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: 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 Cc: Petr Machata Cc: Roopa Prabhu Cc: Nikolay Aleksandrov Cc: Benjamin Poirier Cc: Ido Schimmel --- 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