tcp: md5: Fix overlap between vrf and non-vrf keys

With net.ipv4.tcp_l3mdev_accept=1 it is possible for a listen socket to
accept connection from the same client address in different VRFs. It is
also possible to set different MD5 keys for these clients which differ
only in the tcpm_l3index field.

This appears to work when distinguishing between different VRFs but not
between non-VRF and VRF connections. In particular:

* tcp_md5_do_lookup_exact will match a non-vrf key against a vrf key.
This means that adding a key with l3index != 0 after a key with l3index
== 0 will cause the earlier key to be deleted. Both keys can be present
if the non-vrf key is added later.
* _tcp_md5_do_lookup can match a non-vrf key before a vrf key. This
casues failures if the passwords differ.

Fix this by making tcp_md5_do_lookup_exact perform an actual exact
comparison on l3index and by making __tcp_md5_do_lookup perfer
vrf-bound keys above other considerations like prefixlen.

Fixes: dea53bb80e07 ("tcp: Add l3index to tcp_md5sig_key and md5 functions")
Signed-off-by: Leonard Crestez <[e-mail]>
Reviewed-by: David Ahern <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/tcp_ipv4.c

···
 DEFINE_STATIC_KEY_FALSE(tcp_md5_needed);
 EXPORT_SYMBOL(tcp_md5_needed);
 
+static bool better_md5_match(struct tcp_md5sig_key *old, struct tcp_md5sig_key *new)
+{
+	if (!old)
+		return true;
+
+	/* l3index always overrides non-l3index */
+	if (old->l3index && new->l3index == 0)
+		return false;
+	if (old->l3index == 0 && new->l3index)
+		return true;
+
+	return old->prefixlen < new->prefixlen;
+}
+
 /* Find the Key structure for an address.  */
 struct tcp_md5sig_key *__tcp_md5_do_lookup(const struct sock *sk, int l3index,
 					   const union tcp_md5_addr *addr,
···
 			match = false;
 		}
 
-		if (match && (!best_match ||
-			      key->prefixlen > best_match->prefixlen))
+		if (match && better_md5_match(best_match, key))
 			best_match = key;
 	}
 	return best_match;
···
 				 lockdep_sock_is_held(sk)) {
 		if (key->family != family)
 			continue;
-		if (key->l3index && key->l3index != l3index)
+		if (key->l3index != l3index)
 			continue;
 		if (!memcmp(&key->addr, addr, size) &&
 		    key->prefixlen == prefixlen)