Re: [PATCH net v2] net: bridge: fix soft lockup in br_multicast_query_expired()

[Ido Schimmel <idosch@nvidia.com>]

On Tue, Aug 12, 2025 at 05:18:18PM +0800, 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
> 
> The multicast_startup_query_interval can also cause this issue. Similar to
> the commit 99b40610956a("net: bridge: mcast: add and enforce query interval
                         ^ missing space

> minimum"), add check for the query interval maximum to fix this issue.
> 
> Link: https://lore.kernel.org/netdev/20250806094941.1285944-1-wangliang74@huawei.com/
> Fixes: 7e4df51eb35d ("bridge: netlink: add support for igmp's intervals")

Probably doesn't matter in practice given how old both commits are, but
I think you should blame d902eee43f19 ("bridge: Add multicast
count/interval sysfs entries") instead. The commit message also uses the
sysfs path and not the netlink one.

> Suggested-by: Nikolay Aleksandrov <razor@blackwall.org>
> Signed-off-by: Wang Liang <wangliang74@huawei.com>

Code looks fine to me.