Re: emacs-29 3c1693d08b0: Fix Elisp code injection vulnerability in emacsclient-mail.desktop

[Jim Porter]

On 3/8/2023 9:20 AM, Robert Pluim wrote:
> > > > > > On Wed, 8 Mar 2023 09:03:30 -0800, Jim Porter <jporterbugs@gmail.com> said:
>      Jim> In bug#57752, we'd discussed adding --apply to emacs and emacsclient,
>      Jim> which might work better for this case, as well as to make other
>      Jim> similar cases easier: Org mode uses some pretty extensive hacks in
>      Jim> order to get org-protocol:// URLs working in emacsclient, and
>      Jim> eliminating that would be very nice.
>
> Thanks for the reference. Iʼve re-read the report, and the
> sort-of-consensus was that we needed '--apply' and a `set-arg'
> function.  Eli, would that be acceptable? (my patch called `apply'
> anyway, so itʼs not too big a change :-) )

'set-arg' is probably simple enough that we could expect users to write 
it themselves. '--apply' is a bit tricky (for emacsclient at least), 
since we'd need to properly escape strings. I guess the complexity of 
doing this would depend on how we did the escaping though.

For reference for this thread, the conclusion we came to in bug#57752 
was an interface like this:

  emacs --apply func1 arg1 arg2 -- --apply func2 arg3 arg4

(Ditto for emacsclient.)

----------------------------------------

For Org mode, the problem is that it wants to support "org-protocol": 
this is a special URL protocol that lets you capture bits of text (or 
whatever, really) into an Org file[1]. In order to avoid the escaping 
issues mentioned in this thread, Org has to jump through a lot of hoops, 
advising several functions in server.el (see org-protocol.el and this 
thread[2]). Note: This also uses .desktop files on systems using XDG 
(sorry, Eli), but that's just how you register URL protocols on those 
systems; not much we can do about that.

That said, the '--apply' argument would (debatably) be useful in other 
places too: for example, if I wanted a shell command to open a link in 
EWW, I could define an alias like:

  alias eww="emacs --apply eww"
  # or
  alias eww="emacsclient --apply eww"

Or you could use it with 'view-file' to make an alias to open a file in 
Emacs just for viewing. (And you could do similar things any time you 
want to pass an arbitrary string to Emacs from a script.)

Currently, you can do all this with the main emacs binary by writing 
your own function that calls '(pop command-line-args-left)' (see 
'message-mailto'), but as the commit from this thread suggests, that's 
not possible with emacsclient currently. It also means that even for the 
main emacs binary, you need to specially-write your function to use 
'command-line-args-left' instead of being able to call existing 
functions directly.

[1] https://orgmode.org/worg/org-contrib/org-protocol.html

[2] https://lists.gnu.org/archive/html/emacs-orgmode/2022-02/msg00056.html