options: Added an option to set server HostKey algorithms

The added option SSH_BIND_OPTIONS_HOSTKEY_ALGORITHMS allows restricting
the signature algorithms to offer to the client for host authentication.
The list set is used as a filter of allowed algorithms.

First a list of possible signature algorithms to offer is created from
the keys set and then such list is filtered against the allowed
algorithms.

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

include/libssh/server.h

@@ -54,6 +54,7 @@ enum ssh_bind_options_e {
   SSH_BIND_OPTIONS_HMAC_S_C,
   SSH_BIND_OPTIONS_CONFIG_DIR,
   SSH_BIND_OPTIONS_PUBKEY_ACCEPTED_KEY_TYPES,
+  SSH_BIND_OPTIONS_HOSTKEY_ALGORITHMS,
 };
 
 typedef struct ssh_bind_struct* ssh_bind;

src/options.c

@@ -1612,6 +1612,15 @@ static int ssh_bind_set_algo(ssh_bind sshbind,
  *                        Set the public key algorithm accepted by the server
  *                        (const char *, comma-separated list).
  *
+ *                      - SSH_BIND_OPTIONS_HOSTKEY_ALGORITHMS:
+ *                        Set the list of allowed hostkey signatures algorithms
+ *                        to offer to the client, ordered by preference. This
+ *                        list is used as a filter when creating the list of
+ *                        algorithms to offer to the client: first the list of
+ *                        possible algorithms is created from the list of keys
+ *                        set and then filtered against this list.
+ *                        (const char *, comma-separated list).
+ *
  * @param  value        The value to set. This is a generic pointer and the
  *                      datatype which should be used is described at the
  *                      corresponding value of type above.
@@ -1934,6 +1943,18 @@ int ssh_bind_options_set(ssh_bind sshbind, enum ssh_bind_options_e type,
             sshbind->pubkey_accepted_key_types = p;
         }
         break;
+    case SSH_BIND_OPTIONS_HOSTKEY_ALGORITHMS:
+        v = value;
+        if (v == NULL || v[0] == '\0') {
+            ssh_set_error_invalid(sshbind);
+            return -1;
+        } else {
+            rc = ssh_bind_set_algo(sshbind, SSH_HOSTKEYS, v);
+            if (rc < 0) {
+                return -1;
+            }
+        }
+        break;
     default:
       ssh_set_error(sshbind, SSH_REQUEST_DENIED, "Unknown ssh option %d", type);
       return -1;

src/server.c

@@ -59,6 +59,7 @@
 #include "libssh/messages.h"
 #include "libssh/options.h"
 #include "libssh/curve25519.h"
+#include "libssh/token.h"
 
 #define set_status(session, status) do {\
         if (session->common.callbacks && session->common.callbacks->connect_status_function) \
@@ -84,7 +85,8 @@ int server_set_kex(ssh_session session)
 {
     struct ssh_kex_struct *server = &session->next_crypto->server_kex;
     int i, j, rc;
-  const char *wanted;
+    const char *wanted, *allowed;
+    char *kept;
     char hostkeys[128] = {0};
     enum ssh_keytypes_e keytype;
     size_t len;
@@ -137,9 +139,25 @@ int server_set_kex(ssh_session session)
         return -1;
     }
 
+    if (session->opts.wanted_methods[SSH_HOSTKEYS]) {
+        allowed = session->opts.wanted_methods[SSH_HOSTKEYS];
+    } else {
+        allowed = ssh_kex_get_supported_method(SSH_HOSTKEYS);
+    }
+
+    /* It is expected for the list of allowed hostkeys to be ordered by
+     * preference */
+    kept = ssh_find_all_matching(hostkeys[0] == ',' ? hostkeys + 1 : hostkeys,
+                                 allowed);
+    if (kept == NULL) {
+        /* Nothing was allowed */
+        return -1;
+    }
+
     rc = ssh_options_set_algo(session,
                               SSH_HOSTKEYS,
-                            hostkeys[0] == ',' ? hostkeys + 1 : hostkeys);
+                              kept);
+    SAFE_FREE(kept);
     if (rc < 0) {
         return -1;
     }

tests/unittests/torture_options.c

@@ -1413,6 +1413,61 @@ static void torture_bind_options_set_pubkey_accepted_key_types(void **state)
         "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
 }
 
+static void torture_bind_options_set_hostkey_algorithms(void **state)
+{
+    struct bind_st *test_state;
+    ssh_bind bind;
+    int rc;
+
+    assert_non_null(state);
+    test_state = *((struct bind_st **)state);
+    assert_non_null(test_state);
+    assert_non_null(test_state->bind);
+    bind = test_state->bind;
+
+    /* Test known Pubkey Types */
+    rc = ssh_bind_options_set(bind, SSH_BIND_OPTIONS_HOSTKEY_ALGORITHMS,
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+    assert_int_equal(rc, 0);
+    assert_non_null(bind->wanted_methods[SSH_HOSTKEYS]);
+    assert_string_equal(bind->wanted_methods[SSH_HOSTKEYS],
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+
+    SAFE_FREE(bind->wanted_methods[SSH_HOSTKEYS]);
+
+    /* Test with some unknown type */
+    rc = ssh_bind_options_set(bind, SSH_BIND_OPTIONS_HOSTKEY_ALGORITHMS,
+        "ssh-ed25519,ecdsa-sha2-nistp384,unknown-type,ssh-rsa");
+    assert_int_equal(rc, 0);
+    assert_non_null(bind->wanted_methods[SSH_HOSTKEYS]);
+    assert_string_equal(bind->wanted_methods[SSH_HOSTKEYS],
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+
+    SAFE_FREE(bind->wanted_methods[SSH_HOSTKEYS]);
+
+    /* Test with only unknown type */
+    rc = ssh_bind_options_set(bind, SSH_BIND_OPTIONS_HOSTKEY_ALGORITHMS,
+        "unknown-type");
+    assert_int_equal(rc, -1);
+    assert_null(bind->wanted_methods[SSH_HOSTKEYS]);
+
+    /* Test with something set and then try unknown type */
+    rc = ssh_bind_options_set(bind, SSH_BIND_OPTIONS_HOSTKEY_ALGORITHMS,
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+    assert_int_equal(rc, 0);
+    assert_non_null(bind->wanted_methods[SSH_HOSTKEYS]);
+    assert_string_equal(bind->wanted_methods[SSH_HOSTKEYS],
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+    rc = ssh_bind_options_set(bind, SSH_BIND_OPTIONS_HOSTKEY_ALGORITHMS,
+        "unknown-type");
+    assert_int_equal(rc, -1);
+
+    /* Check that nothing changed */
+    assert_non_null(bind->wanted_methods[SSH_HOSTKEYS]);
+    assert_string_equal(bind->wanted_methods[SSH_HOSTKEYS],
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+}
+
 #endif /* WITH_SERVER */
 
 
@@ -1484,6 +1539,8 @@ int torture_run_tests(void) {
                 sshbind_setup, sshbind_teardown),
         cmocka_unit_test_setup_teardown(torture_bind_options_set_pubkey_accepted_key_types,
                                         sshbind_setup, sshbind_teardown),
+        cmocka_unit_test_setup_teardown(torture_bind_options_set_hostkey_algorithms,
+                                        sshbind_setup, sshbind_teardown),
     };
 #endif /* WITH_SERVER */