unbreak hostkey rotation; attempting to sign with a desired signature

algorithm of kex->hostkey_alg is incorrect when the key type isn't
capable of making those signatures. ok markus@

2 files changed, 23 insertions, 6 deletions

diff --git a/usr.bin/ssh/clientloop.c b/usr.bin/ssh/clientloop.c
index c7b9ff234ec..a4c056077a0 100644
--- a/usr.bin/ssh/clientloop.c
+++ b/usr.bin/ssh/clientloop.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: clientloop.c,v 1.308 2017/12/18 02:25:15 djm Exp $ */
+/* $OpenBSD: clientloop.c,v 1.309 2017/12/18 23:16:23 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -1878,7 +1878,7 @@ client_global_hostkeys_private_confirm(struct ssh *ssh, int type,
 	struct hostkeys_update_ctx *ctx = (struct hostkeys_update_ctx *)_ctx;
 	size_t i, ndone;
 	struct sshbuf *signdata;
-	int r;
+	int r, kexsigtype, use_kexsigtype;
 	const u_char *sig;
 	size_t siglen;
 
@@ -1890,6 +1890,9 @@ client_global_hostkeys_private_confirm(struct ssh *ssh, int type,
 		hostkeys_update_ctx_free(ctx);
 		return;
 	}
+	kexsigtype = sshkey_type_plain(
+	    sshkey_type_from_name(ssh->kex->hostkey_alg));
+
 	if ((signdata = sshbuf_new()) == NULL)
 		fatal("%s: sshbuf_new failed", __func__);
 	/* Don't want to accidentally accept an unbound signature */
@@ -1918,9 +1921,15 @@ client_global_hostkeys_private_confirm(struct ssh *ssh, int type,
 			    __func__, ssh_err(r));
 			goto out;
 		}
+		/*
+		 * For RSA keys, prefer to use the signature type negotiated
+		 * during KEX to the default (SHA1).
+		 */
+		use_kexsigtype = kexsigtype == KEY_RSA &&
+		    sshkey_type_plain(ctx->keys[i]->type) == KEY_RSA;
 		if ((r = sshkey_verify(ctx->keys[i], sig, siglen,
 		    sshbuf_ptr(signdata), sshbuf_len(signdata),
-		    ssh->kex->hostkey_alg, 0)) != 0) {
+		    use_kexsigtype ? ssh->kex->hostkey_alg : NULL, 0)) != 0) {
 			error("%s: server gave bad signature for %s key %zu",
 			    __func__, sshkey_type(ctx->keys[i]), i);
 			goto out;
diff --git a/usr.bin/ssh/serverloop.c b/usr.bin/ssh/serverloop.c
index 853c0fc5eaa..887ffaee312 100644
--- a/usr.bin/ssh/serverloop.c
+++ b/usr.bin/ssh/serverloop.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: serverloop.c,v 1.201 2017/12/18 02:25:15 djm Exp $ */
+/* $OpenBSD: serverloop.c,v 1.202 2017/12/18 23:16:24 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -660,7 +660,7 @@ server_input_hostkeys_prove(struct ssh *ssh, struct sshbuf **respp)
 	struct sshbuf *resp = NULL;
 	struct sshbuf *sigbuf = NULL;
 	struct sshkey *key = NULL, *key_pub = NULL, *key_prv = NULL;
-	int r, ndx, success = 0;
+	int r, ndx, kexsigtype, use_kexsigtype, success = 0;
 	const u_char *blob;
 	u_char *sig = 0;
 	size_t blen, slen;
@@ -668,6 +668,8 @@ server_input_hostkeys_prove(struct ssh *ssh, struct sshbuf **respp)
 	if ((resp = sshbuf_new()) == NULL || (sigbuf = sshbuf_new()) == NULL)
 		fatal("%s: sshbuf_new", __func__);
 
+	kexsigtype = sshkey_type_plain(
+	    sshkey_type_from_name(ssh->kex->hostkey_alg));
 	while (ssh_packet_remaining(ssh) > 0) {
 		sshkey_free(key);
 		key = NULL;
@@ -698,6 +700,12 @@ server_input_hostkeys_prove(struct ssh *ssh, struct sshbuf **respp)
 		sshbuf_reset(sigbuf);
 		free(sig);
 		sig = NULL;
+		/*
+		 * For RSA keys, prefer to use the signature type negotiated
+		 * during KEX to the default (SHA1).
+		 */
+		use_kexsigtype = kexsigtype == KEY_RSA &&
+		    sshkey_type_plain(key->type) == KEY_RSA;
 		if ((r = sshbuf_put_cstring(sigbuf,
 		    "hostkeys-prove-00@openssh.com")) != 0 ||
 		    (r = sshbuf_put_string(sigbuf,
@@ -705,7 +713,7 @@ server_input_hostkeys_prove(struct ssh *ssh, struct sshbuf **respp)
 		    (r = sshkey_puts(key, sigbuf)) != 0 ||
 		    (r = ssh->kex->sign(key_prv, key_pub, &sig, &slen,
 		    sshbuf_ptr(sigbuf), sshbuf_len(sigbuf),
-		    ssh->kex->hostkey_alg, 0)) != 0 ||
+		    use_kexsigtype ? ssh->kex->hostkey_alg : NULL, 0)) != 0 ||
 		    (r = sshbuf_put_string(resp, sig, slen)) != 0) {
 			error("%s: couldn't prepare signature: %s",
 			    __func__, ssh_err(r));