From: charles@aurox.ch (Charles A. Roelli)
To: Eli Zaretskii <eliz@gnu.org>
Cc: 28350@debbugs.gnu.org
Subject: bug#28350: enriched.el code execution
Date: Sat, 09 Sep 2017 22:37:29 +0200 [thread overview]
Message-ID: <m260cra9rq.fsf@aurox.ch> (raw)
In-Reply-To: <83wp57vmk6.fsf@gnu.org> (message from Eli Zaretskii on Sat, 09 Sep 2017 19:55:37 +0300)
[-- Attachment #1: Type: text/plain, Size: 2391 bytes --]
> Date: Sat, 09 Sep 2017 19:55:37 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> CC: 28350@debbugs.gnu.org
>
> > > > +See Info node `(elisp)Display Property' for the use of these
> > > > +display specifications."
> > > > + (ignore-errors
> > > > + (or (stringp prop)
> > > ^^^^^^^^^^^^
> > > What about an image spec (including a slice spec)?
> >
> > Okay, I see that image specs can be safe. But are they all safe?
>
> I think they are. Does anyone know different?
I read over the documentation some more and they do look alright.
> > And I don't understand how a slice spec is used together with an image
> > spec. Is the slice spec used inside of IMAGE-PROPS, i.e. as you might
> > gather from the manual:
> >
> > ‘(image . IMAGE-PROPS)’
> > This kind of display specification is an image descriptor (*note
> > Images). When used as a display specification, it means to
> > display the image instead of the text that has the display
> > specification.
> >
> > ‘(slice X Y WIDTH HEIGHT)’
> > This specification together with ‘image’ specifies a “slice” (a
> > partial area) of the image to display.
> >
> > ?
>
> AFAIU, like this:
>
> ((slice X Y WIDTH HEIGHT) (image . IMAGE-PROPS))
>
> You can see examples of this in image.el and image-mode.el.
Thanks. I forgot that the display property can be set to a list or
vector of display specifications. I've updated the patch to reflect
this:
+ (and (seqp prop) (seq-every-p 'enriched-display-prop-safe-p prop)))))
and I've added slice/image specifications.
> > At this point it seems that unsafe display specs are more the
> > exception than the rule, so it might make sense to define the
> > `enriched-display-prop-safe-p' function by excluding the unsafe
> > specifications instead of including the safe ones. What do you
> > think?
>
> I'm not sure. The display spec can be complex, so to make sure none
> of these exceptions sneak through, you will have to recursively unpack
> the spec data structure and examine each of the elements, which smells
> too similar to emulating 'eval'. No?
Thank you. I've kept the current approach. Please see again the
attached patch.
Also, should the left-fringe/right-fringe display specifications be
considered safe? They seem innocuous.
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From baf533eeddc185a0e65c641022f7be2be2cbcb09 Mon Sep 17 00:00:00 2001
From: "Charles A. Roelli" <charles@aurox.ch>
Date: Sat, 9 Sep 2017 14:03:58 +0200
Subject: [PATCH] Prevent code execution by text/enriched files (Bug#28350)
* lisp/textmodes/enriched.el (enriched-allow-unsafe-display-props):
New customizable option.
(enriched-display-prop-safe-p): New function.
(enriched-decode-display-prop): Use the new function to prevent unsafe
display properties from being applied.
---
lisp/textmodes/enriched.el | 84 +++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 83 insertions(+), 1 deletion(-)
diff --git a/lisp/textmodes/enriched.el b/lisp/textmodes/enriched.el
index 7ace2a5..74a1229 100644
--- a/lisp/textmodes/enriched.el
+++ b/lisp/textmodes/enriched.el
@@ -147,6 +147,20 @@ enriched-mode-hook
:type 'hook
:group 'enriched)
+(defcustom enriched-allow-unsafe-display-props nil
+ "Variable determining whether to decode arbitrary display properties.
+
+Enriched mode recognizes display properties of text stored using
+an extension command to the text/enriched format, \"x-display\".
+These properties must, by default, satisfy
+`enriched-display-prop-safe-p' (q.v.), otherwise they are not
+applied. Customize this option to t to turn off this safety
+feature. Note, however, that applying unsafe display properties
+can execute arbitrary Lisp code."
+ :risky t
+ :type 'boolean
+ :group 'enriched)
+
(defvar enriched-old-bindings nil
"Store old variable values that we change when entering mode.
