upstream: add CASignatureAlgorithms option for the client, allowing

it to specify which signature algorithms may be used by CAs when signing
certificates. Useful if you want to ban RSA/SHA1; ok markus@

OpenBSD-Commit-ID: 9159e5e9f67504829bf53ff222057307a6e3230f

4 files changed, 42 insertions, 10 deletions

diff --git a/readconf.c b/readconf.c
index db5f2d547..057726d0e 100644
--- a/readconf.c
+++ b/readconf.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.c,v 1.297 2018/08/12 20:19:13 djm Exp $ */
+/* $OpenBSD: readconf.c,v 1.298 2018/09/20 03:30:44 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -172,7 +172,7 @@ typedef enum {
         oCanonicalizeFallbackLocal, oCanonicalizePermittedCNAMEs,
         oStreamLocalBindMask, oStreamLocalBindUnlink, oRevokedHostKeys,
         oFingerprintHash, oUpdateHostkeys, oHostbasedKeyTypes,
-        oPubkeyAcceptedKeyTypes, oProxyJump,
+        oPubkeyAcceptedKeyTypes, oCASignatureAlgorithms, oProxyJump,
         oIgnore, oIgnoredUnknownOption, oDeprecated, oUnsupported
 } OpCodes;
 
@@ -266,6 +266,7 @@ static struct {
         { "dynamicforward", oDynamicForward },
         { "preferredauthentications", oPreferredAuthentications },
         { "hostkeyalgorithms", oHostKeyAlgorithms },
+        { "casignaturealgorithms", oCASignatureAlgorithms },
         { "bindaddress", oBindAddress },
         { "bindinterface", oBindInterface },
         { "clearallforwardings", oClearAllForwardings },
@@ -1221,6 +1222,10 @@ parse_keytypes:
                         *charptr = xstrdup(arg);
                 break;
 
+        case oCASignatureAlgorithms:
+                charptr = &options->ca_sign_algorithms;
+                goto parse_keytypes;
+
         case oLogLevel:
                 log_level_ptr = &options->log_level;
                 arg = strdelim(&s);
@@ -1836,6 +1841,7 @@ initialize_options(Options * options)
         options->macs = NULL;
         options->kex_algorithms = NULL;
         options->hostkeyalgorithms = NULL;
+        options->ca_sign_algorithms = NULL;
         options->num_identity_files = 0;
         options->num_certificate_files = 0;
         options->hostname = NULL;
@@ -1924,7 +1930,7 @@ fill_default_options_for_canonicalization(Options *options)
 void
 fill_default_options(Options * options)
 {
-        char *all_cipher, *all_mac, *all_kex, *all_key;
+        char *all_cipher, *all_mac, *all_kex, *all_key, *all_sig;
         int r;
 
         if (options->forward_agent == -1)
@@ -2077,6 +2083,7 @@ fill_default_options(Options * options)
         all_mac = mac_alg_list(',');
         all_kex = kex_alg_list(',');
         all_key = sshkey_alg_list(0, 0, 1, ',');
+        all_sig = sshkey_alg_list(0, 1, 1, ',');
 #define ASSEMBLE(what, defaults, all) \
         do { \
                 if ((r = kex_assemble_names(&options->what, \
@@ -2088,11 +2095,13 @@ fill_default_options(Options * options)
         ASSEMBLE(kex_algorithms, KEX_SERVER_KEX, all_kex);
         ASSEMBLE(hostbased_key_types, KEX_DEFAULT_PK_ALG, all_key);
         ASSEMBLE(pubkey_key_types, KEX_DEFAULT_PK_ALG, all_key);
+        ASSEMBLE(ca_sign_algorithms, SSH_ALLOWED_CA_SIGALGS, all_sig);
 #undef ASSEMBLE
         free(all_cipher);
         free(all_mac);
         free(all_kex);
         free(all_key);
+        free(all_sig);
 
