options: Add option to set server accepted pubkey types

The added option SSH_BIND_OPTIONS_PUBKEY_ACCEPTED_KEY_TYPES allows
restricting the allowed public key types accepted by the server for
authentication.

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

include/libssh/bind.h

@@ -48,6 +48,7 @@ struct ssh_bind_struct {
   int toaccept;
   bool config_processed;
   char *config_dir;
+  char *pubkey_accepted_key_types;
 };
 
 struct ssh_poll_handle_struct *ssh_bind_get_poll(struct ssh_bind_struct

include/libssh/server.h

@@ -53,6 +53,7 @@ enum ssh_bind_options_e {
   SSH_BIND_OPTIONS_HMAC_C_S,
   SSH_BIND_OPTIONS_HMAC_S_C,
   SSH_BIND_OPTIONS_CONFIG_DIR,
+  SSH_BIND_OPTIONS_PUBKEY_ACCEPTED_KEY_TYPES,
 };
 
 typedef struct ssh_bind_struct* ssh_bind;

src/bind.c

@@ -38,6 +38,7 @@
 #include "libssh/buffer.h"
 #include "libssh/socket.h"
 #include "libssh/session.h"
+#include "libssh/token.h"
 
 /**
  * @addtogroup libssh_server
@@ -402,6 +403,7 @@ void ssh_bind_free(ssh_bind sshbind){
   SAFE_FREE(sshbind->banner);
   SAFE_FREE(sshbind->bindaddr);
   SAFE_FREE(sshbind->config_dir);
+  SAFE_FREE(sshbind->pubkey_accepted_key_types);
 
   SAFE_FREE(sshbind->dsakey);
   SAFE_FREE(sshbind->rsakey);
@@ -456,6 +458,29 @@ int ssh_bind_accept_fd(ssh_bind sshbind, ssh_session session, socket_t fd){
       }
     }
 
+    if (sshbind->pubkey_accepted_key_types != NULL) {
+        if (session->opts.pubkey_accepted_types == NULL) {
+            session->opts.pubkey_accepted_types = strdup(sshbind->pubkey_accepted_key_types);
+            if (session->opts.pubkey_accepted_types == NULL) {
+                ssh_set_error_oom(sshbind);
+                return SSH_ERROR;
+            }
+        } else {
+            char *p;
+            /* If something was set to the session prior to calling this
+             * function, keep only what is allowed by the options set in
+             * sshbind */
+            p = ssh_find_all_matching(sshbind->pubkey_accepted_key_types,
+                                      session->opts.pubkey_accepted_types);
+            if (p == NULL) {
+                return SSH_ERROR;
+            }
+
+            SAFE_FREE(session->opts.pubkey_accepted_types);
+            session->opts.pubkey_accepted_types = p;
+        }
+    }
+
     session->common.log_verbosity = sshbind->common.log_verbosity;
     if(sshbind->banner != NULL)
     	session->opts.custombanner = strdup(sshbind->banner);

src/messages.c

@@ -868,11 +868,28 @@ SSH_PACKET_CALLBACK(ssh_packet_userauth_request){
                                            msg->auth_request.pubkey,
                                            &sig);
         if (rc == SSH_OK) {
-            rc = ssh_pki_signature_verify(session,
-                                          sig,
-                                          msg->auth_request.pubkey,
-                                          ssh_buffer_get(digest),
-                                          ssh_buffer_get_len(digest));
+            /* Check if the signature from client matches server preferences */
+            if (session->opts.pubkey_accepted_types) {
+                if (!ssh_match_group(session->opts.pubkey_accepted_types,
+                            sig->type_c))
+                {
+                    ssh_set_error(session,
+                            SSH_FATAL,
+                            "Public key from client (%s) doesn't match server "
+                            "preference (%s)",
+                            sig->type_c,
+                            session->opts.pubkey_accepted_types);
+                    rc = SSH_ERROR;
+                }
+            }
+
+            if (rc == SSH_OK) {
+                rc = ssh_pki_signature_verify(session,
+                                              sig,
+                                              msg->auth_request.pubkey,
+                                              ssh_buffer_get(digest),
+                                              ssh_buffer_get_len(digest));
+            }
         }
         ssh_string_free(sig_blob);
         ssh_buffer_free(digest);

