From: Eric Dumazet <edumazet@google.com>
Date: Sat, 27 Jan 2018 10:58:43 -0800
Subject: net_sched: gen_estimator: fix lockdep splat
Patch-mainline: v4.16-rc1
Git-commit: 40ca54e3a686f13117f3de0c443f8026dadf7c44
References: bsc#1056787

syzbot reported a lockdep splat in gen_new_estimator() /
est_fetch_counters() when attempting to lock est->stats_lock.

Since est_fetch_counters() is called from BH context from timer
interrupt, we need to block BH as well when calling it from process
context.

Most qdiscs use per cpu counters and are immune to the problem,
but net/sched/act_api.c and net/netfilter/xt_RATEEST.c are using
a spinlock to protect their data. They both call gen_new_estimator()
while object is created and not yet alive, so this bug could
not trigger a deadlock, only a lockdep splat.

Fixes: 1c0d32fde5bd ("net_sched: gen_estimator: complete rewrite of rate estimators")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 net/core/gen_estimator.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/net/core/gen_estimator.c
+++ b/net/core/gen_estimator.c
@@ -159,7 +159,11 @@ int gen_new_estimator(struct gnet_stats_
 	est->intvl_log = intvl_log;
 	est->cpu_bstats = cpu_bstats;
 
+	if (stats_lock)
+		local_bh_disable();
 	est_fetch_counters(est, &b);
+	if (stats_lock)
+		local_bh_enable();
 	est->last_bytes = b.bytes;
 	est->last_packets = b.packets;
 	old = rcu_dereference_protected(*rate_est, 1);