 #define CLEAR_ON_NONE(v) \
         do { \
@@ -2614,6 +2623,7 @@ dump_client_config(Options *o, const char *host)
         dump_cfg_string(oIgnoreUnknown, o->ignored_unknown);
         dump_cfg_string(oKbdInteractiveDevices, o->kbd_interactive_devices);
         dump_cfg_string(oKexAlgorithms, o->kex_algorithms ? o->kex_algorithms : KEX_CLIENT_KEX);
+        dump_cfg_string(oCASignatureAlgorithms, o->ca_sign_algorithms ? o->ca_sign_algorithms : SSH_ALLOWED_CA_SIGALGS);
         dump_cfg_string(oLocalCommand, o->local_command);
         dump_cfg_string(oRemoteCommand, o->remote_command);
         dump_cfg_string(oLogLevel, log_level_name(o->log_level));
diff --git a/readconf.h b/readconf.h
index c56887816..fc7e38251 100644
--- a/readconf.h
+++ b/readconf.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.h,v 1.127 2018/07/19 10:28:47 dtucker Exp $ */
+/* $OpenBSD: readconf.h,v 1.128 2018/09/20 03:30:44 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -67,6 +67,7 @@ typedef struct {
         char   *macs;           /* SSH2 macs in order of preference. */
         char   *hostkeyalgorithms;      /* SSH2 server key types in order of preference. */
         char   *kex_algorithms; /* SSH2 kex methods in order of preference. */
+        char   *ca_sign_algorithms;     /* Allowed CA signature algorithms */
         char   *hostname;       /* Real host to connect. */
         char   *host_key_alias; /* hostname alias for .ssh/known_hosts */
         char   *proxy_command;  /* Proxy command for connecting the host. */
diff --git a/ssh_config.5 b/ssh_config.5
index f499396a3..a9b44cc44 100644
--- a/ssh_config.5
+++ b/ssh_config.5
@@ -33,8 +33,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: ssh_config.5,v 1.281 2018/07/23 19:02:49 kn Exp $
-.Dd $Mdocdate: July 23 2018 $
+.\" $OpenBSD: ssh_config.5,v 1.282 2018/09/20 03:30:44 djm Exp $
+.Dd $Mdocdate: September 20 2018 $
 .Dt SSH_CONFIG 5
 .Os
 .Sh NAME
@@ -261,6 +261,18 @@ Only useful on systems with more than one address.
 .It Cm BindInterface
 Use the address of the specified interface on the local machine as the
 source address of the connection.
+.It Cm CASignatureAlgorithms
+Specifies which algorithms are allowed for signing of certificates
+by certificate authorities (CAs).
+The default is:
+.Bd -literal -offset indent
+ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
+ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
+.Ed
+.Pp
+.Xr ssh 1
+will not accept host certificates signed using algorithms other than those
+specified.
 .It Cm CanonicalDomains
 When
 .Cm CanonicalizeHostname
diff --git a/sshconnect.c b/sshconnect.c
index 78813c164..6d819279e 100644
--- a/sshconnect.c
+++ b/sshconnect.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect.c,v 1.304 2018/07/27 05:34:42 dtucker Exp $ */
+/* $OpenBSD: sshconnect.c,v 1.305 2018/09/20 03:30:44 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -734,19 +734,28 @@ confirm(const char *prompt)
 }
 
 static int
-check_host_cert(const char *host, const struct sshkey *host_key)
+check_host_cert(const char *host, const struct sshkey *key)
 {
         const char *reason;
+        int r;
 
-        if (sshkey_cert_check_authority(host_key, 1, 0, host, &reason) != 0) {
+        if (sshkey_cert_check_authority(key, 1, 0, host, &reason) != 0) {
                 error("%s", reason);
                 return 0;
         }
-        if (sshbuf_len(host_key->cert->critical) != 0) {
+        if (sshbuf_len(key->cert->critical) != 0) {
                 error("Certificate for %s contains unsupported "
                     "critical options(s)", host);
                 return 0;
         }
+        if ((r = sshkey_check_cert_sigtype(key,
+            options.ca_sign_algorithms)) != 0) {
+                logit("%s: certificate signature algorithm %s: %s", __func__,
+                    (key->cert == NULL || key->cert->signature_type == NULL) ?
+                    "(null)" : key->cert->signature_type, ssh_err(r));
+                return 0;
+        }
+
         return 1;
 }