src/options.c

@@ -1608,6 +1608,10 @@ static int ssh_bind_set_algo(ssh_bind sshbind,
  *                        paths of configuration files to
  *                        ssh_bind_options_parse_config().
  *
+ *                      - SSH_BIND_OPTIONS_PUBKEY_ACCEPTED_KEY_TYPES:
+ *                        Set the public key algorithm accepted by the server
+ *                        (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.
@@ -1912,6 +1916,24 @@ int ssh_bind_options_set(ssh_bind sshbind, enum ssh_bind_options_e type,
             }
         }
         break;
+    case SSH_BIND_OPTIONS_PUBKEY_ACCEPTED_KEY_TYPES:
+        v = value;
+        if (v == NULL || v[0] == '\0') {
+            ssh_set_error_invalid(sshbind);
+            return -1;
+        } else {
+            p = ssh_keep_known_algos(SSH_HOSTKEYS, v);
+            if (p == NULL) {
+                ssh_set_error(sshbind, SSH_REQUEST_DENIED,
+                    "Setting method: no known public key algorithm (%s)",
+                     v);
+                return -1;
+            }
+
+            SAFE_FREE(sshbind->pubkey_accepted_key_types);
+            sshbind->pubkey_accepted_key_types = p;
+        }
+        break;
     default:
       ssh_set_error(sshbind, SSH_REQUEST_DENIED, "Unknown ssh option %d", type);
       return -1;

src/server.c

@@ -180,11 +180,14 @@ static int ssh_server_send_extensions(ssh_session session) {
     const char *hostkey_algorithms;
 
     SSH_LOG(SSH_LOG_PACKET, "Sending SSH_MSG_EXT_INFO");
-    /*
-     * We can list here all the default hostkey methods, since
-     * they already contain the SHA2 extension algorithms
-     */
-    hostkey_algorithms = ssh_kex_get_default_methods(SSH_HOSTKEYS);
+
+    if (session->opts.pubkey_accepted_types) {
+        hostkey_algorithms = session->opts.pubkey_accepted_types;
+    } else {
+        /* There are no restrictions to the accepted public keys */
+        hostkey_algorithms = ssh_kex_get_default_methods(SSH_HOSTKEYS);
+    }
+
     rc = ssh_buffer_pack(session->out_buffer,
                          "bdss",
                          SSH2_MSG_EXT_INFO,

tests/unittests/torture_options.c

@@ -1358,6 +1358,60 @@ static void torture_bind_options_config_dir(void **state)
     assert_string_equal(bind->config_dir, replacement_dir);
 }
 
+static void torture_bind_options_set_pubkey_accepted_key_types(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_PUBKEY_ACCEPTED_KEY_TYPES,
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+    assert_int_equal(rc, 0);
+    assert_non_null(bind->pubkey_accepted_key_types);
+    assert_string_equal(bind->pubkey_accepted_key_types,
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+
+    SAFE_FREE(bind->pubkey_accepted_key_types);
+
+    /* Test with some unknown type */
+    rc = ssh_bind_options_set(bind, SSH_BIND_OPTIONS_PUBKEY_ACCEPTED_KEY_TYPES,
+        "ssh-ed25519,ecdsa-sha2-nistp384,unknown-type,ssh-rsa");
+    assert_int_equal(rc, 0);
+    assert_non_null(bind->pubkey_accepted_key_types);
+    assert_string_equal(bind->pubkey_accepted_key_types,
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+
+    SAFE_FREE(bind->pubkey_accepted_key_types);
+
+    /* Test with only unknown type */
+    rc = ssh_bind_options_set(bind, SSH_BIND_OPTIONS_PUBKEY_ACCEPTED_KEY_TYPES,
+        "unknown-type");
+    assert_int_equal(rc, -1);
+    assert_null(bind->pubkey_accepted_key_types);
+
+    /* Test with something set and then try unknown type */
+    rc = ssh_bind_options_set(bind, SSH_BIND_OPTIONS_PUBKEY_ACCEPTED_KEY_TYPES,
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+    assert_int_equal(rc, 0);
+    assert_non_null(bind->pubkey_accepted_key_types);
+    assert_string_equal(bind->pubkey_accepted_key_types,
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+    rc = ssh_bind_options_set(bind, SSH_BIND_OPTIONS_PUBKEY_ACCEPTED_KEY_TYPES,
+        "unknown-type");
+    assert_int_equal(rc, -1);
+
+    /* Check that nothing changed */
+    assert_non_null(bind->pubkey_accepted_key_types);
+    assert_string_equal(bind->pubkey_accepted_key_types,
+        "ssh-ed25519,ecdsa-sha2-nistp384,ssh-rsa");
+}
 
 #endif /* WITH_SERVER */
 
@@ -1387,7 +1441,7 @@ int torture_run_tests(void) {
         cmocka_unit_test_setup_teardown(torture_options_copy, setup, teardown),
         cmocka_unit_test_setup_teardown(torture_options_config_host, setup, teardown),
         cmocka_unit_test_setup_teardown(torture_options_config_match,
-                                        setup, teardown)
+                                        setup, teardown),
     };
 
 #ifdef WITH_SERVER
@@ -1428,6 +1482,8 @@ int torture_run_tests(void) {
                 sshbind_setup, sshbind_teardown),
         cmocka_unit_test_setup_teardown(torture_bind_options_config_dir,
                 sshbind_setup, sshbind_teardown),
+        cmocka_unit_test_setup_teardown(torture_bind_options_set_pubkey_accepted_key_types,
+                                        sshbind_setup, sshbind_teardown),
     };
 #endif /* WITH_SERVER */