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Re: "Local variables" denial-of-service attack
From: |
Ulrich Mueller |
Subject: |
Re: "Local variables" denial-of-service attack |
Date: |
Thu, 2 Apr 2009 10:15:00 +0200 |
>>>>> On Fri, 09 Jan 2009, Stefan Monnier wrote:
>> Local Variab*les:
>> byte-compile-warnings: #1=("circular" "object" . #1#)
>> End:
>> Visiting such a file will result in Emacs hanging and consuming all
>> CPU time. (Normally, aborting with C-g is possible. However, if the
>> file is opened with emacsclient there seems to be no way to recover.)
> Hmm... that's not good. It's not the end of the world since it's "only"
> a DoS, but we should try and fix it. I see two ways:
> 1 - don't allow (in file-local vars and .dir-local.el) the # escapes
> that permit creation of those circular objects.
> 2 - be more careful about circularities in the
> safe-local-variable predicates.
> Nb 1 is easier and will solve it "for all cases", so it's probably
> a better choice.
Coming back to this, the following patch works for me:
etc/NEWS entry:
** Reading of circular structures in file-local and directory-local
variable lists has been disabled.
lisp/ChangeLog entry:
2009-04-02 Ulrich Mueller <address@hidden>
* files.el (hack-local-variables-prop-line)
(hack-local-variables, dir-locals-read-from-file):
Bind read-circle to nil, in order to disable reading of recursive
structures in file-local and dir-local variables.
src/ChangeLog entry:
2009-04-02 Ulrich Mueller <address@hidden>
* lread.c (read_circle): New variable.
(syms_of_lread): DEFVAR_BOOL read-circle, initialize to t.
(read1): Disable #N= and #N# syntax for recursive structures if
read-circle is nil.
--- emacs/lisp/files.el~
+++ emacs/lisp/files.el
@@ -2896,10 +2896,11 @@
;; There used to be a downcase here,
;; but the manual didn't say so,
;; and people want to set var names that aren't all lc.
- (let ((key (intern (match-string 1)))
- (val (save-restriction
- (narrow-to-region (point) end)
- (read (current-buffer)))))
+ (let* ((read-circle nil)
+ (key (intern (match-string 1)))
+ (val (save-restriction
+ (narrow-to-region (point) end)
+ (read (current-buffer)))))
;; It is traditional to ignore
;; case when checking for `mode' in set-auto-mode,
;; so we must do that here as well.
@@ -3044,7 +3045,9 @@
(skip-chars-forward "^:\n")
(if (eolp) (error "Missing colon in local variables entry"))
(skip-chars-backward " \t")
- (let* ((str (buffer-substring beg (point)))
+ ;; Bind `read-circle' to nil to disable recursive structures.
+ (let* ((read-circle nil)
+ (str (buffer-substring beg (point)))
(var (read str))
val)
;; Read the variable value.
@@ -3323,7 +3326,8 @@
;; We should probably store the modtime of FILE and then
;; reload it whenever it changes.
(insert-file-contents file)
- (let* ((dir-name (file-name-directory file))
+ (let* ((read-circle nil)
+ (dir-name (file-name-directory file))
(class-name (intern dir-name))
(variables (read (current-buffer))))
(dir-locals-set-class-variables class-name variables)
--- emacs/src/lread.c~
+++ emacs/src/lread.c
@@ -156,6 +156,9 @@
/* List of (SYMBOL . POSITION) accumulated so far. */
Lisp_Object Vread_symbol_positions_list;
+/* Nonzero means allow #N= and #N# syntax for recursive structures. */
+static int read_circle;
+
/* List of descriptors now open for Fload. */
static Lisp_Object load_descriptor_list;
@@ -2558,7 +2561,7 @@
c = READCHAR;
}
/* #n=object returns object, but associates it with n for #n#. */
- if (c == '=')
+ if (c == '=' && read_circle)
{
/* Make a placeholder for #n# to use temporarily */
Lisp_Object placeholder;
@@ -2580,7 +2583,7 @@
return tem;
}
/* #n# returns a previously read object. */
- if (c == '#')
+ if (c == '#' && read_circle)
{
tem = Fassq (make_number (n), read_objects);
if (CONSP (tem))
@@ -4215,6 +4218,12 @@
were read in. */);
Vread_symbol_positions_list = Qnil;
+ DEFVAR_BOOL ("read-circle", &read_circle,
+ doc: /* Non-nil means allow #N= and #N# syntax for recursive
structures.
+This is normally bound by functions like `hack-local-variables' to
+disable this syntax in `read', and is not meant for users to change. */);
+ read_circle = 1;
+
DEFVAR_LISP ("load-path", &Vload_path,
doc: /* *List of directories to search for files to load.
Each element is a string (directory name) or nil (try default directory).
- Re: "Local variables" denial-of-service attack,
Ulrich Mueller <=