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, 7 insertions, 6 deletions

diff --git a/auth2-pubkey.c b/auth2-pubkey.c
index 3ccc3a213..4feeae3e2 100644
--- a/auth2-pubkey.c
+++ b/auth2-pubkey.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: auth2-pubkey.c,v 1.79 2018/06/06 18:29:18 markus Exp $ */
+/* $OpenBSD: auth2-pubkey.c,v 1.80 2018/07/03 11:39:54 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  *
@@ -109,7 +109,7 @@ userauth_pubkey(struct ssh *ssh)
         pktype = sshkey_type_from_name(pkalg);
         if (pktype == KEY_UNSPEC) {
                 /* this is perfectly legal */
-                logit("%s: unsupported public key algorithm: %s",
+                verbose("%s: unsupported public key algorithm: %s",
                     __func__, pkalg);
                 goto done;
         }
@@ -136,8 +136,7 @@ userauth_pubkey(struct ssh *ssh)
                 logit("refusing previously-used %s key", sshkey_type(key));
                 goto done;
         }
-        if (match_pattern_list(sshkey_ssh_name(key),
-            options.pubkey_key_types, 0) != 1) {
+        if (match_pattern_list(pkalg, options.pubkey_key_types, 0) != 1) {
                 logit("%s: key type %s not in PubkeyAcceptedKeyTypes",
                     __func__, sshkey_ssh_name(key));
                 goto done;
@@ -188,8 +187,10 @@ userauth_pubkey(struct ssh *ssh)
                 /* test for correct signature */
                 authenticated = 0;
                 if (PRIVSEP(user_key_allowed(ssh, pw, key, 1, &authopts)) &&
-                    PRIVSEP(sshkey_verify(key, sig, slen, sshbuf_ptr(b),
-                    sshbuf_len(b), NULL, ssh->compat)) == 0) {
+                    PRIVSEP(sshkey_verify(key, sig, slen,
+                    sshbuf_ptr(b), sshbuf_len(b),
+                    (ssh->compat & SSH_BUG_SIGTYPE) == 0 ? pkalg : NULL,
+                    ssh->compat)) == 0) {
                         authenticated = 1;
                 }
                 sshbuf_free(b);