[PATCH] memcg: fix per_node_info cleanup

[Michal Hocko]

From: Michal Hocko <mhocko@xxxxxxxx>

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
initialized.

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")
Reviewed-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---

Hi!
Previously posted [1] based on the syzkaller report [2]. I haven't heard
back from syzkaller but this seems like the right fix. Andrew could you
add this to the pile?

[1] http://lkml.kernel.org/r/20180403105048.GK5501@xxxxxxxxxxxxxx
[2] http://lkml.kernel.org/r/001a113fe4c0a623b10568bb75ea@xxxxxxxxxx

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

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7667ea9daf4f..8c2ed1c2b72c 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;
+
 	free_percpu(pn->lruvec_stat_cpu);
 	kfree(pn);
 }
-- 
2.16.3