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, 19 insertions, 8 deletions

diff --git a/compat.c b/compat.c
index 1c0e08732..1c9890aa8 100644
--- a/compat.c
+++ b/compat.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: compat.c,v 1.107 2018/04/16 22:50:44 djm Exp $ */
+/* $OpenBSD: compat.c,v 1.108 2018/07/03 11:39:54 djm Exp $ */
 /*
  * Copyright (c) 1999, 2000, 2001, 2002 Markus Friedl.  All rights reserved.
  *
@@ -52,16 +52,27 @@ compat_datafellows(const char *version)
         } check[] = {
                 { "OpenSSH_2.*,"
                   "OpenSSH_3.0*,"
-                  "OpenSSH_3.1*",       SSH_BUG_EXTEOF|SSH_OLD_FORWARD_ADDR},
-                { "OpenSSH_3.*",        SSH_OLD_FORWARD_ADDR },
-                { "Sun_SSH_1.0*",       SSH_BUG_NOREKEY|SSH_BUG_EXTEOF},
+                  "OpenSSH_3.1*",       SSH_BUG_EXTEOF|SSH_OLD_FORWARD_ADDR|
+                                        SSH_BUG_SIGTYPE},
+                { "OpenSSH_3.*",        SSH_OLD_FORWARD_ADDR|SSH_BUG_SIGTYPE },
+                { "Sun_SSH_1.0*",       SSH_BUG_NOREKEY|SSH_BUG_EXTEOF|
+                                        SSH_BUG_SIGTYPE},
                 { "OpenSSH_2*,"
                   "OpenSSH_3*,"
-                  "OpenSSH_4*",         0 },
-                { "OpenSSH_5*",         SSH_NEW_OPENSSH|SSH_BUG_DYNAMIC_RPORT},
-                { "OpenSSH_6.6.1*",     SSH_NEW_OPENSSH},
+                  "OpenSSH_4*",         SSH_BUG_SIGTYPE },
+                { "OpenSSH_5*",         SSH_NEW_OPENSSH|SSH_BUG_DYNAMIC_RPORT|
+                                        SSH_BUG_SIGTYPE},
+                { "OpenSSH_6.6.1*",     SSH_NEW_OPENSSH|SSH_BUG_SIGTYPE},
                 { "OpenSSH_6.5*,"
-                  "OpenSSH_6.6*",       SSH_NEW_OPENSSH|SSH_BUG_CURVE25519PAD},
+                  "OpenSSH_6.6*",       SSH_NEW_OPENSSH|SSH_BUG_CURVE25519PAD|
+                                        SSH_BUG_SIGTYPE},
+                { "OpenSSH_7.0*,"
+                  "OpenSSH_7.1*,"
+                  "OpenSSH_7.2*,"
+                  "OpenSSH_7.3*,"
+                  "OpenSSH_7.4*,"
+                  "OpenSSH_7.5*,"
+                  "OpenSSH_7.6*",       SSH_NEW_OPENSSH|SSH_BUG_SIGTYPE},
                 { "OpenSSH*",           SSH_NEW_OPENSSH },
                 { "*MindTerm*",         0 },
                 { "3.0.*",              SSH_BUG_DEBUG },