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Subject: bug#77341: [PATCH] ; (find-function-search-for-symbol): Be cautious with macros.
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--=-=-=
Content-Type: text/plain

Tags: patch

Hi,

find-function may expand Lisp macros in a source file when it fails to
find a definition otherwise.  This patch restricts this fallback to
trusted buffers only, to protect against possibly harmful macros.

Eshel

--=-=-=
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 filename=0001-find-function-search-for-symbol-Be-cautious-with-mac.patch

From a29b2c0888a19069aef8ef248979c4b463e26f5c Mon Sep 17 00:00:00 2001
From: Eshel Yaron <me@HIDDEN>
Date: Fri, 28 Mar 2025 17:57:35 +0100
Subject: [PATCH] ; (find-function-search-for-symbol): Be cautious with macros.

* lisp/emacs-lisp/find-func.el
(find-function-search-for-symbol): Only attempt to expand
macros in trusted buffers.
---
 lisp/emacs-lisp/find-func.el | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/lisp/emacs-lisp/find-func.el b/lisp/emacs-lisp/find-func.el
index 8f488a9c00a..6bdfb4bc38b 100644
--- a/lisp/emacs-lisp/find-func.el
+++ b/lisp/emacs-lisp/find-func.el
@@ -487,11 +487,14 @@ find-function-search-for-symbol
                 ;; If the regexp search didn't find the location of
                 ;; the symbol (for example, because it is generated by
                 ;; a macro), try a slightly more expensive search that
-                ;; expands macros until it finds the symbol.
+                ;; expands macros until it finds the symbol.  Since
+                ;; macro-expansion involves arbitrary code execution,
+                ;; only attempt it in trusted buffers.
                 (cons (current-buffer)
-                      (find-function--search-by-expanding-macros
-                       (current-buffer) symbol type
-                       form-matcher-factory))))))))))
+                      (when (trusted-content-p)
+                        (find-function--search-by-expanding-macros
+                         (current-buffer) symbol type
+                         form-matcher-factory)))))))))))

 ;;;###autoload
 (defun find-function-update-type-alist (symbol type variable)
-- 
2.48.1

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Subject: bug#77341: Acknowledgement ([PATCH] ; (find-function-search-for-symbol):
 Be cautious with macros.)
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Subject: bug#77341: [PATCH] ; (find-function-search-for-symbol): Be cautious with macros.
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On March 28, 2025 1:28:06 PM EDT, "Eshel Yaron via Bug reports for GNU Ema=
cs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu=2Eorg> wrote:
>Tags: patch
>
>Hi,
>
>find-function may expand Lisp macros in a source file when it fails to
>find a definition otherwise=2E  This patch restricts this fallback to
>trusted buffers only, to protect against possibly harmful macros=2E

I get not wanting to execute code from random files I'm just visiting, but=
 if I've already actually evaluated a macro function and installed it in my=
 Emacs function namespace as something I can call, is it all that dangerous=
 to call it? Instead of a blanket prohibition on macro expansion, I'd rathe=
r have macros declare that they're safe to run on untrusted inputs, which m=
eans mostly they don't eval their arguments=2E

Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#77341: [PATCH] ; (find-function-search-for-symbol): Be cautious with macros.
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On March 28, 2025 1:28:06 PM EDT, "Eshel Yaron via Bug reports for GNU Ema=
cs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu=2Eorg> wrote:
>Tags: patch
>
>Hi,
>
>find-function may expand Lisp macros in a source file when it fails to
>find a definition otherwise=2E  This patch restricts this fallback to
>trusted buffers only, to protect against possibly harmful macros=2E

I get not wanting to execute code from random files I'm just visiting, but=
 if I've already actually evaluated a macro function and installed it in my=
 Emacs function namespace as something I can call, is it all that dangerous=
 to call it? Instead of a blanket prohibition on macro expansion, I'd rathe=
r have macros declare that they're safe to run on untrusted inputs, which m=
eans mostly they don't eval their arguments=2E

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Daniel Colascione <dancol@HIDDEN> writes:

> On March 28, 2025 1:28:06 PM EDT, Eshel Yaron wrote:
>>Tags: patch
>>
>>find-function may expand Lisp macros in a source file when it fails to
>>find a definition otherwise.  This patch restricts this fallback to
>>trusted buffers only, to protect against possibly harmful macros.
>
> I get not wanting to execute code from random files I'm just visiting,
> but if I've already actually evaluated a macro function and installed
> it in my Emacs function namespace as something I can call, is it all
> that dangerous to call it? 

find-function searches through code you haven't evaluated/loaded too.
Even for loaded libraries, the source file/buffer contents may be
different than the loaded code.  Either way, if you trust some files,
you can add them to trusted-content.  If you haven't, that means they
are untrusted.

