ipv4: dst_entry leak in ip_send_unicast_reply()

commit 4062090e upstream.

ip_setup_cork() called inside ip_append_data() steals dst entry from rt to cork
and in case errors in __ip_append_data() nobody frees stolen dst entry

Fixes: 2e77d89b ("net: avoid a pair of dst_hold()/dst_release() in ip_append_data()")
Signed-off-by: Vasily Averin <vvs@parallels.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/ipv4/ip_output.c

@@ -1472,6 +1472,7 @@ void ip_send_reply(struct sock *sk, struct sk_buff *skb, __be32 daddr,
 	struct ipcm_cookie ipc;
 	struct flowi4 fl4;
 	struct rtable *rt = skb_rtable(skb);
+	int err;
 
 	if (ip_options_echo(&replyopts.opt.opt, skb))
 		return;
@@ -1509,8 +1510,13 @@ void ip_send_reply(struct sock *sk, struct sk_buff *skb, __be32 daddr,
 	sk->sk_priority = skb->priority;
 	sk->sk_protocol = ip_hdr(skb)->protocol;
 	sk->sk_bound_dev_if = arg->bound_dev_if;
-	ip_append_data(sk, &fl4, ip_reply_glue_bits, arg->iov->iov_base, len, 0,
-		       &ipc, &rt, MSG_DONTWAIT);
+	err = ip_append_data(sk, &fl4, ip_reply_glue_bits, arg->iov->iov_base,
+			     len, 0, &ipc, &rt, MSG_DONTWAIT);
+	if (unlikely(err)) {
+		ip_flush_pending_frames(sk);
+		goto out;
+	}
+
 	if ((skb = skb_peek(&sk->sk_write_queue)) != NULL) {
 		if (arg->csumoffset >= 0)
 			*((__sum16 *)skb_transport_header(skb) +
@@ -1519,7 +1525,7 @@ void ip_send_reply(struct sock *sk, struct sk_buff *skb, __be32 daddr,
 		skb->ip_summed = CHECKSUM_NONE;
 		ip_push_pending_frames(sk, &fl4);
 	}
-
+out:
 	bh_unlock_sock(sk);
 
 	ip_rt_put(rt);