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, 5 insertions, 12 deletions

diff --git a/kex.c b/kex.c
index 15ea28b07..d0a5f1b66 100644
--- a/kex.c
+++ b/kex.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.c,v 1.136 2018/02/07 02:06:50 jsing Exp $ */
+/* $OpenBSD: kex.c,v 1.137 2018/07/03 11:39:54 djm Exp $ */
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
  *
@@ -342,6 +342,7 @@ kex_send_ext_info(struct ssh *ssh)
 
         if ((algs = sshkey_alg_list(0, 1, 1, ',')) == NULL)
                 return SSH_ERR_ALLOC_FAIL;
+        /* XXX filter algs list by allowed pubkey/hostbased types */
         if ((r = sshpkt_start(ssh, SSH2_MSG_EXT_INFO)) != 0 ||
             (r = sshpkt_put_u32(ssh, 1)) != 0 ||
             (r = sshpkt_put_cstring(ssh, "server-sig-algs")) != 0 ||
@@ -378,7 +379,7 @@ kex_input_ext_info(int type, u_int32_t seq, struct ssh *ssh)
 {
         struct kex *kex = ssh->kex;
         u_int32_t i, ninfo;
-        char *name, *found;
+        char *name;
         u_char *val;
         size_t vlen;
         int r;
@@ -401,16 +402,8 @@ kex_input_ext_info(int type, u_int32_t seq, struct ssh *ssh)
                                 return SSH_ERR_INVALID_FORMAT;
                         }
                         debug("%s: %s=<%s>", __func__, name, val);
-                        found = match_list("rsa-sha2-256", val, NULL);
-                        if (found) {
-                                kex->rsa_sha2 = 256;
-                                free(found);
-                        }
-                        found = match_list("rsa-sha2-512", val, NULL);
-                        if (found) {
-                                kex->rsa_sha2 = 512;
-                                free(found);
-                        }
+                        kex->server_sig_algs = val;
+                        val = NULL;
                 } else
                         debug("%s: %s (unrecognised)", __func__, name);
                 free(name);