Merge "Allow users to block the user that blocked them."

diff --combined includes/specials/SpecialBlock.php
index e043a72,03006f3..78b8f8a
--- 1/includes/specials/SpecialBlock.php
--- 2/includes/specials/SpecialBlock.php
+++ b/includes/specials/SpecialBlock.php
@@@ -66,7 -66,6 +66,6 @@@ class SpecialBlock extends FormSpecialP
         */
        protected function checkExecutePermissions( User $user ) {
                parent::checkExecutePermissions( $user );
- 
                # T17810: blocked admins should have limited access here
                $status = self::checkUnblockSelf( $this->target, $user );
                if ( $status !== true ) {
@@@ -74,6 -73,15 +73,15 @@@
                }
        }
  
+       /**
+        * We allow certain special cases where user is blocked
+        *
+        * @return bool
+        */
+       public function requiresUnblock() {
+               return false;
+       }
+ 
        /**
         * Handle some magic here
         *
@@@ -386,7 -394,6 +394,7 @@@
         * @return string
         */
        protected function preText() {
 +              $this->getOutput()->addModuleStyles( 'mediawiki.widgets.TitlesMultiselectWidget.styles' );
                $this->getOutput()->addModules( [ 'mediawiki.special.block' ] );
  
                $blockCIDRLimit = $this->getConfig()->get( 'BlockCIDRLimit' );
@@@ -1022,7 -1029,11 +1030,11 @@@
         * T17810: blocked admins should not be able to block/unblock
         * others, and probably shouldn't be able to unblock themselves
         * either.
-        * @param User|int|string $user
+        *
+        * Exception: Users can block the user who blocked them, to reduce
+        * advantage of a malicious account blocking all admins (T150826)
+        *
+        * @param User|int|string $user Target to block or unblock
         * @param User $performer User doing the request
         * @return bool|string True or error message key
         */
@@@ -1032,7 -1043,6 +1044,6 @@@
                } elseif ( is_string( $user ) ) {
                        $user = User::newFromName( $user );
                }
- 
                if ( $performer->isBlocked() ) {
                        if ( $user instanceof User && $user->getId() == $performer->getId() ) {
                                # User is trying to unblock themselves
@@@ -1044,6 -1054,19 +1055,19 @@@
                                } else {
                                        return 'ipbnounblockself';
                                }
+                       } elseif (
+                               $user instanceof User &&
+                               $performer->getBlock() instanceof Block &&
+                               $performer->getBlock()->getBy() &&
+                               $performer->getBlock()->getBy() === $user->getId()
+                       ) {
+                               // Allow users to block the user that blocked them.
+                               // This is to prevent a situation where a malicious user
+                               // blocks all other users. This way, the non-malicious
+                               // user can block the malicious user back, resulting
+                               // in a stalemate.
+                               return true;
+ 
                        } else {
                                # User is trying to block/unblock someone else
                                return 'ipbblocked';