In general, as long as macro-expansion remains unsafe, we should avoid
expanding untrusted macros in commands that merely edit/browse Lisp code
(in contrast with compiling/evaluating it).

> Instead of a blanket prohibition on macro expansion,

(To be clear, I wouldn't say there's a prohibition on macro expansion,
just a restriction to trusted code, similarly to proper code evaluation,
since they're not that different in practice.)

> I'd rather have macros declare that they're safe to run on untrusted
> inputs, which means mostly they don't eval their arguments.

Yes, please :)
Even better, we should have a safe evaluation sandbox that can be used
for safe macro-expansion among other things.  Indeed, any solution that
allows us to safely expand (most) macros would be a great improvement.
But until we have something like that, we should guard macro-expansion
behind trusted-content-p checks.


Regards,

Eshel

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Daniel Colascione <dancol@HIDDEN> writes:

> On March 28, 2025 1:28:06 PM EDT, Eshel Yaron wrote:
>>Tags: patch
>>
>>find-function may expand Lisp macros in a source file when it fails to
>>find a definition otherwise.  This patch restricts this fallback to
>>trusted buffers only, to protect against possibly harmful macros.
>
> I get not wanting to execute code from random files I'm just visiting,
> but if I've already actually evaluated a macro function and installed
> it in my Emacs function namespace as something I can call, is it all
> that dangerous to call it? 

find-function searches through code you haven't evaluated/loaded too.
Even for loaded libraries, the source file/buffer contents may be
different than the loaded code.  Either way, if you trust some files,
you can add them to trusted-content.  If you haven't, that means they
are untrusted.

In general, as long as macro-expansion remains unsafe, we should avoid
expanding untrusted macros in commands that merely edit/browse Lisp code
(in contrast with compiling/evaluating it).

> Instead of a blanket prohibition on macro expansion,

(To be clear, I wouldn't say there's a prohibition on macro expansion,
just a restriction to trusted code, similarly to proper code evaluation,
since they're not that different in practice.)

> I'd rather have macros declare that they're safe to run on untrusted
> inputs, which means mostly they don't eval their arguments.

Yes, please :)
Even better, we should have a safe evaluation sandbox that can be used
for safe macro-expansion among other things.  Indeed, any solution that
allows us to safely expand (most) macros would be a great improvement.
But until we have something like that, we should guard macro-expansion
behind trusted-content-p checks.


Regards,

Eshel

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On March 29, 2025 2:54:26 AM EDT, Eshel Yaron <me@eshelyaron=2Ecom> wrote:
>Daniel Colascione <dancol@dancol=2Eorg> writes:
>
>> On March 28, 2025 1:28:06 PM EDT, Eshel Yaron wrote:
>>>Tags: patch
>>>
>>>find-function may expand Lisp macros in a source file when it fails to
>>>find a definition otherwise=2E  This patch restricts this fallback to
>>>trusted buffers only, to protect against possibly harmful macros=2E
>>
>> I get not wanting to execute code from random files I'm just visiting,
>> but if I've already actually evaluated a macro function and installed
>> it in my Emacs function namespace as something I can call, is it all
>> that dangerous to call it?=20
>
>find-function searches through code you haven't evaluated/loaded too=2E
>Even for loaded libraries, the source file/buffer contents may be
>different than the loaded code=2E  Either way, if you trust some files,
>you can add them to trusted-content=2E  If you haven't, that means they
>are untrusted=2E
>
>In general, as long as macro-expansion remains unsafe, we should avoid
>expanding untrusted macros in commands that merely edit/browse Lisp code
>(in contrast with compiling/evaluating it)=2E
>
>> Instead of a blanket prohibition on macro expansion,
>
>(To be clear, I wouldn't say there's a prohibition on macro expansion,
>just a restriction to trusted code, similarly to proper code evaluation,
>since they're not that different in practice=2E)
>
>> I'd rather have macros declare that they're safe to run on untrusted
>> inputs, which means mostly they don't eval their arguments=2E
>
>Yes, please :)


Macros are just transformers from lists to lists=2E If a macro doesn't exe=
cute its input data, you can pass radioactive waste through it just fine=2E=
 If we can trust a macro to handle untrusted data safely, we don't need to =
trust its inputs=2E

It should work like this:

1) presumptively, expansion of a trusted form is safe no matter which macr=
o is doing the expanding=20

