Re: [PATCH net] net: bridge: fix soft lockup in br_multicast_query_expired()

[Nikolay Aleksandrov <razor@blackwall.org>]

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 <wangliang74@huawei.com>
> ---
>  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 <razor@blackwall.org>
 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