Re: Patch: Add support for chroot to Worker#user

[Eric Wong <e@80x24.org>]

Jeremy Evans <code@jeremyevans.net> wrote:
> Any chrooting would need to happen inside Worker#user, because
> you can't chroot until after you have parsed the list of groups,
> and you must chroot before dropping root privileges.
> 
> chroot is an important security feature, so that if the unicorn
> process is exploited, file system access is limited to the chroot
> directory instead of the entire system.

I'm hesitant to document this as "an important security feature".

Perhaps "an extra layer of security" is more accurate,
as there are ways of breaking out of a chroot.

> This is not a complete patch as it does not include tests. I can
> add tests if you would consider accepting this feature.

I wouldn't worry about automated tests if elevated privileges
are required.

> -  # Changes the worker process to the specified +user+ and +group+
> +  # Changes the worker process to the specified +user+ and +group+,
> +  # and chroots to the current working directory if +chroot+ is set.
>    # This is only intended to be called from within the worker
>    # process from the +after_fork+ hook.  This should be called in
>    # the +after_fork+ hook after any privileged functions need to be
> @@ -123,7 +124,7 @@ def close # :nodoc:
>    # directly back to the caller (usually the +after_fork+ hook.
>    # These errors commonly include ArgumentError for specifying an
>    # invalid user/group and Errno::EPERM for insufficient privileges
> -  def user(user, group = nil)
> +  def user(user, group = nil, chroot = false)
>      # we do not protect the caller, checking Process.euid == 0 is
>      # insufficient because modern systems have fine-grained
>      # capabilities.  Let the caller handle any and all errors.
> @@ -134,6 +135,10 @@ def user(user, group = nil)
>        Process.initgroups(user, gid)
>        Process::GID.change_privilege(gid)
>      end
> +    if chroot
> +      Dir.chroot(Dir.pwd)
> +      Dir.chdir('/')
> +    end

Perhaps this can be made more flexible by allowing chroot to
any specified directory, not just pwd.  Something like:

    chroot = Dir.pwd if chroot == true
    if chroot
      Dir.chroot(chroot)
      Dir.chdir('/')
    end

Anyways I'm inclined to accept this feature.