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

[Nikolay Aleksandrov <razor@blackwall.org>]

On 8/7/25 13:36, Wang Liang wrote:
> 
> 在 2025/8/6 18:08, Nikolay Aleksandrov 写道:
>> 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
> 
> 
> Thanks very much for your suggestions!
> 
> Similar to the commit 99b40610956a("net: bridge: mcast: add and enforce
> query interval minimum"), it is indeed a better choice to add query
> interval maximum:
> 
> diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
> index 1377f31b719c..8ce145938b02 100644
> --- a/net/bridge/br_multicast.c
> +++ b/net/bridge/br_multicast.c
> @@ -4818,6 +4818,14 @@ void br_multicast_set_query_intvl(struct net_bridge_mcast *brmctx,
>                  intvl_jiffies = BR_MULTICAST_QUERY_INTVL_MIN;
>          }
> 
> +       if (intvl_jiffies > BR_MULTICAST_QUERY_INTVL_MAX) {
> +               br_info(brmctx->br,
> +                       "trying to set multicast query interval above maximum, setting to %lu (%ums)\n",
> + jiffies_to_clock_t(BR_MULTICAST_QUERY_INTVL_MAX),
> + jiffies_to_msecs(BR_MULTICAST_QUERY_INTVL_MAX));
> +               intvl_jiffies = BR_MULTICAST_QUERY_INTVL_MAX;
> +       }
> +
>          brmctx->multicast_query_interval = intvl_jiffies;
>   }
> 
> @@ -4834,6 +4842,14 @@ void br_multicast_set_startup_query_intvl(struct net_bridge_mcast *brmctx,
>                  intvl_jiffies = BR_MULTICAST_STARTUP_QUERY_INTVL_MIN;
>          }
> 
> +       if (intvl_jiffies > BR_MULTICAST_STARTUP_QUERY_INTVL_MAX) {
> +               br_info(brmctx->br,
> +                       "trying to set multicast startup query interval above maximum, setting to %lu (%ums)\n",
> + jiffies_to_clock_t(BR_MULTICAST_STARTUP_QUERY_INTVL_MAX),
> + jiffies_to_msecs(BR_MULTICAST_STARTUP_QUERY_INTVL_MAX));
> +               intvl_jiffies = BR_MULTICAST_STARTUP_QUERY_INTVL_MAX;
> +       }
> +
>          brmctx->multicast_startup_query_interval = intvl_jiffies;
>   }
> 
> diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
> index b159aae594c0..8de0904b9627 100644
> --- a/net/bridge/br_private.h
> +++ b/net/bridge/br_private.h
> @@ -31,6 +31,8 @@
>   #define BR_MULTICAST_DEFAULT_HASH_MAX 4096
>   #define BR_MULTICAST_QUERY_INTVL_MIN msecs_to_jiffies(1000)
>   #define BR_MULTICAST_STARTUP_QUERY_INTVL_MIN BR_MULTICAST_QUERY_INTVL_MIN
> +#define BR_MULTICAST_QUERY_INTVL_MAX msecs_to_jiffies(86400000) /* 24 hours */
> +#define BR_MULTICAST_STARTUP_QUERY_INTVL_MAX BR_MULTICAST_QUERY_INTVL_MAX
> 
>   #define BR_HWDOM_MAX BITS_PER_LONG
> 
> Set the interval maximum to 24 hours, it is big enough.
> Please check it. If the above code is ok, I will send a v2 patch then.
> 
> ------
> Best regards
> Wang Liang
> 

yeah, that is what I suggested, looks good to me

Cheers,
  Nik