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>

4 files changed, 77 insertions, 10 deletions

diff --git a/src/bind.c b/src/bind.c
index a1f25de2..d5193f77 100644
--- a/src/bind.c
+++ b/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);
diff --git a/src/messages.c b/src/messages.c
index f65a484c..f0c6d781 100644
--- a/src/messages.c
+++ b/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);
diff --git a/src/options.c b/src/options.c
index e9d8a721..244f3edf 100644
--- a/src/options.c
+++ b/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;
diff --git a/src/server.c b/src/server.c
index b2c81b60..2937d0f5 100644
--- a/src/server.c
+++ b/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,