[PATCH] memcg: fix per_node_info cleanup

From: Michal Hocko
Date: Tue Apr 03 2018 - 06:40:41 EST

syzbot has triggered a NULL ptr dereference when allocation fault
injection enforces a failure and alloc_mem_cgroup_per_node_info
initializes memcg->nodeinfo only half way through. __mem_cgroup_free
still tries to free all per-node data and dereferences pn->lruvec_stat_cpu
unconditioanlly even if the specific per-node data hasn't been

The bug is quite unlikely to hit because small allocations do not fail
and we would need quite some numa nodes to make struct mem_cgroup_per_node
large enough to cross the costly order.

Reported-by: syzbot+8a5de3cce7cdc70e9ebe@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 00f3ca2c2d66 ("mm: memcontrol: per-lruvec stats infrastructure")
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
mm/memcontrol.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e3d5a0a7917f..0a9c4d5194f3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4340,6 +4340,9 @@ static void free_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
struct mem_cgroup_per_node *pn = memcg->nodeinfo[node];

+ if (!pn)
+ return;

Michal Hocko