Subject: [net-next-2.6 PATCH RFC] TCPCT part 1d: generate Responder Cookie

From: William Allen Simpson <[email-masked]>
Date: Fri, 30 Oct 2009 07:00:51 -0400

First of all, this is my first attempt at locks, so I'd like early review.

Secondly, scripts/checkpatch.pl tells me:

ERROR: do not initialise statics to 0 or NULL
#95: FILE: net/ipv4/tcp.c:2977:
+static struct tcp_cookie_secret *tcp_secret_generating = NULL;

They need to be NULL, and I'm not planning on exporting them, so what's
the preferred mechanism?

(I've grep'd many other instances of statics = 0 or NULL, so I'm not alone.)
diff --git a/include/linux/cryptohash.h b/include/linux/cryptohash.h
index c118b2a..ec78a4b 100644
--- a/include/linux/cryptohash.h
+++ b/include/linux/cryptohash.h
@@ -2,6 +2,7 @@
 #define __CRYPTOHASH_H
 
 #define SHA_DIGEST_WORDS 5
+#define SHA_MESSAGE_BYTES (512 /*bits*/ / 8)
 #define SHA_WORKSPACE_WORDS 80
 
 void sha_init(__u32 *buf);
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 51b7426..f669c43 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1526,12 +1526,18 @@ static inline int tcp_s_data_size(const struct tcp_sock *tp)
 		: 0;
 }
 
+/* Using SHA1 for now, define some constants.
+ */
+#define COOKIE_DIGEST_WORDS (SHA_DIGEST_WORDS)
+#define COOKIE_MESSAGE_WORDS (SHA_MESSAGE_BYTES / 4)
+#define COOKIE_WORKSPACE_WORDS (COOKIE_DIGEST_WORDS + COOKIE_MESSAGE_WORDS)
+
 /* As tcp_request_sock has already been extended in other places, the
  * only remaining method is to pass stack values along as function
  * parameters.  These parameters are not needed after sending SYNACK.
  */
 struct tcp_extend_values {
-	u8				cookie_bakery[TCP_COOKIE_MAX];
+	u32				cookie_bakery[COOKIE_WORKSPACE_WORDS];
 	u8				cookie_plus;
 	u8				cookie_in_always:1,
 					cookie_out_never:1;
@@ -1542,6 +1548,8 @@ static inline struct tcp_extend_values *tcp_xv(const struct request_values *rvp)
 	return (struct tcp_extend_values *)rvp;
 }
 
+extern int tcp_cookie_generator(struct tcp_extend_values *xvp);
+
 extern void tcp_v4_init(void);
 extern void tcp_init(void);
 
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 12409df..32f8711 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2933,6 +2933,126 @@ EXPORT_SYMBOL(tcp_md5_hash_key);
 