The value is a list of \(VAR VALUE VAR VALUE...).")
@@ -503,6 +517,74 @@ enriched-decode-display-prop
(error nil)))))
(unless prop
(message "Warning: invalid <x-display> parameter %s" param))
- (list start end 'display prop)))
+ (if (enriched-display-prop-safe-p prop)
+ (list start end 'display prop)
+ (message "Warning: unsafe <x-display> parameter %s not applied" param)
+ (list start end))))
+
+(defun enriched-display-prop-safe-p (prop)
+ "Return t if display property PROP is safe to apply to text.
+
+This function always returns t when
+`enriched-allow-unsafe-display-props' is set to t.
+
+A safe display property is either:
+
+ - a string,
+
+ - an image display specification, (image . image-props), where
+ IMAGE-PROPS is a property list,
+
+ - a slice display specification, (slice x y width height),
+ where X and Y are integers, and WIDTH and HEIGHT are either
+ integers or floats,
+
+ - a space display specification, (space . props), where PROPS
+ is a property list,
+
+ - a space-width display specification, (space-width factor),
+ where FACTOR is an integer or a float,
+
+ - a margin display specification, ((margin right-margin) spec)
+ or ((margin left-margin) spec), where SPEC is a string or an
+ image display specification as above,
+
+ - a height display specification, (height spec), where SPEC is
+ of the form (+ n), (- n) or n, and N is an integer or a
+ float,
+
+ - a raise display specification, (raise factor), where
+ FACTOR is an integer or a float,
+
+ - or a list/vector containing safe display specifications, as
+ above.
+
+See Info node `(elisp)Display Property' for the use of these
+display specifications."
+ (ignore-errors
+ (or enriched-allow-unsafe-display-props
+ (stringp prop)
+ (and (consp prop) (eq (car prop) 'image))
+ (and (consp prop)
+ (eq (car prop) 'slice)
+ (integerp (elt prop 1)) ; x
+ (integerp (elt prop 2)) ; y
+ (numberp (elt prop 3)) ; width
+ (numberp (elt prop 4))) ; height
+ (and (consp prop) (eq (car prop) 'space))
+ (and (eq (car prop) 'space-width) (numberp (cadr prop)))
+ (and (consp (car prop))
+ (eq (caar prop) 'margin)
+ (or (eq (cadar prop) 'right-margin)
+ (eq (cadar prop) 'left-margin))
+ (enriched-display-prop-safe-p (cadr prop)))
+ (and (eq (car prop) 'height)
+ (or (numberp (cadr prop))
+ (and (listp (cadr prop))
+ (or (eq (elt (cadr prop) 0) '+) (elt (cadr prop) 0) '-)
+ (integerp (elt (cadr prop) 1)))))
+ (and (eq (car prop) 'raise)
+ (numberp (cadr prop)))
+ (and (seqp prop) (seq-every-p 'enriched-display-prop-safe-p prop)))))
;;; enriched.el ends here
--
2.9.4
next prev parent reply other threads:[~2017-09-09 20:37 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-04 19:24 bug#28350: enriched.el code execution Charles A. Roelli
2017-09-06 19:25 ` Charles A. Roelli
2017-09-07 2:34 ` Eli Zaretskii
2017-09-09 12:23 ` Charles A. Roelli
2017-09-09 13:45 ` Eli Zaretskii
2017-09-09 15:57 ` Charles A. Roelli
2017-09-09 16:55 ` Eli Zaretskii
2017-09-09 20:37 ` Charles A. Roelli [this message]
2017-09-10 17:01 ` Eli Zaretskii
2017-09-11 16:32 ` Glenn Morris
2017-09-11 17:01 ` Eli Zaretskii
2017-09-09 22:43 ` Paul Eggert
2017-09-10 18:54 ` Charles A. Roelli
2017-09-10 21:46 ` Paul Eggert
2017-09-11 2:39 ` Eli Zaretskii
2017-09-11 14:22 ` Eli Zaretskii
2017-09-11 15:18 ` Eli Zaretskii
2017-09-11 18:44 ` Charles A. Roelli
2017-09-11 19:07 ` Eli Zaretskii
2017-09-16 9:48 ` Eli Zaretskii
2017-09-11 15:33 ` Glenn Morris
2017-09-11 16:38 ` Paul Eggert
2017-09-11 21:16 ` Glenn Morris
2017-09-12 19:59 ` Paul Eggert
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