[PATCH] tcp: verify the checksum of the first data segment in a new connection

[Frank van der Linden <fllinden@amazon.com>]

commit 079096f103fa ("tcp/dccp: install syn_recv requests into ehash
table") introduced an optimization for the handling of child sockets
created for a new TCP connection.

But this optimization passes any data associated with the last ACK of the
connection handshake up the stack without verifying its checksum, because it
calls tcp_child_process(), which in turn calls tcp_rcv_state_process()
directly.  These lower-level processing functions do not do any checksum
verification.

Insert a tcp_checksum_complete call in the TCP_NEW_SYN_RECEIVE path to
fix this.

Signed-off-by: Frank van der Linden <fllinden@amazon.com>
---
 net/ipv4/tcp_ipv4.c | 8 +++++++-
 net/ipv6/tcp_ipv6.c | 8 +++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index f70586b50838..1ec4c0d4aba5 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1703,7 +1703,13 @@ int tcp_v4_rcv(struct sk_buff *skb)
 			th = (const struct tcphdr *)skb->data;
 			iph = ip_hdr(skb);
 			tcp_v4_fill_cb(skb, iph, th);
-			nsk = tcp_check_req(sk, skb, req, false, &req_stolen);
+
+			if (tcp_checksum_complete(skb)) {
+				__TCP_INC_STATS(net, TCP_MIB_CSUMERRORS);
+			} else {
+				nsk = tcp_check_req(sk, skb, req, false,
+						    &req_stolen);
+			}
 		}
 		if (!nsk) {
 			reqsk_put(req);
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 6d664d83cd16..a12b694d3d1e 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1486,7 +1486,13 @@ static int tcp_v6_rcv(struct sk_buff *skb)
 			th = (const struct tcphdr *)skb->data;
 			hdr = ipv6_hdr(skb);
 			tcp_v6_fill_cb(skb, hdr, th);
-			nsk = tcp_check_req(sk, skb, req, false, &req_stolen);
+
+			if (tcp_checksum_complete(skb)) {
+				__TCP_INC_STATS(net, TCP_MIB_CSUMERRORS);
+			} else {
+				nsk = tcp_check_req(sk, skb, req, false,
+						    &req_stolen);
+			}
 		}
 		if (!nsk) {
 			reqsk_put(req);
-- 
2.14.4