[PATCH] tcp: md5: Fix overlap between vrf and non-vrf keys

[Leonard Crestez <cdleonard@gmail.com>]

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
different 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.

This was found while working on TCP-AO tests, the exact failing tests
are among test_vrf_overlap*_md5 from this file:
https://github.com/cdleonard/tcp-authopt-test/blob/main/tcp_authopt_test/test_vrf_bind.py

There is also a test for overlapping between vrfs
(test_vrf_overlap12_md5) and that works correctly without this change.

Reproducing this with nettest and fcnal-test.sh would require support
for multiple keys inside the nettest server.

Signed-off-by: Leonard Crestez <cdleonard@gmail.com>

---
 net/ipv4/tcp_ipv4.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

The fact that keys with l3index==0 affect VRF connections might not be
desirable at all. It might make more sense to have an option to completely
ignore keys outside the vrf for connections inside the VRF.

At least this patch makes it behave in a well defined manner that doesn't
depend on the order of key addition.

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 29a57bd159f0..a9a6a6d598c6 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1035,10 +1035,24 @@ static void tcp_v4_reqsk_destructor(struct request_sock *req)
  */
 
 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,
 					   int family)
 {
@@ -1072,12 +1086,11 @@ struct tcp_md5sig_key *__tcp_md5_do_lookup(const struct sock *sk, int l3index,
 #endif
 		} else {
 			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;
 }
 EXPORT_SYMBOL(__tcp_md5_do_lookup);
@@ -1103,11 +1116,11 @@ static struct tcp_md5sig_key *tcp_md5_do_lookup_exact(const struct sock *sk,
 #endif
 	hlist_for_each_entry_rcu(key, &md5sig->head, node,
 				 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)
 			return key;
 	}

base-commit: 9cbfc51af026f5b721a1b36cf622ada591b3c5de
-- 
2.25.1