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Date:   Tue, 3 Apr 2018 12:50:48 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     syzbot <syzbot+8a5de3cce7cdc70e9ebe@syzkaller.appspotmail.com>
Cc:     cgroups@vger.kernel.org, hannes@cmpxchg.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        syzkaller-bugs@googlegroups.com, vdavydov.dev@gmail.com,
        Andrey Ryabinin <aryabinin@virtuozzo.com>
Subject: Re: general protection fault in __mem_cgroup_free
Message-ID: <20180403105048.GK5501@dhcp22.suse.cz>
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 <20180403093733.GI5501@dhcp22.suse.cz>
 <20180403094329.GJ5501@dhcp22.suse.cz>
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Here we go

From 38f0f08a3f9f19c106ae53350e43dc97e2e3a4d8 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Tue, 3 Apr 2018 12:40:41 +0200
Subject: [PATCH] memcg: fix per_node_info cleanup

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@syzkaller.appspotmail.com
Fixes: 00f3ca2c2d66 ("mm: memcontrol: per-lruvec stats infrastructure")
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 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;
+
 	free_percpu(pn->lruvec_stat_cpu);
 	kfree(pn);
 }
-- 
2.16.3

-- 
Michal Hocko
SUSE Labs