netdev - [RFC v2 03/14] tcp: Allow tcp_fast_parse

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Date:   Wed, 31 Jan 2018 16:07:05 -0800
From:   Christoph Paasch <cpaasch@...le.com>
To:     netdev@...r.kernel.org
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Mat Martineau <mathew.j.martineau@...ux.intel.com>
Subject: [RFC v2 03/14] tcp: Allow tcp_fast_parse_options to drop segments

After parsing the TCP-options, some option-kinds might trigger a drop of
the segment (e.g., as is the case for TCP_MD5). As we are moving to
consolidate the TCP_MD5-code in follow-up patches, we need to add the
capability to drop a segment right after parsing the options in
tcp_fast_parse_options().

Originally, tcp_fast_parse_options() returned false, when there is no
timestamp option, except in the case of the slow-path processing through
tcp_parse_options() where it always returns true.

So, the return-value of tcp_fast_parse_options() was kind of
inconsistent. With this patch, we make it return true when the segment
should get dropped based on the parsed options, and false otherwise.

In tcp_validate_incoming, we will then just check for
tp->rx_opt.saw_tstamp to see if we should verify PAWS.

The goto will be used in a follow-up patch to check whether one of the
options triggers a drop of the segment.

Signed-off-by: Christoph Paasch <cpaasch@...le.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@...ux.intel.com>
---
 net/ipv4/tcp_input.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index cfa51cfd2d99..1fbabcc99b62 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3847,6 +3847,8 @@ static bool tcp_parse_aligned_timestamp(struct tcp_sock *tp, const struct tcphdr
 
 /* Fast parse options. This hopes to only see timestamps.
  * If it is wrong it falls back on tcp_parse_options().
+ *
+ * Returns true if we should drop this packet based on present TCP-options.
  */
 static bool tcp_fast_parse_options(const struct net *net,
 				   const struct sk_buff *skb,
@@ -3857,18 +3859,19 @@ static bool tcp_fast_parse_options(const struct net *net,
 	 */
 	if (th->doff == (sizeof(*th) / 4)) {
 		tp->rx_opt.saw_tstamp = 0;
-		return false;
+		goto extra_opt_check;
 	} else if (tp->rx_opt.tstamp_ok &&
 		   th->doff == ((sizeof(*th) + TCPOLEN_TSTAMP_ALIGNED) / 4)) {
 		if (tcp_parse_aligned_timestamp(tp, th))
-			return true;
+			goto extra_opt_check;
 	}
 
 	tcp_parse_options(net, skb, &tp->rx_opt, 1, NULL);
 	if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
 		tp->rx_opt.rcv_tsecr -= tp->tsoffset;
 
-	return true;
+extra_opt_check:
+	return false;
 }
 
 #ifdef CONFIG_TCP_MD5SIG
@@ -5188,9 +5191,11 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	struct tcp_sock *tp = tcp_sk(sk);
 	bool rst_seq_match = false;
 
+	if (tcp_fast_parse_options(sock_net(sk), skb, th, tp))
+		goto discard;
+
 	/* RFC1323: H1. Apply PAWS check first. */
-	if (tcp_fast_parse_options(sock_net(sk), skb, th, tp) &&
-	    tp->rx_opt.saw_tstamp &&
+	if (tp->rx_opt.saw_tstamp &&
 	    tcp_paws_discard(sk, skb)) {
 		if (!th->rst) {
 			NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
-- 
2.16.1