net: fix sk_forward_alloc corruptions

As David found out, sock_queue_err_skb() should be called with socket
lock hold, or we risk sk_forward_alloc corruption, since we use non
atomic operations to update this field.

This patch adds bh_lock_sock()/bh_unlock_sock() pair to three spots.
(BH already disabled)

1) skb_tstamp_tx() 
2) Before calling ip_icmp_error(), in __udp4_lib_err() 
3) Before calling ipv6_icmp_error(), in __udp6_lib_err()
Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -2992,7 +2992,11 @@ void skb_tstamp_tx(struct sk_buff *orig_skb,
 	memset(serr, 0, sizeof(*serr));
 	serr->ee.ee_errno = ENOMSG;
 	serr->ee.ee_origin = SO_EE_ORIGIN_TIMESTAMPING;
+
+	bh_lock_sock(sk);
 	err = sock_queue_err_skb(sk, skb);
+	bh_unlock_sock(sk);
+
 	if (err)
 		kfree_skb(skb);
 }

net/ipv4/udp.c

@@ -634,7 +634,9 @@ void __udp4_lib_err(struct sk_buff *skb, u32 info, struct udp_table *udptable)
 		if (!harderr || sk->sk_state != TCP_ESTABLISHED)
 			goto out;
 	} else {
+		bh_lock_sock(sk);
 		ip_icmp_error(sk, skb, err, uh->dest, info, (u8 *)(uh+1));
+		bh_unlock_sock(sk);
 	}
 	sk->sk_err = err;
 	sk->sk_error_report(sk);

net/ipv6/udp.c

@@ -466,9 +466,11 @@ void __udp6_lib_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 	if (sk->sk_state != TCP_ESTABLISHED && !np->recverr)
 		goto out;
 
-	if (np->recverr)
+	if (np->recverr) {
+		bh_lock_sock(sk);
 		ipv6_icmp_error(sk, skb, err, uh->dest, ntohl(info), (u8 *)(uh+1));
-
+		bh_unlock_sock(sk);
+	}
 	sk->sk_err = err;
 	sk->sk_error_report(sk);
 out: