[PATCH] tcp: fix authentication bypass via unsupported TCP auth options

["Jérémy Jean" <Jeremy.Jean@oss.cyber.gouv.fr>]

tcp_inbound_hash() parses both TCP-MD5 and TCP-AO options when at
least one of the two features is enabled. With TCP_MD5SIG=y and
TCP_AO=n, a packet with an AO option reaches the disabled
tcp_inbound_ao_hash() stub. That stub returns success, so sockets
configured to require MD5 authentication end up accepting
unauthenticated data.

The inverse configuration with TCP_MD5SIG=n and TCP_AO=y has the same
problem for an MD5 option on AO sockets.

Make the disabled stubs reject options for the unsupported
authentication scheme.

Fixes: 0a3a809089eb ("net/tcp: Verify inbound TCP-AO signed segments")
Cc: stable@vger.kernel.org
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
---
 include/net/tcp_ao.h | 2 +-
 net/ipv4/tcp.c       | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/net/tcp_ao.h b/include/net/tcp_ao.h
index 9a2333e62e994..bb249d6df8723 100644
--- a/include/net/tcp_ao.h
+++ b/include/net/tcp_ao.h
@@ -303,7 +303,7 @@ static inline enum skb_drop_reason tcp_inbound_ao_hash(struct sock *sk,
 		const struct request_sock *req, int l3index,
 		const struct tcp_ao_hdr *aoh)
 {
-	return SKB_NOT_DROPPED_YET;
+	return SKB_DROP_REASON_TCP_AOUNEXPECTED;
 }
 
 static inline struct tcp_ao_key *tcp_ao_do_lookup(const struct sock *sk,
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 87ef6d5cbfeba..7d800b6283bd2 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -4962,7 +4962,7 @@ tcp_inbound_md5_hash(const struct sock *sk, const struct sk_buff *skb,
 		     const void *saddr, const void *daddr,
 		     int family, int l3index, const __u8 *hash_location)
 {
-	return SKB_NOT_DROPPED_YET;
+	return SKB_DROP_REASON_TCP_MD5UNEXPECTED;
 }
 
 #endif
-- 
2.47.3