[PATCH] tcp: md5: fix md5 RST when both sides have listener

Shawn Lu
2012-02-01T00:50:35Z

TCP RST mechanism is broken in TCP md5(RFC2385). When
connection is gone, md5 key is lost, sending RST
without md5 hash is deem to ignored by peer. This can
be a problem since RST help protocal like bgp to fast
recove from peer crash.

In most case, users of tcp md5, such as bgp and ldp,
have listeners on both sides. md5 keys for peers are
saved in listening socket. When passive side connection
is gone, we can still get md5 key from listening socket.
When active side of connection is gone, we can try to
find listening socket through source port, and then md5
key.
we are not loosing sercuriy here:
packet is valified checked with md5 hash. No RST is generated
if md5 hash doesn't match or no md5 key can be found.

Signed-off-by: Shawn Lu <shawn.lu@ericsson.com>
---
net/ipv4/tcp_ipv4.c | 45 ++++++++++++++++++++++++++++++++++++++++++---
net/ipv6/tcp_ipv6.c | 43 +++++++++++++++++++++++++++++++++++++++++--
2 files changed, 83 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index da5d322..1a761c8 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -593,6 +593,10 @@ static void tcp_v4_send_reset(struct sock *sk, struct sk_buff *skb)
struct ip_reply_arg arg;
#ifdef CONFIG_TCP_MD5SIG
struct tcp_md5sig_key *key;
+ const __u8 *hash_location = NULL;
+ unsigned char newhash[16];
+ int genhash;
+ struct sock *sk1 = NULL;
#endif
struct net *net;

@@ -623,9 +627,36 @@ static void tcp_v4_send_reset(struct sock *sk, struct sk_buff *skb)
arg.iov[0].iov_len = sizeof(rep.th);

#ifdef CONFIG_TCP_MD5SIG
- key = sk ? tcp_md5_do_lookup(sk,
- (union tcp_md5_addr *)&ip_hdr(skb)->saddr,
- AF_INET) : NULL;
+ hash_location = tcp_parse_md5sig_option(th);
+ if (!sk && hash_location) {
+ /*
+ * active side is lost. Try to find listening socket through
+ * source port, and then find md5 key through listening socket.
+ * we are not loose security here:
+ * Incoming packet is checked with md5 hash with finding key,
+ * no RST generated if md5 hash doesn't match.
+ */
+ sk1 = __inet_lookup_listener(dev_net(skb_dst(skb)->dev),
+ &tcp_hashinfo, ip_hdr(skb)->daddr,
+ ntohs(th->source), inet_iif(skb));
+ /* don't send rst if it can't find key */
+ if (!sk1)
+ return;
+ rcu_read_lock();
+ key = tcp_md5_do_lookup(sk1, (union tcp_md5_addr *)
+ &ip_hdr(skb)->saddr, AF_INET);
+ if (!key)
+ goto release_sk1;
+
+ genhash = tcp_v4_md5_hash_skb(newhash, key, NULL, NULL, skb);
+ if (genhash || memcmp(hash_location, newhash, 16) != 0)
+ goto release_sk1;
+ } else {
+ key = sk ? tcp_md5_do_lookup(sk, (union tcp_md5_addr *)
+ &ip_hdr(skb)->saddr,
+ AF_INET) : NULL;
+ }
+
if (key) {
rep.opt[0] = htonl((TCPOPT_NOP << 24) |
(TCPOPT_NOP << 16) |
@@ -653,6 +684,14 @@ static void tcp_v4_send_reset(struct sock *sk, struct sk_buff *skb)

TCP_INC_STATS_BH(net, TCP_MIB_OUTSEGS);
TCP_INC_STATS_BH(net, TCP_MIB_OUTRSTS);
+
+#ifdef CONFIG_TCP_MD5SIG
+release_sk1:
+ if (sk1) {
+ rcu_read_unlock();
+ sock_put(sk1);
+ }
+#endif
}

/* The code following below sending ACKs in SYN-RECV and TIME-WAIT states
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index bec41f9..3b139e2 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -925,6 +925,13 @@ static void tcp_v6_send_reset(struct sock *sk, struct sk_buff *skb)
const struct tcphdr *th = tcp_hdr(skb);
u32 seq = 0, ack_seq = 0;
struct tcp_md5sig_key *key = NULL;
+#ifdef CONFIG_TCP_MD5SIG
+ const __u8 *hash_location = NULL;
+ struct ipv6hdr *ipv6h = ipv6_hdr(skb);
+ unsigned char newhash[16];
+ int genhash;
+ struct sock *sk1 = NULL;
+#endif

if (th->rst)
return;
@@ -933,8 +940,32 @@ static void tcp_v6_send_reset(struct sock *sk, struct sk_buff *skb)
return;

#ifdef CONFIG_TCP_MD5SIG
- if (sk)
- key = tcp_v6_md5_do_lookup(sk, &ipv6_hdr(skb)->saddr);
+ hash_location = tcp_parse_md5sig_option(th);
+ if (!sk && hash_location) {
+ /*
+ * active side is lost. Try to find listening socket through
+ * source port, and then find md5 key through listening socket.
+ * we are not loose security here:
+ * Incoming packet is checked with md5 hash with finding key,
+ * no RST generated if md5 hash doesn't match.
+ */
+ sk1 = inet6_lookup_listener(dev_net(skb_dst(skb)->dev),
+ &tcp_hashinfo, &ipv6h->daddr,
+ ntohs(th->source), inet6_iif(skb));
+ if (!sk1)
+ return;
+
+ rcu_read_lock();
+ key = tcp_v6_md5_do_lookup(sk1, &ipv6h->saddr);
+ if (!key)
+ goto release_sk1;
+
+ genhash = tcp_v6_md5_hash_skb(newhash, key, NULL, NULL, skb);
+ if (genhash || memcmp(hash_location, newhash, 16) != 0)
+ goto release_sk1;
+ } else {
+ key = sk ? tcp_v6_md5_do_lookup(sk, &ipv6h->saddr) : NULL;
+ }
#endif

if (th->ack)
@@ -944,6 +975,14 @@ static void tcp_v6_send_reset(struct sock *sk, struct sk_buff *skb)
(th->doff << 2);

tcp_v6_send_response(skb, seq, ack_seq, 0, 0, key, 1, 0);
+
+#ifdef CONFIG_TCP_MD5SIG
+release_sk1:
+ if (sk1) {
+ rcu_read_unlock();
+ sock_put(sk1);
+ }
+#endif
}

static void tcp_v6_send_ack(struct sk_buff *skb, u32 seq, u32 ack, u32 win, u32 ts,
--
1.7.0.4