[PATCH net] net_sched: gen_estimator: fix lockdep splat

[PATCH net] net_sched: gen_estimator: fix lockdep splat

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.com>

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>
---
 net/core/gen_estimator.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/gen_estimator.c b/net/core/gen_estimator.c
index 9834cfa21b21168a7654290dc2a999e41937b534..0a3f88f08727f1f1217560407ff539c8a8c17496 100644
--- a/net/core/gen_estimator.c
+++ b/net/core/gen_estimator.c
@@ -159,7 +159,11 @@ int gen_new_estimator(struct gnet_stats_basic_packed *bstats,
 	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);

Re: [PATCH net] net_sched: gen_estimator: fix lockdep splat

[Cong Wang]

On Sat, Jan 27, 2018 at 10:58 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> From: Eric Dumazet <edumazet@google.com>
>
> 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>

Nit: perhaps it is better to move the spin_lock out of est_fetch_counters()
and let callers decide to use spin_lock or spin_lock_bh.

Re: [PATCH net] net_sched: gen_estimator: fix lockdep splat

[David Miller]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Sat, 27 Jan 2018 10:58:43 -0800

> From: Eric Dumazet <edumazet@google.com>
> 
> 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>

Applied, thanks.