bridge: mcast: Fix a false positive lockdep splat

Connecting two bridges on the same system [1] can result in a lockdep
splat [2].

The report is a false positive. Multicast queries are built and
transmitted under the bridge multicast lock. When the outgoing port of
one bridge is configured on top of another bridge, the transmit path
re-enters bridge code and acquires the other bridge's multicast lock in
order to snoop the query. Both lock instances share a single lockdep
class, so lockdep flags the nested acquisition as an AA deadlock.

Giving each bridge its own lock class will not solve the problem: the
reverse topology would produce an ABBA splat with the same pair of
classes. It also consumes a lockdep key per bridge.

Instead, fix the problem by deferring the transmission of the queries to
a workqueue. Build the skb and update querier state under the lock as
before, then enqueue the skb on a per multicast context queue and
schedule the work.

Purge the queue when the multicast context is de-initialized. At this
stage the work cannot be requeued. There is no need to take a reference
on skb->dev since the work cannot outlive the bridge or the bridge port.

Use the high priority workqueue to reduce the delay between the enqueue
time and the transmission time. With default settings (i.e., querier
interval - 255 seconds, query interval - 125 seconds) the extra delay
should not be a problem.

Avoid the unlikely case of the queue growing endlessly by limiting it to
1,000 skbs. Use this number for the simple reason that this is the
default Tx queue length.

Use local_bh_{disable,enable}() to disable/enable softIRQs and migration
in order to avoid corrupting the multicast statistics (per-CPU
u64_stats).

[1]
ip link add name br1 up type bridge mcast_snooping 1 mcast_querier 1
ip link add name br0 up type bridge mcast_snooping 1 mcast_querier 1
ip link add link br0 name br0.10 up master br1 type vlan id 10

[2]
WARNING: possible recursive locking detected
7.0.0-virtme-gb50c64a58a90 #1 Not tainted
[...]
ip/339 is trying to acquire lock:
ffff888104f0b480 (&br->multicast_lock){+.-.}-{3:3}, at: br_ip6_multicast_query (net/bridge/br_multicast.c:3584)

but task is already holding lock:
ffff888104f03480 (&br->multicast_lock){+.-.}-{3:3}, at: br_multicast_port_query_expired (net/bridge/br_multicast.c:1904)

[...]

Call Trace:
[...]
br_ip6_multicast_query (net/bridge/br_multicast.c:3584)
br_multicast_ipv6_rcv (net/bridge/br_multicast.c:3988)
br_dev_xmit (net/bridge/br_device.c:98 (discriminator 1))
dev_hard_start_xmit (net/core/dev.c:3904)
__dev_queue_xmit (net/core/dev.c:4871)
vlan_dev_hard_start_xmit (net/8021q/vlan_dev.c:131 (discriminator 1))
dev_hard_start_xmit (net/core/dev.c:3904)
__dev_queue_xmit (net/core/dev.c:4871)
br_dev_queue_push_xmit (net/bridge/br_forward.c:60)
__br_multicast_send_query (net/bridge/br_multicast.c:1811 (discriminator 1))
br_multicast_send_query (net/bridge/br_multicast.c:1889)
br_multicast_port_query_expired (net/bridge/br_multicast.c:1914)
call_timer_fn (kernel/time/timer.c:1749)
[...]

Reported-by: syzbot+d7b7f1412c02134efa6d@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/000000000000c4c9d405f2643e01@google.com/
Reviewed-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260708122820.1298718-2-idosch@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

0 files changed, 0 insertions, 0 deletions