Add FileUploader.root to allowed upload paths

Currently we check if uploaded file is under
`Gitlab.config.uploads.storage_path`, the problem is that
uploads are placed in `uploads` subdirectory which is symlink.

In allow_path? method we check real (expanded) paths, which causes
that `Gitlab.config.uploads.storage_path` is expaned into symlink
path and there is a mismatch with upload file path.

By adding `Gitlab.config.uploads.storage_path/uploads` into allowed
paths, this path is expaned during path check.

`Gitlab.config.uploads.storage_path` is left there intentionally in case
some uploader wouldn't use `uploads` subdir.

changelogs/unreleased/jprovazn-upload-symlink.yml

+---
+title: Add /uploads subdirectory to allowed upload paths.
+merge_request:
+author:
+type: fixed

lib/gitlab/middleware/multipart.rb

@@ -84,7 +84,7 @@ module Gitlab
         def open_file(params, key)
           ::UploadedFile.from_params(
             params, key,
-            Gitlab.config.uploads.storage_path)
+            [FileUploader.root, Gitlab.config.uploads.storage_path])
         end
       end
 

lib/uploaded_file.rb

@@ -28,7 +28,7 @@ class UploadedFile
     @tempfile = File.new(path, 'rb')
   end
 
-  def self.from_params(params, field, upload_path)
+  def self.from_params(params, field, upload_paths)
     unless params["#{field}.path"]
       raise InvalidPathError, "file is invalid" if params["#{field}.remote_id"]
 
@@ -37,7 +37,8 @@ class UploadedFile
 
     file_path = File.realpath(params["#{field}.path"])
 
-    unless self.allowed_path?(file_path, [upload_path, Dir.tmpdir].compact)
+    paths = Array.wrap(upload_paths) << Dir.tmpdir
+    unless self.allowed_path?(file_path, paths.compact)
       raise InvalidPathError, "insecure path used '#{file_path}'"
     end