: avoid potential NULL pointer dereference [Linux 5.1]

mm/page_alloc.c: avoid potential NULL pointer dereference [Linux 5.1]

This Linux kernel change "mm/page_alloc.c: avoid potential NULL pointer dereference" is included in the Linux 5.1 release. This change is authored by Andrey Ryabinin <aryabinin [at] virtuozzo.com> on Thu Apr 25 22:23:58 2019 -0700. The commit for this change in Linux stable tree is 8139ad0 (patch).

mm/page_alloc.c: avoid potential NULL pointer dereference

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@virtuozzo.com
Fixes: 6bb154504f8b ("mm, page_alloc: spread allocations across zones before introducing fragmentation")
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

There are 3 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 mm/page_alloc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d167c48..9992ca7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3432,6 +3432,9 @@ static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
        alloc_flags |= ALLOC_KSWAPD;

 #ifdef CONFIG_ZONE_DMA32
+   if (!zone)
+       return alloc_flags;
+
    if (zone_idx(zone) != ZONE_NORMAL)
        goto out;

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