tcp: do not mangle skb->cb[] in tcp_make_synack()

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@google.com>
Reported-by: Christoph Paasch <cpaasch@apple.com>
Reviewed-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 7 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 823003eef3a2..478909f4694d 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3180,13 +3180,8 @@ 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);