Prevent extraction from escaping destination_dir via pre-existing symlinks

lib/rubygems/package.rb

@@ -448,13 +448,18 @@ def extract_tar_gz(io, destination_dir, pattern = "*") # :nodoc:
             File.dirname destination
           end
 
-        unless directories.include?(mkdir)
-          FileUtils.mkdir_p mkdir, mode: dir_mode ? 0o755 : (entry.header.mode if entry.directory?)
-          directories << mkdir
-        end
+unless directories.include?(mkdir)
+  FileUtils.mkdir_p mkdir, mode: dir_mode ? 0o755 : (entry.header.mode if entry.directory?)
+  directories << mkdir
+end
+
+real_mkdir = File.realpath(mkdir)
+unless real_mkdir == destination_dir || normalize_path(real_mkdir).start_with?(normalize_path(destination_dir + "/"))
+  raise Gem::Package::PathError.new(real_mkdir, destination_dir)
+end
 
-        if entry.file?
-          File.open(destination, "wb") do |out|
+if entry.file?
+  File.open(destination, "wb") do |out|
             copy_stream(tar.io, out, entry.size)
             # Flush needs to happen before chmod because there could be data
             # in the IO buffer that needs to be written, and that could be

test/rubygems/test_gem_package.rb

@@ -615,6 +615,33 @@ def test_extract_tar_gz_symlink_directory
                  File.read(extracted)
   end
 
+
+def test_extract_tar_gz_rejects_preexisting_symlink_escape
+  omit "Symlinks not supported or not enabled" unless symlink_supported?
+
+  package = Gem::Package.new @gem
+
+  tgz_io = util_tar_gz do |tar|
+    tar.add_file "lib/owned.txt", 0o644 do |io|
+      io.write "poc-content"
+    end
+  end
+
+  escape_dir = File.join(@tempdir, "escape")
+  FileUtils.mkdir_p escape_dir
+
+  FileUtils.rm_rf File.join(@destination, "lib")
+  File.symlink escape_dir, File.join(@destination, "lib")
+
+  escaped = File.join(escape_dir, "owned.txt")
+
+  assert_raise Gem::Package::PathError do
+    package.extract_tar_gz tgz_io, @destination
+  end
+
+  refute File.exist?(escaped), "must not write outside extraction root via symlink"
+end
+
   def test_extract_symlink_into_symlink_dir
     omit "Symlinks not supported or not enabled" unless symlink_supported?
     package = Gem::Package.new @gem