Subject: bug#75017: 31.0.50; Untrusted user lisp files

From: Björn Bidar <[email-masked]>
Date: Mon, 23 Dec 2024 21:15:32 +0200

Stefan Kangas <[email-masked]> writes:

> Eli Zaretskii <[email-masked]> writes:
>
>>> From: Stefan Kangas <[email-masked]>
>>> Date: Sun, 22 Dec 2024 17:36:15 +0000
>>> Cc: [email-masked], [email-masked], [email-masked]
>>>
>>> Eli Zaretskii <[email-masked]> writes:
>>>
>>> >> From: Stefan Monnier <[email-masked]>
>>> >> Cc: john muhl <[email-masked]>,  [email-masked],  Eli Zaretskii
>>> >>  <[email-masked]>,  Andrea Corallo <[email-masked]>
>>> >> Date: Sat, 21 Dec 2024 22:16:05 -0500
>>> >>
>>> >> > Maybe we should install something like the below?
>>> >>
>>> >> Fine by me, but I think this should be added via a new
>>> >> `trusted-content-function(s)` and added buffer-locally only in
>>> >> elisp-mode buffers.
>>> >
>>> > Sorry, but this is slippery slope.  For starters, no one said that
>>> > site-run-file is installed by a sysadmin -- that is only so on certain
>>> > systems.  For example, MS-Windows is generally not in that category.
>>>
>>> It doesn't matter who can edit it.  `site-run-file` is already trusted,
>>> since it is loaded at run-time before `user-init-file`.
>>
>> It is loaded if it is there.  On my system, there's no such file, and
>> I don't expect to have it.
>
> This seems orthogonal to the issue at hand.
>
> If you don't want to load `site-run-file`, you should use the
> --no-site-file flag.  (We should probably take that flag into account
> when saying if that file is `trusted-content-p` though.)

How does it make sense to not trust site-run-file when we trust the
site-lisp?
Further it is very likely or on Unix systems almost always the case that
Emacs was built by those who control the site-run-file. How is it
possible to trust them on the Emacs binary or anything elese included in
the Emacs package but not site-run-file?

>
> Without that flag, we load files in these well-known locations
> unconditionally.  In my view, it then makes little sense to worry about
> loading any `eval-when-compile` forms (or similar) in these files when
> byte-compiling them.  If they contain malicious code, that code has
> already been run when Emacs started, or it will be run the next time
> Emacs starts (e.g., if it has been modified after Emacs started).
>
> In other words, this case is quite analogous to `user-init-file`.
>
>> So if such a file somehow materializes there, I want to know, pronto.
>
> First, I note that it's likely already game over if an attacker can
> write to `site-init-file`, because they can then just as easily write to
> your init file (or other relevant files in `load-path`) instead.

Also by that point the attacker could already manipulate other files
such as the Emacs binary itself.

> But to do what you suggest, we would need to start with deciding under
> what circumstances it is not expected to find a file in this location,
> and then not just warn but refuse to load it if it meets that criteria.
> I don't know how to design such criteria.
>
> If we can figure out a way to do that, then I agree that it would be
> consistent not to treat this file as `trusted-content-p`, when it exists
> unexpectedly.

What about checking if the location of site-run-file machtes with the
location of the fiel during compilation e.g. by taking the value from
the pdump or configuring the check value into the executable without
pdump if that is better?