2) by default, expansion of untrusted forms is unsafe=20

3) however, #2 can be overridden if a macro declares (probably via oclosur=
e slot) "I am safely process untrusted input"

Now we can expand most macros in practice even in untrusted code because m=
ost of the time the macros will be coming from the Emacs core, the macros o=
f which we'll annotate as safe=2E No reason whatsoever that we shouldn't be=
 able to expand, say, a cl-defgeneric form in an untrusted file=2E

(We should use an oclosure instead of a plist property so we can easily at=
tach the attribute to macrolet macros too)

Probably simplest to make the safety annotation just another declare form=
=2E I'd love to integrate it into edebug specs, but sadly edebug doesn't ha=
ve an API other components can use to extract and use the information embed=
ded in debug specs, and current edebug syntax doesn't have all the informat=
ion we'd need for safety=2E (Oh, this macro takes a sexp? I can still eval =
it during expansion!)

>Even better, we should have a safe evaluation sandbox that can be used
>for safe macro-expansion among other things=2E  Indeed, any solution that
>allows us to safely expand (most) macros would be a great improvement=2E


>But until we have something like that, we should guard macro-expansion
>behind trusted-content-p checks=2E

I think we need a little more nuance than that


------EMCF8KR1KFEKRJWKYNZXRNI8RKV9CO
Content-Type: text/html;
 charset=utf-8
Content-Transfer-Encoding: quoted-printable

<!DOCTYPE html><html><body><div dir=3D"auto"><br><br>On March 29, 2025 2:54=
:26 AM EDT, Eshel Yaron &lt;me@eshelyaron=2Ecom&gt; wrote:<br>&gt;Daniel Co=
lascione &lt;dancol@dancol=2Eorg&gt; writes:<br>&gt;<br>&gt;&gt; On March 2=
8, 2025 1:28:06 PM EDT, Eshel Yaron wrote:<br>&gt;&gt;&gt;Tags: patch<br>&g=
t;&gt;&gt;<br>&gt;&gt;&gt;find-function may expand Lisp macros in a source =
file when it fails to<br>&gt;&gt;&gt;find a definition otherwise=2E=C2=A0 T=
his patch restricts this fallback to<br>&gt;&gt;&gt;trusted buffers only, t=
o protect against possibly harmful macros=2E<br>&gt;&gt;<br>&gt;&gt; I get =
not wanting to execute code from random files I'm just visiting,<br>&gt;&gt=
; but if I've already actually evaluated a macro function and installed<br>=
&gt;&gt; it in my Emacs function namespace as something I can call, is it a=
ll<br>&gt;&gt; that dangerous to call it? <br>&gt;<br>&gt;find-function sea=
rches through code you haven't evaluated/loaded too=2E<br>&gt;Even for load=
ed libraries, the source file/buffer contents may be<br>&gt;different than =
the loaded code=2E=C2=A0 Either way, if you trust some files,<br>&gt;you ca=
n add them to trusted-content=2E=C2=A0 If you haven't, that means they<br>&=
gt;are untrusted=2E<br>&gt;<br>&gt;In general, as long as macro-expansion r=
emains unsafe, we should avoid<br>&gt;expanding untrusted macros in command=
s that merely edit/browse Lisp code<br>&gt;(in contrast with compiling/eval=
uating it)=2E<br>&gt;<br>&gt;&gt; Instead of a blanket prohibition on macro=
 expansion,<br>&gt;<br>&gt;(To be clear, I wouldn't say there's a prohibiti=
on on macro expansion,<br>&gt;just a restriction to trusted code, similarly=
 to proper code evaluation,<br>&gt;since they're not that different in prac=
tice=2E)<br>&gt;<br>&gt;&gt; I'd rather have macros declare that they're sa=
fe to run on untrusted<br>&gt;&gt; inputs, which means mostly they don't ev=
al their arguments=2E<br>&gt;<br>&gt;Yes, please :)<br><br><br>Macros are j=
ust transformers from lists to lists=2E If a macro doesn't execute its inpu=
t data, you can pass radioactive waste through it just fine=2E If we can tr=
ust a macro to handle untrusted data safely, we don't need to trust its inp=
uts=2E<br><br>It should work like this:<br><br>1) presumptively, expansion =
of a trusted form is safe no matter which macro is doing the expanding <br>=
<br>2) by default, expansion of untrusted forms is unsafe <br><br>3) howeve=
r, #2 can be overridden if a macro declares (probably via oclosure slot) "I=
 am safely process untrusted input"<br><br>Now we can expand most macros in=
 practice even in untrusted code because most of the time the macros will b=
e coming from the Emacs core, the macros of which we'll annotate as safe=2E=
 No reason whatsoever that we shouldn't be able to expand, say, a cl-defgen=
eric form in an untrusted file=2E<br><br>(We should use an oclosure instead=
 of a plist property so we can easily attach the attribute to macrolet macr=
os too)<br><br>Probably simplest to make the safety annotation just another=
 declare form=2E I'd love to integrate it into edebug specs, but sadly edeb=
ug doesn't have an API other components can use to extract and use the info=
rmation embedded in debug specs, and current edebug syntax doesn't have all=
 the information we'd need for safety=2E (Oh, this macro takes a sexp? I ca=
n still eval it during expansion!)<br><br>&gt;Even better, we should have a=
 safe evaluation sandbox that can be used<br>&gt;for safe macro-expansion a=
mong other things=2E=C2=A0 Indeed, any solution that<br>&gt;allows us to sa=
fely expand (most) macros would be a great improvement=2E<br><br><br>&gt;Bu=
t until we have something like that, we should guard macro-expansion<br>&gt=
;behind trusted-content-p checks=2E<br><br>I think we need a little more nu=
ance than that<br><br></div></body></html>
------EMCF8KR1KFEKRJWKYNZXRNI8RKV9CO--

