upstream: Improve strictness and control over RSA-SHA2 signature

In ssh, when an agent fails to return a RSA-SHA2 signature when
requested and falls back to RSA-SHA1 instead, retry the signature to
ensure that the public key algorithm sent in the SSH_MSG_USERAUTH
matches the one in the signature itself.

In sshd, strictly enforce that the public key algorithm sent in the
SSH_MSG_USERAUTH message matches what appears in the signature.

Make the sshd_config PubkeyAcceptedKeyTypes and
HostbasedAcceptedKeyTypes options control accepted signature algorithms
(previously they selected supported key types). This allows these
options to ban RSA-SHA1 in favour of RSA-SHA2.

Add new signature algorithms "rsa-sha2-256-cert-v01@openssh.com" and
"rsa-sha2-512-cert-v01@openssh.com" to force use of RSA-SHA2 signatures
with certificate keys.

feedback and ok markus@

OpenBSD-Commit-ID: c6e9f6d45eed8962ad502d315d7eaef32c419dde

1 files changed, 2 insertions, 3 deletions

diff --git a/auth2-hostbased.c b/auth2-hostbased.c
index 8996f7e05..f70609cb0 100644
--- a/auth2-hostbased.c
+++ b/auth2-hostbased.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: auth2-hostbased.c,v 1.33 2018/01/23 05:27:21 djm Exp $ */
+/* $OpenBSD: auth2-hostbased.c,v 1.34 2018/07/03 11:39:54 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  *
@@ -111,8 +111,7 @@ userauth_hostbased(struct ssh *ssh)
                     "signature format");
                 goto done;
         }
-        if (match_pattern_list(sshkey_ssh_name(key),
-            options.hostbased_key_types, 0) != 1) {
+        if (match_pattern_list(pkalg, options.hostbased_key_types, 0) != 1) {
                 logit("%s: key type %s not in HostbasedAcceptedKeyTypes",
                     __func__, sshkey_type(key));
                 goto done;