tmpio.rb and taint mode

[Terry Scheingeld]

tmpio.rb causes an "insecure operation" error when being run in taint
mode. This is due not to a problem in tmpio.rb but in Ruby's File
class. Here are the details on the problem and a simple workaround for
it.

I filed this bug report in February 2018:
https://bugs.ruby-lang.org/issues/14485. The problem is that when a
File object is created using an untainted string for the path, File
nevertheless changes that path to tainted. It is agreed thatit's a
bug: File should not taint an untainted path. However, efforts to fix
the bug seem to have stalled out.

Now, in tmpio.rb, a random, untainted path is generated and stored in
the Unicorn::TmpIO object. Then, a few lines later, the class attempts
to unlink that file using the path stored in the object. Because of
the bug in File, the path is now tainted, resulting in an insecure
operation error.

I propose a simple workaround. Store the path in its own variable.
Pass the variable to the Unicorn::TmpIO object, but use the original
variable to unlink the file. This technique worked in experimentation
for me. Here's a modified version of tmpio.rb.

# -*- encoding: binary -*-
# :stopdoc:
require 'tmpdir'

# some versions of Ruby had a broken Tempfile which didn't work
# well with unlinked files.  This one is much shorter, easier
# to understand, and slightly faster.
class Unicorn::TmpIO < File

    # creates and returns a new File object.  The File is unlinked
    # immediately, switched to binary mode, and userspace output
    # buffering is disabled
    def self.new
        path = nil

        fp = begin
            path = "#{Dir::tmpdir}/#{rand}"
            super(path, RDWR|CREAT|EXCL, 0600)
        rescue Errno::EEXIST
            retry
        end

        unlink(path)
        fp.binmode
        fp.sync = true
        fp
    end

    # pretend we're Tempfile for Rack::TempfileReaper
    alias close! close
end