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, 13 insertions, 11 deletions

diff --git a/authfd.c b/authfd.c
index 3ee7dffa5..f24230b7c 100644
--- a/authfd.c
+++ b/authfd.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: authfd.c,v 1.109 2018/04/10 00:10:49 djm Exp $ */
+/* $OpenBSD: authfd.c,v 1.110 2018/07/03 11:39:54 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -343,8 +343,8 @@ ssh_agent_sign(int sock, const struct sshkey *key,
     const u_char *data, size_t datalen, const char *alg, u_int compat)
 {
         struct sshbuf *msg;
-        u_char *blob = NULL, type;
-        size_t blen = 0, len = 0;
+        u_char *sig = NULL, type = 0;
+        size_t len = 0;
         u_int flags = 0;
         int r = SSH_ERR_INTERNAL_ERROR;
 
@@ -355,11 +355,9 @@ ssh_agent_sign(int sock, const struct sshkey *key,
                 return SSH_ERR_INVALID_ARGUMENT;
         if ((msg = sshbuf_new()) == NULL)
                 return SSH_ERR_ALLOC_FAIL;
-        if ((r = sshkey_to_blob(key, &blob, &blen)) != 0)
-                goto out;
         flags |= agent_encode_alg(key, alg);
         if ((r = sshbuf_put_u8(msg, SSH2_AGENTC_SIGN_REQUEST)) != 0 ||
-            (r = sshbuf_put_string(msg, blob, blen)) != 0 ||
+            (r = sshkey_puts(key, msg)) != 0 ||
             (r = sshbuf_put_string(msg, data, datalen)) != 0 ||
             (r = sshbuf_put_u32(msg, flags)) != 0)
                 goto out;
@@ -374,15 +372,19 @@ ssh_agent_sign(int sock, const struct sshkey *key,
                 r = SSH_ERR_INVALID_FORMAT;
                 goto out;
         }
-        if ((r = sshbuf_get_string(msg, sigp, &len)) != 0)
+        if ((r = sshbuf_get_string(msg, &sig, &len)) != 0)
+                goto out;
+        /* Check what we actually got back from the agent. */
+        if ((r = sshkey_check_sigtype(sig, len, alg)) != 0)
                 goto out;
+        /* success */
+        *sigp = sig;
         *lenp = len;
+        sig = NULL;
+        len = 0;
         r = 0;
  out:
-        if (blob != NULL) {
-                explicit_bzero(blob, blen);
-                free(blob);
-        }
+        freezero(sig, len);
         sshbuf_free(msg);
         return r;
 }