TCP: MTUprobe: fix potential sk_send_head corruption

[TCP] MTUprobe: fix potential sk_send_head corruption

[ Upstream commit: 6e421410 ]

When the abstraction functions got added, conversion here was
made incorrectly. As a result, the skb may end up pointing
to skb which got included to the probe skb and then was freed.
For it to trigger, however, skb_transmit must fail sending as
well.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/net/tcp.h

@@ -1256,6 +1256,9 @@ static inline void tcp_insert_write_queue_before(struct sk_buff *new,
 						  struct sock *sk)
 {
 	__skb_insert(new, skb->prev, skb, &sk->sk_write_queue);
+
+	if (sk->sk_send_head == skb)
+		sk->sk_send_head = new;
 }
 
 static inline void tcp_unlink_write_queue(struct sk_buff *skb, struct sock *sk)

net/ipv4/tcp_output.c

@@ -1287,7 +1287,6 @@ static int tcp_mtu_probe(struct sock *sk)
 
 	skb = tcp_send_head(sk);
 	tcp_insert_write_queue_before(nskb, skb, sk);
-	tcp_advance_send_head(sk, skb);
 
 	TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
 	TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;