Subject: [PATCH 03/22] tcp: do not mangle skb->cb[] in tcp_make_synack()

From: Eric Dumazet <[email-masked]>
Date: Thu, 2 Nov 2017 12:30:25 -0700

[ Upstream commit 3b11775033dc87c3d161996c54507b15ba26414a ]

Christoph Paasch sent a patch to address the following issue :

tcp_make_synack() is leaving some TCP private info in skb->cb[],
then send the packet by other means than tcp_transmit_skb()

tcp_transmit_skb() makes sure to clear skb->cb[] to not confuse
IPv4/IPV6 stacks, but we have no such cleanup for SYNACK.

tcp_make_synack() should not use tcp_init_nondata_skb() :

tcp_init_nondata_skb() really should be limited to skbs put in write/rtx
queues (the ones that are only sent via tcp_transmit_skb())

This patch fixes the issue and should even save few cpu cycles ;)

Fixes: 971f10eca186 ("tcp: better TCP_SKB_CB layout to reduce cache line misses")
Signed-off-by: Eric Dumazet <[email-masked]>
Reported-by: Christoph Paasch <[email-masked]>
Reviewed-by: Christoph Paasch <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/ipv4/tcp_output.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 58587b0e2b5d..e359840f46c0 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3207,13 +3207,8 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 	th->source = htons(ireq->ir_num);
 	th->dest = ireq->ir_rmt_port;
 	skb->mark = ireq->ir_mark;
-	/* Setting of flags are superfluous here for callers (and ECE is
-	 * not even correctly set)
-	 */
-	tcp_init_nondata_skb(skb, tcp_rsk(req)->snt_isn,
-			     TCPHDR_SYN | TCPHDR_ACK);
-
-	th->seq = htonl(TCP_SKB_CB(skb)->seq);
+	skb->ip_summed = CHECKSUM_PARTIAL;
+	th->seq = htonl(tcp_rsk(req)->snt_isn);
 	/* XXX data is queued and acked as is. No buffer/window check */
 	th->ack_seq = htonl(tcp_rsk(req)->rcv_nxt);
 
-- 
2.13.6