[PATCH 4.4 01/16] tcp: do not mangle skb->cb[] in tcp_make_synack()

From: Greg Kroah-Hartman
Date: Wed Nov 22 2017 - 05:34:32 EST

4.4-stable review patch. If anyone has any objections, please let me know.


From: Eric Dumazet <edumazet@xxxxxxxxxx>

[ 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 <edumazet@xxxxxxxxxx>
Reported-by: Christoph Paasch <cpaasch@xxxxxxxxx>
Reviewed-by: Christoph Paasch <cpaasch@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
net/ipv4/tcp_output.c | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)

--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3018,13 +3018,8 @@ struct sk_buff *tcp_make_synack(const st
tcp_ecn_make_synack(req, th);
th->source = htons(ireq->ir_num);
th->dest = ireq->ir_rmt_port;
- /* 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,
- 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);