bug#28350: enriched.el code execution

[Charles A. Roelli]

> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sat, 9 Sep 2017 15:43:30 -0700
> 
> Thanks for reporting this bug. Since it is a serious security hole I have 
> installed a patch by Lars Ingebrigtsen that temporarily disables the 
> problematic 
> translations, and that also changes Gnus to not call enriched-decode. For the 
> emacs-25 branch the patch is here:
> 
> https://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-25&id=9ad0fcc54442a9a01d41be19880250783426db70
> 
> and for the master branch the patch is here:
> 
> https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=19584f13b1e2e4a778602a8302619ef5c675e68b
> 
> As this patch is merely a workaround to close the security hole, I am not 
> marking the underlying bug as fixed.
> 
> Thank you for reporting the problem.

Thanks for these fixes.  I have some comments:

> branch: master
> commit 19584f13b1e2e4a778602a8302619ef5c675e68b
> Author: Lars Ingebrigtsen <larsi@gnus.org>
> Commit: Paul Eggert <eggert@cs.ucla.edu>
>
> [...]
>
> --- a/lisp/textmodes/enriched.el
> +++ b/lisp/textmodes/enriched.el
> @@ -117,12 +117,7 @@ expression, which is evaluated to get the string to 
> insert.")
>                  (full        "flushboth")
>                  (center      "center"))
>      (PARAMETER     (t           "param")) ; Argument of preceding annotation
> -    ;; The following are not part of the standard:
> -    (FUNCTION      (enriched-decode-foreground "x-color")
> -                (enriched-decode-background "x-bg-color")

Do we know that "x-color" and/or "x-bg-color" are vulnerable to a
similar misuse as "x-display"?  If not, I can still re-add them at a
later time.

> branch: emacs-25
> commit b6389930146882a77c22901a4357e287826fc7ff
> Author: Paul Eggert <eggert@cs.ucla.edu>
> Commit: Paul Eggert <eggert@cs.ucla.edu>
>
> [...]
>
> +** Enriched text mode no longer supports the 'FUNCTION' and 'display'
> +translations, and Gnus no longer processes enriched text when
> +inlining.  This fixes bugs introduced in Emacs 19.29.  To work around
> +these bugs in Emacs versions 19.29 through 25.2, append the following
> +to your ~/.emacs file:
> +
> +  (provide 'enriched)
> +  (defun enriched-mode (&optional arg))
> +  (defun enriched-decode (from to))

This fix is very safe, at the cost of disabling Enriched mode.  Could
we do any better?  I had suggested the following (in
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=28350#16):

  (eval-after-load "enriched"
    '(defun enriched-decode-display-prop (start end &optional param)
       (list start end)))

But it may not work in Emacs earlier than 23 (I can't test it).