[PATCH net-next] net: ipv4: guard ip_mr_output() with rcu

[Eric Dumazet <edumazet@google.com>]

syzbot found at least one path leads to an ip_mr_output()
without RCU being held.

Add guard(rcu)() to fix this in a concise way.

WARNING: CPU: 0 PID: 0 at net/ipv4/ipmr.c:2302 ip_mr_output+0xbb1/0xe70 net/ipv4/ipmr.c:2302
Call Trace:
 <IRQ>
  igmp_send_report+0x89e/0xdb0 net/ipv4/igmp.c:799
 igmp_timer_expire+0x204/0x510 net/ipv4/igmp.c:-1
  call_timer_fn+0x17e/0x5f0 kernel/time/timer.c:1747
  expire_timers kernel/time/timer.c:1798 [inline]
  __run_timers kernel/time/timer.c:2372 [inline]
  __run_timer_base+0x61a/0x860 kernel/time/timer.c:2384
  run_timer_base kernel/time/timer.c:2393 [inline]
  run_timer_softirq+0xb7/0x180 kernel/time/timer.c:2403
  handle_softirqs+0x286/0x870 kernel/softirq.c:579
  __do_softirq kernel/softirq.c:613 [inline]
  invoke_softirq kernel/softirq.c:453 [inline]
  __irq_exit_rcu+0xca/0x1f0 kernel/softirq.c:680
  irq_exit_rcu+0x9/0x30 kernel/softirq.c:696
  instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1050 [inline]
  sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1050

Fixes: 35bec72a24ac ("net: ipv4: Add ip_mr_output()")
Reported-by: syzbot+f02fb9e43bd85c6c66ae@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/685e841a.a00a0220.129264.0002.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Petr Machata <petrm@nvidia.com>
Cc: Roopa Prabhu <roopa@nvidia.com>
Cc: Nikolay Aleksandrov <razor@blackwall.org>
Cc: Benjamin Poirier <bpoirier@nvidia.com>
Cc: Ido Schimmel <idosch@nvidia.com>
---
 net/ipv4/ipmr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index f78c4e53dc8c161e334781970bbff6069c084ebb..3a2044e6033d5683bda678489f6eaf72ea0b8890 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -2299,7 +2299,8 @@ int ip_mr_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 	struct mr_table *mrt;
 	int vif;
 
-	WARN_ON_ONCE(!rcu_read_lock_held());
+	guard(rcu)();
+
 	dev = rt->dst.dev;
 
 	if (IPCB(skb)->flags & IPSKB_FORWARDED)
@@ -2313,7 +2314,6 @@ int ip_mr_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 	if (IS_ERR(mrt))
 		goto mc_output;
 
-	/* already under rcu_read_lock() */
 	cache = ipmr_cache_find(mrt, ip_hdr(skb)->saddr, ip_hdr(skb)->daddr);
 	if (!cache) {
 		vif = ipmr_find_vif(mrt, dev);
-- 
2.50.0.727.gbf7dc18ff4-goog