[PATCH net-next 3/6] tcp: add support to TCP_FASTOPEN_KEY for optional backup key

[Jason Baron <jbaron@akamai.com>]

Add support for get/set of an optional backup key via TCP_FASTOPEN_KEY, in
addition to the current 'primary' key. The primary key is used to encrypt
and decrypt TFO cookies, while the backup is only used to decrpt TFO
cookies. The backup key is used to maximize successful TFO connections when
TFO keys are rotated.

Currently, TCP_FASTOPEN_KEY allows a single 16-byte key to be set. The
first 16 bytes are used as the primary key and the second 16 bytes are used
for the backup key. Similarly, for getsockopt(), we can receive a 32-byte
value as output if requested. If a 16-byte value is used to set the primary
key via TCP_FASTOPEN_KEY, then any previously set backup key will be
removed.

Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Jason Baron <jbaron@akamai.com>
---
 net/ipv4/tcp.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index bca51a3..27ce13e 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2790,16 +2790,24 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 		return err;
 	}
 	case TCP_FASTOPEN_KEY: {
-		__u8 key[TCP_FASTOPEN_KEY_LENGTH];
+		__u8 key[TCP_FASTOPEN_KEY_BUF_LENGTH];
+		__u8 *backup_key = NULL;
 
-		if (optlen != sizeof(key))
+		/* Allow a backup key as well to facilitate key rotation
+		 * First key is the active one.
+		 */
+		if (optlen != TCP_FASTOPEN_KEY_LENGTH &&
+		    optlen != TCP_FASTOPEN_KEY_BUF_LENGTH)
 			return -EINVAL;
 
 		if (copy_from_user(key, optval, optlen))
 			return -EFAULT;
 
-		return tcp_fastopen_reset_cipher(net, sk, key, NULL,
-						 sizeof(key));
+		if (optlen == TCP_FASTOPEN_KEY_BUF_LENGTH)
+			backup_key = key + TCP_FASTOPEN_KEY_LENGTH;
+
+		return tcp_fastopen_reset_cipher(net, sk, key, backup_key,
+						 TCP_FASTOPEN_KEY_LENGTH);
 	}
 	default:
 		/* fallthru */
@@ -3453,21 +3461,23 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 		return 0;
 
 	case TCP_FASTOPEN_KEY: {
-		__u8 key[TCP_FASTOPEN_KEY_LENGTH];
+		__u8 key[TCP_FASTOPEN_KEY_BUF_LENGTH];
 		struct tcp_fastopen_context *ctx;
+		unsigned int key_len = 0;
 
 		if (get_user(len, optlen))
 			return -EFAULT;
 
 		rcu_read_lock();
 		ctx = rcu_dereference(icsk->icsk_accept_queue.fastopenq.ctx);
-		if (ctx)
-			memcpy(key, ctx->key, sizeof(key));
-		else
-			len = 0;
+		if (ctx) {
+			key_len = tcp_fastopen_context_len(ctx) *
+					TCP_FASTOPEN_KEY_LENGTH;
+			memcpy(&key[0], &ctx->key[0], key_len);
+		}
 		rcu_read_unlock();
 
-		len = min_t(unsigned int, len, sizeof(key));
+		len = min_t(unsigned int, len, key_len);
 		if (put_user(len, optlen))
 			return -EFAULT;
 		if (copy_to_user(optval, key, len))
-- 
2.7.4