On 8/6/25 12:49, Wang Liang wrote:
> When set multicast_query_interval to a large value, the local variable
> 'time' in br_multicast_send_query() may overflow. If the time is smaller
> than jiffies, the timer will expire immediately, and then call mod_timer()
> again, which creates a loop and may trigger the following soft lockup
> issue:
>
> watchdog: BUG: soft lockup - CPU#1 stuck for 221s! [rb_consumer:66]
> CPU: 1 UID: 0 PID: 66 Comm: rb_consumer Not tainted 6.16.0+ #259
> PREEMPT(none)
> Call Trace:
> <IRQ>
> __netdev_alloc_skb+0x2e/0x3a0
> br_ip6_multicast_alloc_query+0x212/0x1b70
> __br_multicast_send_query+0x376/0xac0
> br_multicast_send_query+0x299/0x510
> br_multicast_query_expired.constprop.0+0x16d/0x1b0
> call_timer_fn+0x3b/0x2a0
> __run_timers+0x619/0x950
> run_timer_softirq+0x11c/0x220
> handle_softirqs+0x18e/0x560
> __irq_exit_rcu+0x158/0x1a0
> sysvec_apic_timer_interrupt+0x76/0x90
> </IRQ>
>
> This issue can be reproduced with:
> ip link add br0 type bridge
> echo 1 > /sys/class/net/br0/bridge/multicast_querier
> echo 0xffffffffffffffff >
> /sys/class/net/br0/bridge/multicast_query_interval
> ip link set dev br0 up
>
> Fix this by comparing expire time with jiffies, to avoid the timer loop.
>
> Fixes: 7e4df51eb35d ("bridge: netlink: add support for igmp's intervals")
> Signed-off-by: Wang Liang <[email protected]>
> ---
> net/bridge/br_multicast.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
> index 1377f31b719c..631ae3b4c45d 100644
> --- a/net/bridge/br_multicast.c
> +++ b/net/bridge/br_multicast.c
> @@ -1892,7 +1892,8 @@ static void br_multicast_send_query(struct
> net_bridge_mcast *brmctx,
> time += own_query->startup_sent < brmctx->multicast_startup_query_count
> ?
> brmctx->multicast_startup_query_interval :
> brmctx->multicast_query_interval;
> - mod_timer(&own_query->timer, time);
> + if (time_is_after_jiffies(time))
> + mod_timer(&own_query->timer, time);
> }
>
> static void
This is the wrong way to fix it, it is a configuration issue, so we could either
cap the value at something that noone uses, e.g. 24 hours, or we could make
sure time
is at least 1s (that is BR_MULTICAST_QUERY_INTVL_MIN).
The simple fix would be to do a min(time, BR_MULTICAST_QUERY_INTVL_MIN), but
I'd go
for something similar to:
commit 99b40610956a
Author: Nikolay Aleksandrov <[email protected]>
Date: Mon Dec 27 19:21:15 2021 +0200
net: bridge: mcast: add and enforce query interval minimum
for the maximum to avoid the overflow altogether. By the way
multicast_startup_query_interval
would also cause the same issue, so you'd have to cap it.
Cheers,
Nik
