MDEV-3909 remote user enumeration

instead of returning Access denied on the incorrect user name,
emulate the complete failed logic procedure, possibly with
the change plugin packet.

mysql-test/r/failed_auth_3909.result

+optimize table mysql.user;
+Table	Op	Msg_type	Msg_text
+mysql.user	optimize	status	OK
+insert mysql.user (user,plugin) values ('foo','bar'),('bar','bar'),('baz','bar');
+Warnings:
+Warning	1364	Field 'ssl_cipher' doesn't have a default value
+Warning	1364	Field 'x509_issuer' doesn't have a default value
+Warning	1364	Field 'x509_subject' doesn't have a default value
+Warning	1364	Field 'auth_string' doesn't have a default value
+flush privileges;
+connect(localhost,u1,,test,MASTER_PORT,MASTER_SOCKET);
+ERROR HY000: Plugin 'bar' is not loaded
+connect(localhost,u2,,test,MASTER_PORT,MASTER_SOCKET);
+ERROR 28000: Access denied for user 'u2'@'localhost' (using password: NO)
+connect(localhost,u2,password,test,MASTER_PORT,MASTER_SOCKET);
+ERROR 28000: Access denied for user 'u2'@'localhost' (using password: YES)
+ERROR HY000: Plugin 'bar' is not loaded
+ERROR 28000: Access denied for user 'u2'@'localhost' (using password: NO)
+ERROR 28000: Access denied for user 'u2'@'localhost' (using password: YES)
+delete from mysql.user where plugin = 'bar';
+flush privileges;

mysql-test/t/failed_auth_3909.test

+source include/not_embedded.inc;
+
+#
+# MDEV-3909 remote user enumeration
+#
+# verify that for some failed login attemps (with wrong user names)
+# the server requests a plugin
+#
+optimize table mysql.user;
+insert mysql.user (user,plugin) values ('foo','bar'),('bar','bar'),('baz','bar');
+flush privileges;
+
+--replace_result $MASTER_MYSOCK MASTER_SOCKET $MASTER_MYPORT MASTER_PORT
+--error ER_PLUGIN_IS_NOT_LOADED
+connect (fail,localhost,u1);
+
+--replace_result $MASTER_MYSOCK MASTER_SOCKET $MASTER_MYPORT MASTER_PORT
+--error ER_ACCESS_DENIED_ERROR
+connect (fail,localhost,u2);
+
+--replace_result $MASTER_MYSOCK MASTER_SOCKET $MASTER_MYPORT MASTER_PORT
+--error ER_ACCESS_DENIED_ERROR
+connect (fail,localhost,u2,password);
+
+--error ER_PLUGIN_IS_NOT_LOADED
+change_user u1;
+
+--error ER_ACCESS_DENIED_ERROR
+change_user u2;
+
+--error ER_ACCESS_DENIED_ERROR
+change_user u2,password;
+
+delete from mysql.user where plugin = 'bar';
+flush privileges;
+
+

sql/sql_acl.cc

@@ -7056,6 +7056,7 @@ struct MPVIO_EXT : public MYSQL_PLUGIN_VIO
   } cached_server_packet;
   int packets_read, packets_written; ///< counters for send/received packets
   uint connect_errors;      ///< if there were connect errors for this host
+  bool make_it_fail;
   /** when plugin returns a failure this tells us what really happened */
   enum { SUCCESS, FAILURE, RESTART } status;
 };
@@ -7322,14 +7323,14 @@ static bool send_plugin_request_packet(MPVIO_EXT *mpvio,
 /**
    Finds acl entry in user database for authentication purposes.
    
-   Finds a user and copies it into mpvio. Reports an authentication
-   failure if a user is not found.
+   Finds a user and copies it into mpvio. Creates a fake user
+   if no matching user account is found.
 
    @note find_acl_user is not the same, because it doesn't take into
    account the case when user is not empty, but acl_user->user is empty
 
    @retval 0    found
-   @retval 1    not found
+   @retval 1    error
 */
 
 static bool find_mpvio_user(MPVIO_EXT *mpvio, Security_context *sctx)
@@ -7351,8 +7352,27 @@ static bool find_mpvio_user(MPVIO_EXT *mpvio, Security_context *sctx)
 
   if (!mpvio->acl_user)
   {
-    login_failed_error(mpvio->thd);
-    return 1;
+    /*
+      A matching user was not found. Fake it. Take any user, make the
+      authentication fail later.
+      This way we get a realistically looking failure, with occasional
+      "change auth plugin" requests even for nonexistent users. The ratio
+      of "change auth plugin" request will be the same for real and
+      nonexistent users.
+      Note, that we cannot pick any user at random, it must always be
+      the same user account for the incoming sctx->user name.
+    */
+    ulong nr1=1, nr2=4;
+    CHARSET_INFO *cs= &my_charset_latin1;
+    cs->coll->hash_sort(cs, (uchar*) sctx->user, strlen(sctx->user), &nr1, &nr2);
+
+    pthread_mutex_lock(&acl_cache->lock);
+    uint i= nr1 % acl_users.elements;
+    ACL_USER *acl_user_tmp= dynamic_element(&acl_users, i, ACL_USER*);
+    mpvio->acl_user= acl_user_tmp->copy(mpvio->thd->mem_root);
+    pthread_mutex_unlock(&acl_cache->lock);
+
+    mpvio->make_it_fail= true;
   }
 
   /* user account requires non-default plugin and the client is too old */
@@ -7763,8 +7783,8 @@ static ulong parse_client_handshake_packet(MPVIO_EXT *mpvio,
                                    mpvio->cached_server_packet.pkt_len))
       return packet_error;
 
-    passwd_len= my_net_read(&mpvio->thd->net);
-    passwd = (char*)mpvio->thd->net.read_pos;
+    passwd_len= my_net_read(&thd->net);
+    passwd= (char*)thd->net.read_pos;
   }
 
   *buff= (uchar*)passwd;
@@ -7868,6 +7888,10 @@ static int server_mpvio_read_packet(MYSQL_PLUGIN_VIO *param, uchar **buf)
       *buf= (uchar*)mpvio->cached_client_reply.pkt;
       mpvio->cached_client_reply.pkt= 0;
       mpvio->packets_read++;
+
+      if (mpvio->make_it_fail)
+        goto err;
+
       return (int)mpvio->cached_client_reply.pkt_len;
     }
     /*
@@ -7901,12 +7925,18 @@ static int server_mpvio_read_packet(MYSQL_PLUGIN_VIO *param, uchar **buf)
   else
     *buf = mpvio->thd->net.read_pos;
 
+  if (mpvio->make_it_fail)
+    goto err;
+
   return (int)pkt_len;
 
 err:
   if (mpvio->status == MPVIO_EXT::FAILURE && !mpvio->thd->is_error())
   {
     inc_host_errors(&mpvio->thd->net.vio->remote.sin_addr);
+    if (mpvio->make_it_fail)
+      login_failed_error(mpvio->thd);
+    else
       my_error(ER_HANDSHAKE_ERROR, MYF(0));
   }
   return -1;
@@ -8116,6 +8146,7 @@ bool acl_authenticate(THD *thd, uint connect_errors,
   mpvio.thd= thd;
   mpvio.connect_errors= connect_errors;
   mpvio.status= MPVIO_EXT::FAILURE;
+  mpvio.make_it_fail= false;
 
   if (command == COM_CHANGE_USER)
   {