net/tcp: Verify inbound TCP-AO signed segments

Now there is a common function to verify signature on TCP segments:
tcp_inbound_hash(). It has checks for all possible cross-interactions
with MD5 signs as well as with unsigned segments.

The rules from RFC5925 are:
(1) Any TCP segment can have at max only one signature.
(2) TCP connections can't switch between using TCP-MD5 and TCP-AO.
(3) TCP-AO connections can't stop using AO, as well as unsigned
    connections can't suddenly start using AO.

Co-developed-by: Francesco Ruggeri <fruggeri@arista.com>
Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Co-developed-by: Salam Noureddine <noureddine@arista.com>
Signed-off-by: Salam Noureddine <noureddine@arista.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 142 insertions, 0 deletions

diff --git a/net/ipv4/tcp_ao.c b/net/ipv4/tcp_ao.c
index de3710758d55..6c5815713b73 100644
--- a/net/ipv4/tcp_ao.c
+++ b/net/ipv4/tcp_ao.c
@@ -761,6 +761,148 @@ void tcp_ao_syncookie(struct sock *sk, const struct sk_buff *skb,
 	treq->maclen = tcp_ao_maclen(key);
 }
 
+static enum skb_drop_reason
+tcp_ao_verify_hash(const struct sock *sk, const struct sk_buff *skb,
+		   unsigned short int family, struct tcp_ao_info *info,
+		   const struct tcp_ao_hdr *aoh, struct tcp_ao_key *key,
+		   u8 *traffic_key, u8 *phash, u32 sne)
+{
+	u8 maclen = aoh->length - sizeof(struct tcp_ao_hdr);
+	const struct tcphdr *th = tcp_hdr(skb);
+	void *hash_buf = NULL;
+
+	if (maclen != tcp_ao_maclen(key))
+		return SKB_DROP_REASON_TCP_AOFAILURE;
+
+	hash_buf = kmalloc(tcp_ao_digest_size(key), GFP_ATOMIC);
+	if (!hash_buf)
+		return SKB_DROP_REASON_NOT_SPECIFIED;
+
+	/* XXX: make it per-AF callback? */
+	tcp_ao_hash_skb(family, hash_buf, key, sk, skb, traffic_key,
+			(phash - (u8 *)th), sne);
+	if (memcmp(phash, hash_buf, maclen)) {
+		kfree(hash_buf);
+		return SKB_DROP_REASON_TCP_AOFAILURE;
+	}
+	kfree(hash_buf);
+	return SKB_NOT_DROPPED_YET;
+}
+
+enum skb_drop_reason
+tcp_inbound_ao_hash(struct sock *sk, const struct sk_buff *skb,
+		    unsigned short int family, const struct request_sock *req,
+		    const struct tcp_ao_hdr *aoh)
+{
+	const struct tcphdr *th = tcp_hdr(skb);
+	u8 *phash = (u8 *)(aoh + 1); /* hash goes just after the header */
+	struct tcp_ao_info *info;
+	enum skb_drop_reason ret;
+	struct tcp_ao_key *key;
+	__be32 sisn, disn;
+	u8 *traffic_key;
+	u32 sne = 0;
+
+	info = rcu_dereference(tcp_sk(sk)->ao_info);
+	if (!info)
+		return SKB_DROP_REASON_TCP_AOUNEXPECTED;
+
+	if (unlikely(th->syn)) {
+		sisn = th->seq;
+		disn = 0;
+	}
+
+	/* Fast-path */
+	if (likely((1 << sk->sk_state) & TCP_AO_ESTABLISHED)) {
+		enum skb_drop_reason err;
+		struct tcp_ao_key *current_key;
+
+		/* Check if this socket's rnext_key matches the keyid in the
+		 * packet. If not we lookup the key based on the keyid
+		 * matching the rcvid in the mkt.
+		 */
+		key = READ_ONCE(info->rnext_key);
+		if (key->rcvid != aoh->keyid) {
+			key = tcp_ao_established_key(info, -1, aoh->keyid);
+			if (!key)
+				goto key_not_found;
+		}
+
+		/* Delayed retransmitted SYN */
+		if (unlikely(th->syn && !th->ack))
+			goto verify_hash;
+
+		sne = 0;
+		/* Established socket, traffic key are cached */
+		traffic_key = rcv_other_key(key);
+		err = tcp_ao_verify_hash(sk, skb, family, info, aoh, key,
+					 traffic_key, phash, sne);
+		if (err)
+			return err;
+		current_key = READ_ONCE(info->current_key);
+		/* Key rotation: the peer asks us to use new key (RNext) */
+		if (unlikely(aoh->rnext_keyid != current_key->sndid)) {
+			/* If the key is not found we do nothing. */
+			key = tcp_ao_established_key(info, aoh->rnext_keyid, -1);
+			if (key)
+				/* pairs with tcp_ao_del_cmd */
+				WRITE_ONCE(info->current_key, key);
+		}
+		return SKB_NOT_DROPPED_YET;
+	}
+
+	/* Lookup key based on peer address and keyid.
+	 * current_key and rnext_key must not be used on tcp listen
+	 * sockets as otherwise:
+	 * - request sockets would race on those key pointers
+	 * - tcp_ao_del_cmd() allows async key removal
+	 */
+	key = tcp_ao_inbound_lookup(family, sk, skb, -1, aoh->keyid);
+	if (!key)
+		goto key_not_found;
+
+	if (th->syn && !th->ack)
+		goto verify_hash;
+
+	if ((1 << sk->sk_state) & (TCPF_LISTEN | TCPF_NEW_SYN_RECV)) {
+		/* Make the initial syn the likely case here */
+		if (unlikely(req)) {
+			sne = 0;
+			sisn = htonl(tcp_rsk(req)->rcv_isn);
+			disn = htonl(tcp_rsk(req)->snt_isn);
+		} else if (unlikely(th->ack && !th->syn)) {
+			/* Possible syncookie packet */
+			sisn = htonl(ntohl(th->seq) - 1);
+			disn = htonl(ntohl(th->ack_seq) - 1);
+			sne = 0;
+		} else if (unlikely(!th->syn)) {
+			/* no way to figure out initial sisn/disn - drop */
+			return SKB_DROP_REASON_TCP_FLAGS;
+		}
+	} else if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
+		disn = info->lisn;
+		if (th->syn || th->rst)
+			sisn = th->seq;
+		else
+			sisn = info->risn;
+	} else {
+		WARN_ONCE(1, "TCP-AO: Unexpected sk_state %d", sk->sk_state);
+		return SKB_DROP_REASON_TCP_AOFAILURE;
+	}
+verify_hash:
+	traffic_key = kmalloc(tcp_ao_digest_size(key), GFP_ATOMIC);
+	if (!traffic_key)
+		return SKB_DROP_REASON_NOT_SPECIFIED;
+	tcp_ao_calc_key_skb(key, traffic_key, skb, sisn, disn, family);
+	ret = tcp_ao_verify_hash(sk, skb, family, info, aoh, key,
+				 traffic_key, phash, sne);
+	kfree(traffic_key);
+	return ret;
+
+key_not_found:
+	return SKB_DROP_REASON_TCP_AOKEYNOTFOUND;
+}
+
 static int tcp_ao_cache_traffic_keys(const struct sock *sk,
 				     struct tcp_ao_info *ao,
 				     struct tcp_ao_key *ao_key)