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Date:	Sat, 31 May 2008 14:57:00 -0700
From:	"Adam Langley" <agl@...erialviolet.org>
To:	davem@...emloft.net
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] Fix corrupt TCP packets when options space overflows with MD5SIG enabled

When MD5 signatures are turned on we can end up with syntactically invalid
packets with a header length < 20 bytes. This is because tcp_header_size
overflows with 12 bytes of timestamp, 20 bytes of signature and > 8 bytes of
SACK option.

Since we can't fit any SACK blocks in the final 8 bytes of options space, and
the MD5 signature is more important, we disable including SACK, or even
advertising it, when MD5 is enabled.

Signed-off-by: Adam Langley <agl@...erialviolet.org>

---
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index e399bde..e4fb5d9 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -474,8 +474,9 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 	struct tcp_sock *tp;
 	struct tcp_skb_cb *tcb;
 	int tcp_header_size;
+
+	struct tcp_md5sig_key *md5 = NULL;
 #ifdef CONFIG_TCP_MD5SIG
-	struct tcp_md5sig_key *md5;
 	__u8 *md5_hash_location;
 #endif
 	struct tcphdr *th;
@@ -504,6 +505,16 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 	tcb = TCP_SKB_CB(skb);
 	tcp_header_size = tp->tcp_header_len;
 
+#ifdef CONFIG_TCP_MD5SIG
+	/*
+	 * Are we doing MD5 on this segment? If so - make
+	 * room for it.
+	 */
+	md5 = tp->af_specific->md5_lookup(sk, sk);
+	if (md5)
+		tcp_header_size += TCPOLEN_MD5SIG_ALIGNED;
+#endif
+
 #define SYSCTL_FLAG_TSTAMPS	0x1
 #define SYSCTL_FLAG_WSCALE	0x2
 #define SYSCTL_FLAG_SACK	0x4
@@ -519,12 +530,15 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 			tcp_header_size += TCPOLEN_WSCALE_ALIGNED;
 			sysctl_flags |= SYSCTL_FLAG_WSCALE;
 		}
-		if (sysctl_tcp_sack) {
+		/* We don't include SACK options if we are going to
+		 * include an MD5 signature because they can't fit
+		 * in the options space */
+		if (sysctl_tcp_sack && !md5) {
 			sysctl_flags |= SYSCTL_FLAG_SACK;
 			if (!(sysctl_flags & SYSCTL_FLAG_TSTAMPS))
 				tcp_header_size += TCPOLEN_SACKPERM_ALIGNED;
 		}
-	} else if (unlikely(tp->rx_opt.eff_sacks)) {
+	} else if (unlikely(tp->rx_opt.eff_sacks && !md5)) {
 		/* A SACK is 2 pad bytes, a 2 byte header, plus
 		 * 2 32-bit sequence numbers for each SACK block.
 		 */
@@ -536,16 +550,6 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 	if (tcp_packets_in_flight(tp) == 0)
 		tcp_ca_event(sk, CA_EVENT_TX_START);
 
-#ifdef CONFIG_TCP_MD5SIG
-	/*
-	 * Are we doing MD5 on this segment? If so - make
-	 * room for it.
-	 */
-	md5 = tp->af_specific->md5_lookup(sk, sk);
-	if (md5)
-		tcp_header_size += TCPOLEN_MD5SIG_ALIGNED;
-#endif
-
 	skb_push(skb, tcp_header_size);
 	skb_reset_transport_header(skb);
 	skb_set_owner_w(skb, sk);
--
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