bug#37656: 27.0.50; Arbitrary code execution with special `mode:'

[Stefan Kangas <stefan@marxist.se>]

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Stefan Kangas <stefan@marxist.se> writes:
> The below patch seems to fix it by disabling the feature it exploits.

Here is a more complete patch.  Does it look like the right fix?

I think the relevant node in the documentation is:
(info "(emacs)Choosing Modes")

Best regards,
Stefan Kangas

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From d640efe970ed1fdd5f1262d09bfd6d564c8e7f80 Mon Sep 17 00:00:00 2001
From: Stefan Kangas <stefankangas@gmail.com>
Date: Wed, 16 Oct 2019 00:44:56 +0200
Subject: [PATCH] Remove support for more than one mode in file local variables

* lisp/files.el (set-auto-mode): Remove support for specifying
more than one major mode in file local variables.  (Bug#37656)
* etc/NEWS: Announce it.
---
 etc/NEWS      |  7 +++++++
 lisp/files.el | 13 +++++++------
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index d06f0a5952..d0cec05143 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -197,6 +197,13 @@ the new version of the file again.)
 \f
 * Changes in Emacs 27.1
 
+---
+** File local variables can now specify only one major mode.
+Previously, it was possible to specify more than one major mode using
+file local variables.  A security issue was discovered where an
+attacker could exploit this feature using a specially crafted file,
+which could lead to arbitrary code execution when the file was opened.
+
 ** emacsclient
 
 +++
diff --git a/lisp/files.el b/lisp/files.el
index 40807617fa..41c1ba60ed 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -3066,12 +3066,13 @@ set-auto-mode
 	   try-locals
 	   (setq end (set-auto-mode-1))
 	   (if (save-excursion (search-forward ":" end t))
-	       ;; Find all specifications for the `mode:' variable
-	       ;; and execute them left to right.
-	       (while (let ((case-fold-search t))
-			(or (and (looking-at "mode:")
-				 (goto-char (match-end 0)))
-			    (re-search-forward "[ \t;]mode:" end t)))
+	       ;; Find the specification of the `mode:' variable and
+	       ;; execute it.  We never want to execute more than one
+	       ;; mode for security reasons.  (Bug#37656)
+	       (when (let ((case-fold-search t))
+		       (or (and (looking-at "mode:")
+				(goto-char (match-end 0)))
+			   (re-search-forward "[ \t;]mode:" end t)))
 		 (skip-chars-forward " \t")
 		 (let ((beg (point)))
 		   (if (search-forward ";" end t)
-- 
2.23.0