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On March 29, 2025 2:54:26 AM EDT, Eshel Yaron <me@eshelyaron=2Ecom> wrote:
>Daniel Colascione <dancol@dancol=2Eorg> writes:
>
>> On March 28, 2025 1:28:06 PM EDT, Eshel Yaron wrote:
>>>Tags: patch
>>>
>>>find-function may expand Lisp macros in a source file when it fails to
>>>find a definition otherwise=2E  This patch restricts this fallback to
>>>trusted buffers only, to protect against possibly harmful macros=2E
>>
>> I get not wanting to execute code from random files I'm just visiting,
>> but if I've already actually evaluated a macro function and installed
>> it in my Emacs function namespace as something I can call, is it all
>> that dangerous to call it?=20
>
>find-function searches through code you haven't evaluated/loaded too=2E
>Even for loaded libraries, the source file/buffer contents may be
>different than the loaded code=2E  Either way, if you trust some files,
>you can add them to trusted-content=2E  If you haven't, that means they
>are untrusted=2E
>
>In general, as long as macro-expansion remains unsafe, we should avoid
>expanding untrusted macros in commands that merely edit/browse Lisp code
>(in contrast with compiling/evaluating it)=2E
>
>> Instead of a blanket prohibition on macro expansion,
>
>(To be clear, I wouldn't say there's a prohibition on macro expansion,
>just a restriction to trusted code, similarly to proper code evaluation,
>since they're not that different in practice=2E)
>
>> I'd rather have macros declare that they're safe to run on untrusted
>> inputs, which means mostly they don't eval their arguments=2E
>
>Yes, please :)


Macros are just transformers from lists to lists=2E If a macro doesn't exe=
cute its input data, you can pass radioactive waste through it just fine=2E=
 If we can trust a macro to handle untrusted data safely, we don't need to =
trust its inputs=2E

It should work like this:

1) presumptively, expansion of a trusted form is safe no matter which macr=
o is doing the expanding=20

2) by default, expansion of untrusted forms is unsafe=20

3) however, #2 can be overridden if a macro declares (probably via oclosur=
e slot) "I am safely process untrusted input"

Now we can expand most macros in practice even in untrusted code because m=
ost of the time the macros will be coming from the Emacs core, the macros o=
f which we'll annotate as safe=2E No reason whatsoever that we shouldn't be=
 able to expand, say, a cl-defgeneric form in an untrusted file=2E

(We should use an oclosure instead of a plist property so we can easily at=
tach the attribute to macrolet macros too)

Probably simplest to make the safety annotation just another declare form=
=2E I'd love to integrate it into edebug specs, but sadly edebug doesn't ha=
ve an API other components can use to extract and use the information embed=
ded in debug specs, and current edebug syntax doesn't have all the informat=
ion we'd need for safety=2E (Oh, this macro takes a sexp? I can still eval =
it during expansion!)

