Keep the build context inside the build directory

lib/docker/util.rb

@@ -173,14 +173,25 @@ def docker_context(directory)
   end
 
   def create_relative_dir_tar(directory, output)
+    root = File.realpath(directory)
+
     Gem::Package::TarWriter.new(output) do |tar|
       files = docker_context(directory)
 
       files.each do |prefixed_file_name|
-        stat = File.stat(prefixed_file_name)
+        stat = File.lstat(prefixed_file_name)
+        unprefixed_file_name = prefixed_file_name[directory.length..-1]
+
+        if stat.symlink?
+          tar.add_symlink(
+            unprefixed_file_name, File.readlink(prefixed_file_name), stat.mode
+          )
+          next
+        end
         next unless stat.file?
 
-        unprefixed_file_name = prefixed_file_name[directory.length..-1]
+        next unless contained?(prefixed_file_name, root)
+
         add_file_to_tar(
           tar, unprefixed_file_name, stat.mode, stat.size, stat.mtime
         ) do |tar_file|
@@ -190,6 +201,14 @@ def create_relative_dir_tar(directory, output)
     end
   end
 
+  # True when path resolves, with every symlink and '..' expanded, to
+  # something underneath root.
+  def contained?(path, root)
+    File.realpath(path).start_with?(root + File::SEPARATOR)
+  rescue SystemCallError
+    false
+  end
+
   def add_file_to_tar(tar, name, mode, size, mtime)
     tar.check_closed
 

spec/docker/util_spec.rb

@@ -226,6 +226,52 @@ def expect_tar_entries(*entries)
         expect(files_in_tar(tar)).to eq ['.dockerignore', 'zig']
       end
     end
+
+    describe 'build context containment' do
+      attr_accessor :outside
+
+      def entries_in_tar(tar)
+        Gem::Package::TarReader.new(tar) do |content|
+          return content.map { |e| [e.full_name, e.header.typeflag, e.read.to_s] }
+        end
+      end
+
+      around do |example|
+        Dir.mktmpdir do |dir|
+          self.outside = dir
+          File.write("#{dir}/secret", 'OUTSIDE')
+          example.call
+        end
+      end
+
+      it 'archives a symlink as a symlink rather than following it' do
+        File.symlink("#{outside}/secret", "#{tmpdir}/innocent.txt")
+
+        name, typeflag, body = entries_in_tar(tar).first
+        expect(name).to eq 'innocent.txt'
+        expect(typeflag).to eq '2'
+        expect(body).to_not include 'OUTSIDE'
+      end
+
+      it 'does not raise on a dangling symlink' do
+        File.symlink("#{tmpdir}/no_such_target", "#{tmpdir}/dangling")
+
+        expect { files_in_tar(tar) }.to_not raise_error
+      end
+
+      it 'does not pack a file a negation pattern reaches outside the build directory' do
+        File.write("#{tmpdir}/.dockerignore", "!#{'../' * 12}#{outside[1..]}/secret")
+
+        expect(files_in_tar(tar)).to eq ['.dockerignore']
+      end
+
+      it 'does not pack a file reached through a symlinked directory' do
+        File.symlink(outside, "#{tmpdir}/a_link")
+        File.write("#{tmpdir}/.dockerignore", '!a_link/secret')
+
+        expect(files_in_tar(tar)).to eq ['.dockerignore', 'a_link']
+      end
+    end
   end
 
   describe '.build_auth_header' do