tcp: MD5: Fix MD5 signatures on certain ACK packets

I noticed, looking at tcpdumps, that timewait ACKs were getting sent
with an incorrect MD5 signature when signatures were enabled.

I broke this in 49a72dfb8814c2d65bd9f8c9c6daf6395a1ec58d ("tcp: Fix
MD5 signatures for non-linear skbs"). I didn't take into account that
the skb passed to tcp_*_send_ack was the inbound packet, thus the
source and dest addresses need to be swapped when calculating the MD5
pseudoheader.

Signed-off-by: Adam Langley <agl@imperialviolet.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -655,8 +655,8 @@ static void tcp_v4_send_ack(struct sk_buff *skb, u32 seq, u32 ack,
 		rep.th.doff = arg.iov[0].iov_len/4;
 
 		tcp_v4_md5_hash_hdr((__u8 *) &rep.opt[offset],
-				    key, ip_hdr(skb)->daddr,
-				    ip_hdr(skb)->saddr, &rep.th);
+				    key, ip_hdr(skb)->saddr,
+				    ip_hdr(skb)->daddr, &rep.th);
 	}
 #endif
 	arg.csum = csum_tcpudp_nofold(ip_hdr(skb)->daddr,

net/ipv6/tcp_ipv6.c

@@ -1094,8 +1094,8 @@ static void tcp_v6_send_ack(struct sk_buff *skb, u32 seq, u32 ack, u32 win, u32
 		*topt++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
 				(TCPOPT_MD5SIG << 8) | TCPOLEN_MD5SIG);
 		tcp_v6_md5_hash_hdr((__u8 *)topt, key,
-				    &ipv6_hdr(skb)->daddr,
-				    &ipv6_hdr(skb)->saddr, t1);
+				    &ipv6_hdr(skb)->saddr,
+				    &ipv6_hdr(skb)->daddr, t1);
 	}
 #endif