Add user deletion permission check in `Users::DestroyService`

We saw from a recent incident that the `Users::DestroyService` would
attempt to delete a user over and over. Revoking the permissions
from the current user did not help. We should ensure that the
current user does, in fact, have permissions to delete the user.
Signed-off-by: Rémy Coutable <remy@rymai.me>

app/services/users/destroy_service.rb

@@ -7,6 +7,10 @@ module Users
     end
 
     def execute(user, options = {})
+      unless current_user.admin? || current_user == user
+        raise Gitlab::Access::AccessDeniedError, "#{current_user} tried to destroy user #{user}!"
+      end
+
       if !options[:delete_solo_owned_groups] && user.solo_owned_groups.present?
         user.errors[:base] << 'You must transfer ownership or delete groups before you can remove user'
         return user

app/workers/delete_user_worker.rb

@@ -7,5 +7,7 @@ class DeleteUserWorker
     current_user = User.find(current_user_id)
 
     Users::DestroyService.new(current_user).execute(delete_user, options.symbolize_keys)
+  rescue Gitlab::Access::AccessDeniedError => e
+    Rails.logger.warn("User could not be destroyed: #{e}")
   end
 end

changelogs/unreleased/sh-delete-user-permission-check.yml

+---
+title: Add user deletion permission check in `Users::DestroyService`
+merge_request:
+author:

spec/services/users/destroy_spec.rb

@@ -3,10 +3,10 @@ require 'spec_helper'
 describe Users::DestroyService, services: true do
   describe "Deletes a user and all their personal projects" do
     let!(:user)      { create(:user) }
-    let!(:current_user) { create(:user) }
+    let!(:admin)     { create(:admin) }
     let!(:namespace) { create(:namespace, owner: user) }
     let!(:project)   { create(:project, namespace: namespace) }
-    let(:service)       { described_class.new(current_user) }
+    let(:service)    { described_class.new(admin) }
 
     context 'no options are given' do
       it 'deletes the user' do
@@ -57,5 +57,26 @@ describe Users::DestroyService, services: true do
         expect { User.find(user.id) }.to raise_error(ActiveRecord::RecordNotFound)
       end
     end
+
+    context "deletion permission checks" do
+      it 'does not delete the user when user is not an admin' do
+        other_user = create(:user)
+
+        expect { described_class.new(other_user).execute(user) }.to raise_error(Gitlab::Access::AccessDeniedError)
+        expect(User.exists?(user.id)).to be(true)
+      end
+
+      it 'allows admins to delete anyone' do
+        described_class.new(admin).execute(user)
+
+        expect(User.exists?(user.id)).to be(false)
+      end
+
+      it 'allows users to delete their own account' do
+        described_class.new(user).execute(user)
+
+        expect(User.exists?(user.id)).to be(false)
+      end
+    end
   end
 end