>Even better, we should have a safe evaluation sandbox that can be used
>for safe macro-expansion among other things=2E  Indeed, any solution that
>allows us to safely expand (most) macros would be a great improvement=2E


>But until we have something like that, we should guard macro-expansion
>behind trusted-content-p checks=2E

I think we need a little more nuance than that


------EMCF8KR1KFEKRJWKYNZXRNI8RKV9CO
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<!DOCTYPE html><html><body><div dir=3D"auto"><br><br>On March 29, 2025 2:54=
:26 AM EDT, Eshel Yaron &lt;me@eshelyaron=2Ecom&gt; wrote:<br>&gt;Daniel Co=
lascione &lt;dancol@dancol=2Eorg&gt; writes:<br>&gt;<br>&gt;&gt; On March 2=
8, 2025 1:28:06 PM EDT, Eshel Yaron wrote:<br>&gt;&gt;&gt;Tags: patch<br>&g=
t;&gt;&gt;<br>&gt;&gt;&gt;find-function may expand Lisp macros in a source =
file when it fails to<br>&gt;&gt;&gt;find a definition otherwise=2E=C2=A0 T=
his patch restricts this fallback to<br>&gt;&gt;&gt;trusted buffers only, t=
o protect against possibly harmful macros=2E<br>&gt;&gt;<br>&gt;&gt; I get =
not wanting to execute code from random files I'm just visiting,<br>&gt;&gt=
; but if I've already actually evaluated a macro function and installed<br>=
&gt;&gt; it in my Emacs function namespace as something I can call, is it a=
ll<br>&gt;&gt; that dangerous to call it? <br>&gt;<br>&gt;find-function sea=
rches through code you haven't evaluated/loaded too=2E<br>&gt;Even for load=
ed libraries, the source file/buffer contents may be<br>&gt;different than =
the loaded code=2E=C2=A0 Either way, if you trust some files,<br>&gt;you ca=
n add them to trusted-content=2E=C2=A0 If you haven't, that means they<br>&=
gt;are untrusted=2E<br>&gt;<br>&gt;In general, as long as macro-expansion r=
emains unsafe, we should avoid<br>&gt;expanding untrusted macros in command=
s that merely edit/browse Lisp code<br>&gt;(in contrast with compiling/eval=
uating it)=2E<br>&gt;<br>&gt;&gt; Instead of a blanket prohibition on macro=
 expansion,<br>&gt;<br>&gt;(To be clear, I wouldn't say there's a prohibiti=
on on macro expansion,<br>&gt;just a restriction to trusted code, similarly=
 to proper code evaluation,<br>&gt;since they're not that different in prac=
tice=2E)<br>&gt;<br>&gt;&gt; I'd rather have macros declare that they're sa=
fe to run on untrusted<br>&gt;&gt; inputs, which means mostly they don't ev=
al their arguments=2E<br>&gt;<br>&gt;Yes, please :)<br><br><br>Macros are j=
ust transformers from lists to lists=2E If a macro doesn't execute its inpu=
t data, you can pass radioactive waste through it just fine=2E If we can tr=
ust a macro to handle untrusted data safely, we don't need to trust its inp=
uts=2E<br><br>It should work like this:<br><br>1) presumptively, expansion =
of a trusted form is safe no matter which macro is doing the expanding <br>=
<br>2) by default, expansion of untrusted forms is unsafe <br><br>3) howeve=
r, #2 can be overridden if a macro declares (probably via oclosure slot) "I=
 am safely process untrusted input"<br><br>Now we can expand most macros in=
 practice even in untrusted code because most of the time the macros will b=
e coming from the Emacs core, the macros of which we'll annotate as safe=2E=
 No reason whatsoever that we shouldn't be able to expand, say, a cl-defgen=
eric form in an untrusted file=2E<br><br>(We should use an oclosure instead=
 of a plist property so we can easily attach the attribute to macrolet macr=
os too)<br><br>Probably simplest to make the safety annotation just another=
 declare form=2E I'd love to integrate it into edebug specs, but sadly edeb=
ug doesn't have an API other components can use to extract and use the info=
rmation embedded in debug specs, and current edebug syntax doesn't have all=
 the information we'd need for safety=2E (Oh, this macro takes a sexp? I ca=
n still eval it during expansion!)<br><br>&gt;Even better, we should have a=
 safe evaluation sandbox that can be used<br>&gt;for safe macro-expansion a=
mong other things=2E=C2=A0 Indeed, any solution that<br>&gt;allows us to sa=
fely expand (most) macros would be a great improvement=2E<br><br><br>&gt;Bu=
t until we have something like that, we should guard macro-expansion<br>&gt=
;behind trusted-content-p checks=2E<br><br>I think we need a little more nu=
ance than that<br><br></div></body></html>
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Last modified: Sat, 29 Mar 2025 15:15:02 UTC

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Copyright (C) 1999 Darren O. Benham, 1997 nCipher Corporation Ltd, 1994-97 Ian Jackson.