net: bridge: mcast: wait for previous gc cycles when removing port

syzbot hit a use-after-free[1] which is caused because the bridge doesn't
make sure that all previous garbage has been collected when removing a
port. What happens is:
      CPU 1                   CPU 2
 start gc cycle           remove port
                         acquire gc lock first
 wait for lock
                         call br_multicasg_gc() directly
 acquire lock now but    free port
 the port can be freed
 while grp timers still
 running

Make sure all previous gc cycles have finished by using flush_work before
freeing the port.

[1]
  BUG: KASAN: slab-use-after-free in br_multicast_port_group_expired+0x4c0/0x550 net/bridge/br_multicast.c:861
  Read of size 8 at addr ffff888071d6d000 by task syz.5.1232/9699

  CPU: 1 PID: 9699 Comm: syz.5.1232 Not tainted 6.10.0-rc5-syzkaller-00021-g24ca36a5 #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/07/2024
  Call Trace:
   <IRQ>
   __dump_stack lib/dump_stack.c:88 [inline]
   dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:114
   print_address_description mm/kasan/report.c:377 [inline]
   print_report+0xc3/0x620 mm/kasan/report.c:488
   kasan_report+0xd9/0x110 mm/kasan/report.c:601
   br_multicast_port_group_expired+0x4c0/0x550 net/bridge/br_multicast.c:861
   call_timer_fn+0x1a3/0x610 kernel/time/timer.c:1792
   expire_timers kernel/time/timer.c:1843 [inline]
   __run_timers+0x74b/0xaf0 kernel/time/timer.c:2417
   __run_timer_base kernel/time/timer.c:2428 [inline]
   __run_timer_base kernel/time/timer.c:2421 [inline]
   run_timer_base+0x111/0x190 kernel/time/timer.c:2437

Reported-by: syzbot+263426984509be19c9a0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=263426984509be19c9a0
Fixes: e12cec65 ("net: bridge: mcast: destroy all entries via gc")
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20240802080730.3206303-1-razor@blackwall.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/bridge/br_multicast.c

@@ -2045,16 +2045,14 @@ void br_multicast_del_port(struct net_bridge_port *port)
 {
 	struct net_bridge *br = port->br;
 	struct net_bridge_port_group *pg;
-	HLIST_HEAD(deleted_head);
 	struct hlist_node *n;
 
 	/* Take care of the remaining groups, only perm ones should be left */
 	spin_lock_bh(&br->multicast_lock);
 	hlist_for_each_entry_safe(pg, n, &port->mglist, mglist)
 		br_multicast_find_del_pg(br, pg);
-	hlist_move_list(&br->mcast_gc_list, &deleted_head);
 	spin_unlock_bh(&br->multicast_lock);
-	br_multicast_gc(&deleted_head);
+	flush_work(&br->mcast_gc_work);
 	br_multicast_port_ctx_deinit(&port->multicast_ctx);
 	free_percpu(port->mcast_stats);
 }