 #endif
 
+/**
+ * Each Responder maintains up to two secret values concurrently for
+ * efficient secret rollover.  Each secret value has 4 states:
+ *
+ * Generating.
+ *    Generates new Responder-Cookies, but not yet used for primary
+ *    verification.  This is a short-term state, typically lasting only
+ *    one round trip time (RTT).
+ *
+ * Primary.
+ *    Used both for generation and primary verification.
+ *
+ * Retiring.
+ *    Used for verification, until the first failure that can be
+ *    verified by the newer Generating secret.  At that time, this
+ *    cookie's state is changed to Secondary, and the Generating
+ *    cookie's state is changed to Primary.  This is a short-term state,
+ *    typically lasting only one round trip time (RTT).
+ *
+ * Secondary.
+ *    Used for secondary verification, after primary verification
+ *    failures.  This state lasts no more than twice the Maximum Segment
+ *    Lifetime (2MSL).  Then, the secret is discarded.
+ */
+struct tcp_cookie_secret {
+	/* The secret is divided into two parts.  The digest part is the
+	 * equivalent of previously hashing a secret and saving the state,
+	 * and serves as an initialization vector (IV).  The message part
+	 * serves as the trailing secret.
+	 */
+	u32				secrets[COOKIE_WORKSPACE_WORDS];
+	unsigned long			expires;
+};
+
+#define TCP_SECRET_1MSL (HZ * TCP_PAWS_MSL)
+#define TCP_SECRET_2MSL (HZ * TCP_PAWS_MSL * 2)
+#define TCP_SECRET_LIFE (HZ * 600)
+
+static struct tcp_cookie_secret tcp_secret_one;
+static struct tcp_cookie_secret tcp_secret_two;
+
+static struct tcp_cookie_secret *tcp_secret_generating = NULL;
+static struct tcp_cookie_secret *tcp_secret_primary = NULL;
+static struct tcp_cookie_secret *tcp_secret_retiring = NULL;
+static struct tcp_cookie_secret *tcp_secret_secondary = NULL;
+
+static DEFINE_RWLOCK(tcp_secret_locker);
+
+/* Fill cookie_bakery with current generator.
+ * Returns: 0 o success.
+ */
+int tcp_cookie_generator(struct tcp_extend_values *xvp)
+{
+	u32 secrets[COOKIE_WORKSPACE_WORDS];
+
+	if (unlikely(NULL == tcp_secret_primary)) {
+		get_random_bytes(secrets, sizeof(secrets));
+
+		/* The first time, paranoia assumes that the randomization
+		 * function isn't as strong.  But this secret initialization
+		 * is delayed until the last possible moment (packet arrival).
+		 * Although that time is observable, it is unpredictably
+		 * variable.  Mash in the fastest clock bits available, and
+		 * expire the secret extra quickly.
+		 */
+		secrets[COOKIE_DIGEST_WORDS] ^= (u32)jiffies;
+
+		write_lock(&tcp_secret_locker);
+		if (NULL == tcp_secret_primary) {
+			/* still needs initialization */
+			memcpy((u8 *)&tcp_secret_one.secrets[0],
+			       (u8 *)&secrets[0],
+			       sizeof(secrets));
+			tcp_secret_one.expires = jiffies + TCP_SECRET_1MSL;
+			tcp_secret_primary = &tcp_secret_one;
+			/* unused at this time */
+			memcpy((u8 *)&tcp_secret_two.secrets[0],
+			       (u8 *)&secrets[0],
+			       sizeof(secrets));
+			tcp_secret_two.expires = jiffies; /* past due */
+			tcp_secret_secondary = &tcp_secret_two;
+		}
+		write_unlock(&tcp_secret_locker);
+	}
+
+	if (unlikely(time_after(jiffies, tcp_secret_primary->expires))) {
+		get_random_bytes(secrets, sizeof(secrets));
+
+		write_lock(&tcp_secret_locker);
+		if (time_after(jiffies, tcp_secret_primary->expires)) {
+			/* still needs refreshing */
+			tcp_secret_primary->expires = jiffies
+						    + TCP_SECRET_2MSL;
+			tcp_secret_retiring = tcp_secret_primary;
+			/* new generator at secondary position */
+			memcpy((u8 *)&tcp_secret_secondary->secrets[0],
+			       (u8 *)&secrets[0],
+			       sizeof(secrets));
+			tcp_secret_secondary->expires = jiffies
+						      + TCP_SECRET_LIFE;
+			tcp_secret_generating = tcp_secret_secondary;
+		}
+		write_unlock(&tcp_secret_locker);
+	}
+
+	read_lock(&tcp_secret_locker);
+	if (unlikely(NULL != tcp_secret_generating)) {
+		memcpy((u8 *)&xvp->cookie_bakery[0],
+		       (u8 *)&tcp_secret_generating->secrets[0],
+		       sizeof(tcp_secret_generating->secrets));
+	} else {
+		memcpy((u8 *)&xvp->cookie_bakery[0],
+		       (u8 *)&tcp_secret_primary->secrets[0],
+		       sizeof(tcp_secret_primary->secrets));
+	}
+	read_unlock(&tcp_secret_locker);
+	return 0;
+}
+EXPORT_SYMBOL(tcp_cookie_generator);
+
 void tcp_done(struct sock *sk)
 {
 	if (sk->sk_state == TCP_SYN_SENT || sk->sk_state == TCP_SYN_RECV)