Commit 3b117750 authored Nov 02, 2017 by Eric Dumazet Committed by David S. Miller Nov 03, 2017

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: 971f10ec ("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>

parent 25dd169a

net/ipv4/tcp_output.c

+2 −7

Original line number	Diff line number	Diff line
		@@ -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);

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