[PATCH 4.9 55/62] sch_netem: fix rcu splat in netem_enqueue()

Greg Kroah-Hartman
2019-11-04T21:51:44Z

From: Eric Dumazet <edumazet@google.com>

commit 159d2c7d8106177bd9a986fd005a311fe0d11285 upstream.

qdisc_root() use from netem_enqueue() triggers a lockdep warning.

__dev_queue_xmit() uses rcu_read_lock_bh() which is
not equivalent to rcu_read_lock() + local_bh_disable_bh as far
as lockdep is concerned.

WARNING: suspicious RCU usage
5.3.0-rc7+ #0 Not tainted
-----------------------------
include/net/sch_generic.h:492 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
3 locks held by syz-executor427/8855:
#0: 00000000b5525c01 (rcu_read_lock_bh){....}, at: lwtunnel_xmit_redirect include/net/lwtunnel.h:92 [inline]
#0: 00000000b5525c01 (rcu_read_lock_bh){....}, at: ip_finish_output2+0x2dc/0x2570 net/ipv4/ip_output.c:214
#1: 00000000b5525c01 (rcu_read_lock_bh){....}, at: __dev_queue_xmit+0x20a/0x3650 net/core/dev.c:3804
#2: 00000000364bae92 (&(&sch->q.lock)->rlock){+.-.}, at: spin_lock include/linux/spinlock.h:338 [inline]
#2: 00000000364bae92 (&(&sch->q.lock)->rlock){+.-.}, at: __dev_xmit_skb net/core/dev.c:3502 [inline]
#2: 00000000364bae92 (&(&sch->q.lock)->rlock){+.-.}, at: __dev_queue_xmit+0x14b8/0x3650 net/core/dev.c:3838

stack backtrace:
CPU: 0 PID: 8855 Comm: syz-executor427 Not tainted 5.3.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x172/0x1f0 lib/dump_stack.c:113
lockdep_rcu_suspicious+0x153/0x15d kernel/locking/lockdep.c:5357
qdisc_root include/net/sch_generic.h:492 [inline]
netem_enqueue+0x1cfb/0x2d80 net/sched/sch_netem.c:479
__dev_xmit_skb net/core/dev.c:3527 [inline]
__dev_queue_xmit+0x15d2/0x3650 net/core/dev.c:3838
dev_queue_xmit+0x18/0x20 net/core/dev.c:3902
neigh_hh_output include/net/neighbour.h:500 [inline]
neigh_output include/net/neighbour.h:509 [inline]
ip_finish_output2+0x1726/0x2570 net/ipv4/ip_output.c:228
__ip_finish_output net/ipv4/ip_output.c:308 [inline]
__ip_finish_output+0x5fc/0xb90 net/ipv4/ip_output.c:290
ip_finish_output+0x38/0x1f0 net/ipv4/ip_output.c:318
NF_HOOK_COND include/linux/netfilter.h:294 [inline]
ip_mc_output+0x292/0xf40 net/ipv4/ip_output.c:417
dst_output include/net/dst.h:436 [inline]
ip_local_out+0xbb/0x190 net/ipv4/ip_output.c:125
ip_send_skb+0x42/0xf0 net/ipv4/ip_output.c:1555
udp_send_skb.isra.0+0x6b2/0x1160 net/ipv4/udp.c:887
udp_sendmsg+0x1e96/0x2820 net/ipv4/udp.c:1174
inet_sendmsg+0x9e/0xe0 net/ipv4/af_inet.c:807
sock_sendmsg_nosec net/socket.c:637 [inline]
sock_sendmsg+0xd7/0x130 net/socket.c:657
___sys_sendmsg+0x3e2/0x920 net/socket.c:2311
__sys_sendmmsg+0x1bf/0x4d0 net/socket.c:2413
__do_sys_sendmmsg net/socket.c:2442 [inline]
__se_sys_sendmmsg net/socket.c:2439 [inline]
__x64_sys_sendmmsg+0x9d/0x100 net/socket.c:2439
do_syscall_64+0xfd/0x6a0 arch/x86/entry/common.c:296
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
include/net/sch_generic.h | 5 +++++
net/sched/sch_netem.c | 2 +-
2 files changed, 6 insertions(+), 1 deletion(-)

--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -276,6 +276,11 @@ static inline struct Qdisc *qdisc_root(c
return q;
}

+static inline struct Qdisc *qdisc_root_bh(const struct Qdisc *qdisc)
+{
+ return rcu_dereference_bh(qdisc->dev_queue->qdisc);
+}
+
static inline struct Qdisc *qdisc_root_sleeping(const struct Qdisc *qdisc)
{
return qdisc->dev_queue->qdisc_sleeping;
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -475,7 +475,7 @@ static int netem_enqueue(struct sk_buff
* skb will be queued.
*/
if (count > 1 && (skb2 = skb_clone(skb, GFP_ATOMIC)) != NULL) {
- struct Qdisc *rootq = qdisc_root(sch);
+ struct Qdisc *rootq = qdisc_root_bh(sch);
u32 dupsave = q->duplicate; /* prevent duplicating a dup... */

q->duplicate = 0;