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

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 <[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


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

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