[NETFILTER]: ulog: fix panic on SMP kernels

Fix kernel panic on various SMP machines. The culprit is a null
ub->skb in ulog_send(). If ulog_timer() has already been scheduled on
one CPU and is spinning on the lock, and ipt_ulog_packet() flushes the
queue on another CPU by calling ulog_send() right before it exits,
there will be no skbuff when ulog_timer() acquires the lock and calls
ulog_send(). Cancelling the timer in ulog_send() doesn't help because
it has already been scheduled and is running on the first CPU.

Similar problem exists in ebt_ulog.c and nfnetlink_log.c.
Signed-off-by: Mark Huang <mlhuang@cs.princeton.edu>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/netfilter/ebt_ulog.c

@@ -74,6 +74,9 @@ static void ulog_send(unsigned int nlgroup)
 	if (timer_pending(&ub->timer))
 		del_timer(&ub->timer);
 
+	if (!ub->skb)
+		return;
+
 	/* last nlmsg needs NLMSG_DONE */
 	if (ub->qlen > 1)
 		ub->lastnlh->nlmsg_type = NLMSG_DONE;

net/ipv4/netfilter/ipt_ULOG.c

@@ -115,6 +115,11 @@ static void ulog_send(unsigned int nlgroupnum)
 		del_timer(&ub->timer);
 	}
 
+	if (!ub->skb) {
+		DEBUGP("ipt_ULOG: ulog_send: nothing to send\n");
+		return;
+	}
+
 	/* last nlmsg needs NLMSG_DONE */
 	if (ub->qlen > 1)
 		ub->lastnlh->nlmsg_type = NLMSG_DONE;

net/netfilter/nfnetlink_log.c

@@ -366,6 +366,9 @@ __nfulnl_send(struct nfulnl_instance *inst)
 	if (timer_pending(&inst->timer))
 		del_timer(&inst->timer);
 
+	if (!inst->skb)
+		return 0;
+
 	if (inst->qlen > 1)
 		inst->lastnlh->nlmsg_type = NLMSG_DONE;