Code: static int __br_mdb_del(struct net_bridge *br, struct br_mdb_entry *entry)
{
	struct net_bridge_mdb_htable *mdb;
	struct net_bridge_mdb_entry *mp;
	struct net_bridge_port_group *p;
	struct net_bridge_port_group __rcu **pp;
	struct br_ip ip;
	int err = -EINVAL;

	if (!netif_running(br->dev) || br->multicast_disabled)
		return -EINVAL;

	if (timer_pending(&br->multicast_querier_timer))
		return -EBUSY;

	ip.proto = entry->addr.proto;
	if (ip.proto == htons(ETH_P_IP))
		ip.u.ip4 = entry->addr.u.ip4;
#if IS_ENABLED(CONFIG_IPV6)
	else
		ip.u.ip6 = entry->addr.u.ip6;
#endif

	spin_lock_bh(&br->multicast_lock);
	mdb = mlock_dereference(br->mdb, br);

	mp = br_mdb_ip_get(mdb, &ip);
	if (!mp)
		goto unlock;

	for (pp = &mp->ports;
	     (p = mlock_dereference(*pp, br)) != NULL;
	     pp = &p->next) {
		if (!p->port || p->port->dev->ifindex != entry->ifindex)
			continue;

		if (p->port->state == BR_STATE_DISABLED)
			goto unlock;

		rcu_assign_pointer(*pp, p->next);
		hlist_del_init(&p->mglist);
		del_timer(&p->timer);
 		call_rcu_bh(&p->rcu, br_multicast_free_pg);
 		err = 0;
 
		if (!mp->ports && !mp->mglist &&
 		    netif_running(br->dev))
 			mod_timer(&mp->timer, jiffies);
 		break;
	}

unlock:
	spin_unlock_bh(&br->multicast_lock);
	return err;
}

Vulnerability Type: DoS 
CWE ID: CWE-20
Summary: The bridge multicast implementation in the Linux kernel through 3.10.3 does not check whether a certain timer is armed before modifying the timeout value of that timer, which allows local users to cause a denial of service (BUG and system crash) via vectors involving the shutdown of a KVM virtual machine, related to net/bridge/br_mdb.c and net/bridge/br_multicast.c.
Commit Message: bridge: fix some kernel warning in multicast timer

Several people reported the warning: "kernel BUG at kernel/timer.c:729!"
and the stack trace is:

	#7 [ffff880214d25c10] mod_timer+501 at ffffffff8106d905
	#8 [ffff880214d25c50] br_multicast_del_pg.isra.20+261 at ffffffffa0731d25 [bridge]
	#9 [ffff880214d25c80] br_multicast_disable_port+88 at ffffffffa0732948 [bridge]
	#10 [ffff880214d25cb0] br_stp_disable_port+154 at ffffffffa072bcca [bridge]
	#11 [ffff880214d25ce8] br_device_event+520 at ffffffffa072a4e8 [bridge]
	#12 [ffff880214d25d18] notifier_call_chain+76 at ffffffff8164aafc
	#13 [ffff880214d25d50] raw_notifier_call_chain+22 at ffffffff810858f6
	#14 [ffff880214d25d60] call_netdevice_notifiers+45 at ffffffff81536aad
	#15 [ffff880214d25d80] dev_close_many+183 at ffffffff81536d17
	#16 [ffff880214d25dc0] rollback_registered_many+168 at ffffffff81537f68
	#17 [ffff880214d25de8] rollback_registered+49 at ffffffff81538101
	#18 [ffff880214d25e10] unregister_netdevice_queue+72 at ffffffff815390d8
	#19 [ffff880214d25e30] __tun_detach+272 at ffffffffa074c2f0 [tun]
	#20 [ffff880214d25e88] tun_chr_close+45 at ffffffffa074c4bd [tun]
	#21 [ffff880214d25ea8] __fput+225 at ffffffff8119b1f1
	#22 [ffff880214d25ef0] ____fput+14 at ffffffff8119b3fe
	#23 [ffff880214d25f00] task_work_run+159 at ffffffff8107cf7f
	#24 [ffff880214d25f30] do_notify_resume+97 at ffffffff810139e1
	#25 [ffff880214d25f50] int_signal+18 at ffffffff8164f292

this is due to I forgot to check if mp->timer is armed in
br_multicast_del_pg(). This bug is introduced by
commit 9f00b2e7cf241fa389733d41b6 (bridge: only expire the mdb entry
when query is received).

Same for __br_mdb_del().

Tested-by: poma <pomidorabelisima@gmail.com>
Reported-by: LiYonghua <809674045@qq.com>
Reported-by: Robert Hancock <hancockrwd@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>