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

From: Nikolay Aleksandrov
Date: Sun Aug 10 2025 - 06:02:16 EST

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@xxxxxxxxxx>
---
  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@xxxxxxxxxxxxx>
  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