upstream: more strictly enforce KEX state-machine by banning packet

types once they are received. Fixes memleak caused by duplicate
SSH2_MSG_KEX_DH_GEX_REQUEST (spotted by portable OpenSSH kex_fuzz via
oss-fuzz #30078).

ok markus@

OpenBSD-Commit-ID: 87331c715c095b587d5c88724694cdeb701c9def

5 files changed, 25 insertions, 9 deletions

diff --git a/kex.c b/kex.c
index b73f14d3..30425ab8 100644
--- a/kex.c
+++ b/kex.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.c,v 1.166 2021/01/27 23:49:46 djm Exp $ */
+/* $OpenBSD: kex.c,v 1.167 2021/01/31 22:55:29 djm Exp $ */
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
  *
@@ -405,7 +405,7 @@ kex_prop_free(char **proposal)
 }
 
 /* ARGSUSED */
-static int
+int
 kex_protocol_error(int type, u_int32_t seq, struct ssh *ssh)
 {
 	int r;
diff --git a/kex.h b/kex.h
index 18bfa923..9605ed52 100644
--- a/kex.h
+++ b/kex.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.h,v 1.113 2021/01/27 10:05:28 djm Exp $ */
+/* $OpenBSD: kex.h,v 1.114 2021/01/31 22:55:29 djm Exp $ */
 
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
@@ -193,6 +193,7 @@ int	 kex_verify_host_key(struct ssh *, struct sshkey *);
 int	 kex_send_kexinit(struct ssh *);
 int	 kex_input_kexinit(int, u_int32_t, struct ssh *);
 int	 kex_input_ext_info(int, u_int32_t, struct ssh *);
+int	 kex_protocol_error(int, u_int32_t, struct ssh *);
 int	 kex_derive_keys(struct ssh *, u_char *, u_int, const struct sshbuf *);
 int	 kex_send_newkeys(struct ssh *);
 int	 kex_start_rekex(struct ssh *);
diff --git a/kexgen.c b/kexgen.c
index 39a848f2..35b83ccf 100644
--- a/kexgen.c
+++ b/kexgen.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexgen.c,v 1.5 2020/12/29 00:59:15 djm Exp $ */
+/* $OpenBSD: kexgen.c,v 1.6 2021/01/31 22:55:29 djm Exp $ */
 /*
  * Copyright (c) 2019 Markus Friedl.  All rights reserved.
  *
@@ -148,6 +148,9 @@ input_kex_gen_reply(int type, u_int32_t seq, struct ssh *ssh)
 	size_t slen, hashlen;
 	int r;
 
+	debug("SSH2_MSG_KEX_ECDH_REPLY received");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEX_ECDH_REPLY, &kex_protocol_error);
+
 	/* hostkey */
 	if ((r = sshpkt_getb_froms(ssh, &server_host_key_blob)) != 0)
 		goto out;
@@ -254,6 +257,9 @@ input_kex_gen_init(int type, u_int32_t seq, struct ssh *ssh)
 	size_t slen, hashlen;
 	int r;
 
+	debug("SSH2_MSG_KEX_ECDH_INIT received");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEX_ECDH_INIT, &kex_protocol_error);
+
 	if ((r = kex_load_hostkey(ssh, &server_host_private,
 	    &server_host_public)) != 0)
 		goto out;
diff --git a/kexgexc.c b/kexgexc.c
index c3702c92..4a2e741d 100644
--- a/kexgexc.c
+++ b/kexgexc.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexgexc.c,v 1.36 2021/01/27 09:26:54 djm Exp $ */
+/* $OpenBSD: kexgexc.c,v 1.37 2021/01/31 22:55:29 djm Exp $ */
 /*
  * Copyright (c) 2000 Niels Provos.  All rights reserved.
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
@@ -83,6 +83,7 @@ kexgex_client(struct ssh *ssh)
 	fprintf(stderr, "\nmin = %d, nbits = %d, max = %d\n",
 	    kex->min, kex->nbits, kex->max);
 #endif
+	debug("expecting SSH2_MSG_KEX_DH_GEX_GROUP");
 	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_GROUP,
 	    &input_kex_dh_gex_group);
 	r = 0;
@@ -98,7 +99,8 @@ input_kex_dh_gex_group(int type, u_int32_t seq, struct ssh *ssh)
 	const BIGNUM *pub_key;
 	int r, bits;
 
-	debug("got SSH2_MSG_KEX_DH_GEX_GROUP");
+	debug("SSH2_MSG_KEX_DH_GEX_GROUP received");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_GROUP, &kex_protocol_error);
 
 	if ((r = sshpkt_get_bignum2(ssh, &p)) != 0 ||
 	    (r = sshpkt_get_bignum2(ssh, &g)) != 0 ||
@@ -130,7 +132,7 @@ input_kex_dh_gex_group(int type, u_int32_t seq, struct ssh *ssh)
 	BN_print_fp(stderr, pub_key);
 	fprintf(stderr, "\n");
 #endif
-	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_GROUP, NULL);
+	debug("expecting SSH2_MSG_KEX_DH_GEX_REPLY");
 	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_REPLY, &input_kex_dh_gex_reply);
 	r = 0;
 out:
@@ -153,7 +155,9 @@ input_kex_dh_gex_reply(int type, u_int32_t seq, struct ssh *ssh)
 	size_t slen, hashlen;
 	int r;
 
-	debug("got SSH2_MSG_KEX_DH_GEX_REPLY");
+	debug("SSH2_MSG_KEX_DH_GEX_REPLY received");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_REPLY, &kex_protocol_error);
+
 	/* key, cert */
 	if ((r = sshpkt_getb_froms(ssh, &server_host_key_blob)) != 0)
 		goto out;
diff --git a/kexgexs.c b/kexgexs.c
index 8ee3aacc..f0fbcb91 100644
--- a/kexgexs.c
+++ b/kexgexs.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexgexs.c,v 1.42 2019/01/23 00:30:41 djm Exp $ */
+/* $OpenBSD: kexgexs.c,v 1.43 2021/01/31 22:55:29 djm Exp $ */
 /*
  * Copyright (c) 2000 Niels Provos.  All rights reserved.
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
@@ -77,6 +77,8 @@ input_kex_dh_gex_request(int type, u_int32_t seq, struct ssh *ssh)
 	const BIGNUM *dh_p, *dh_g;
 
 	debug("SSH2_MSG_KEX_DH_GEX_REQUEST received");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_REQUEST, &kex_protocol_error);
+
 	if ((r = sshpkt_get_u32(ssh, &min)) != 0 ||
 	    (r = sshpkt_get_u32(ssh, &nbits)) != 0 ||
 	    (r = sshpkt_get_u32(ssh, &max)) != 0 ||
@@ -136,6 +138,9 @@ input_kex_dh_gex_init(int type, u_int32_t seq, struct ssh *ssh)
 	size_t slen, hashlen;
 	int r;
 
+	debug("SSH2_MSG_KEX_DH_GEX_INIT received");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_INIT, &kex_protocol_error);
+
 	if ((r = kex_load_hostkey(ssh, &server_host_private,
 	    &server_host_public)) != 0)
 		goto out;