[PATCH] mm: fix NULL pointer dereference in alloc_pages_bulk_noprof

[Jinjiang Tu <tujinjiang@huawei.com>]

We triggered a NULL pointer dereference for ac.preferred_zoneref->zone
in alloc_pages_bulk_noprof() when the task is migrated between cpusets.

When cpuset is enabled, in prepare_alloc_pages(), ac->nodemask may be
&current->mems_allowed. when first_zones_zonelist() is called to find
preferred_zoneref, the ac->nodemask may be modified concurrently if the
task is migrated between different cpusets. Assuming we have 2 NUMA Node,
when traversing Node1 in ac->zonelist, the nodemask is 2, and when
traversing Node2 in ac->zonelist, the nodemask is 1. As a result, the
ac->preferred_zoneref points to NULL zone.

In alloc_pages_bulk_noprof(), for_each_zone_zonelist_nodemask() finds a
allowable zone and calls zonelist_node_idx(ac.preferred_zoneref), leading
to NULL pointer dereference.

__alloc_pages_noprof() fixes this issue by checking NULL pointer in commit
ea57485af8f4 ("mm, page_alloc: fix check for NULL preferred_zone") and
commit df76cee6bbeb ("mm, page_alloc: remove redundant checks from alloc
fastpath").

To fix it, check NULL pointer for preferred_zoneref->zone.

Fixes: 387ba26fb1cb ("mm/page_alloc: add a bulk page allocator")
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
---
 mm/page_alloc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c6c7bb3ea71b..4afe8bc06358 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4592,7 +4592,8 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
 	gfp = alloc_gfp;
 
 	/* Find an allowed local zone that meets the low watermark. */
-	for_each_zone_zonelist_nodemask(zone, z, ac.zonelist, ac.highest_zoneidx, ac.nodemask) {
+	z = ac.preferred_zoneref;
+	for_next_zone_zonelist_nodemask(zone, z, ac.highest_zoneidx, ac.nodemask) {
 		unsigned long mark;
 
 		if (cpusets_enabled() && (alloc_flags & ALLOC_CPUSET) &&
-- 
2.34.1