From: Stefan Monnier <monnier@iro.umontreal.ca>
To: Eli Zaretskii <eliz@gnu.org>
Cc: Petteri Hintsanen <petterih@iki.fi>, emacs-devel@gnu.org
Subject: Re: Bindat can exhaust memory when unpacking to vector
Date: Sun, 10 Mar 2024 17:58:14 -0400 [thread overview]
Message-ID: <jwvjzm9ix5f.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <jwvr0l67x40.fsf-monnier+emacs@gnu.org> (Stefan Monnier's message of "Sat, 04 Nov 2023 04:21:25 -0400")
[ Somehow I can't find my previous answer in the `emacs-devel`
archive. In any case, here's an updated (and late) answer :-) ]
> If you then, for example, eval this
>
> (bindat-unpack foo-bindat-spec [255 255 255 255 0 1 2 3 4 5])
>
> it will result in (make-vector 4294967295 0), which makes Emacs hang.
Yup. Validation is needed. And
> If the data being unpacked cannot be trusted, then some kind of sanity
> checking should be done to avoid things like above. In my application
> this is the case, so I embedded validation into the spec itself:
>
> (defconst foo-bindat-spec
> (bindat-type
> (length uint 32)
> (_ unit (when (> length 1234) (error "malicious input")))
> (data vec length)))
>
> Which is somewhat messy, but it works.
You should also be able to do
(defconst foo-bindat-spec
(bindat-type
(length uint 32)
(data vec (if (< length 1234) length (error "malicious input")))))
as well. But it's still not very elegant. We could provide helper
functions so you can write things like
(defconst foo-bindat-spec
(bindat-type
(length uint 32)
(data vec (bounded 1 length 1234))))
so we signal an error if the length is less than 1 or larger than 1234.
> I also played around with the idea of patching bindat.el itself to do
> trivial checking against the input data size, like this:
>
> diff --git a/lisp/emacs-lisp/bindat.el b/lisp/emacs-lisp/bindat.el
> index 6f2af7f975b..c58f6c8f5c7 100644
> --- a/lisp/emacs-lisp/bindat.el
> +++ b/lisp/emacs-lisp/bindat.el
> @@ -204,6 +204,9 @@ bindat--unpack-item
> ('str (bindat--unpack-str len))
> ('strz (bindat--unpack-strz len))
> ('vec
> + (when (> len (length bindat-raw))
> + (error "Vector length %d is greater than raw data length %d."
> + len (length bindat-raw)))
> (let ((v (make-vector len 0)) (vlen 1))
> (if (consp vectype)
> (setq vlen (nth 1 vectype)
>
> but this feels way too intrusive.
Actually, this is a nice solution, I think.
It could conceivably be too conservative if someone manages to use `vec`
with a type that occupies less than one byte (say the input bytes is
a bitvector, or a compressed stream, and the vector output is the result of
uncompressing), but currently I can't see how that could be implemented
with `vec`. I think it can be implemented with a recursive type, like I do in
`bindat-test--LEB128`, but doing it with `vec` would be very cumbersome.
It seems hypothetical enough that I think we should go with your patch.
> Checking should be optional and somehow programmable, perhaps
> a separate "checked vec" type? (I don't have any good, concrete
> ideas, sorry.)
I don't see the benefit of not-checking, to be honest.
Stefan
next prev parent reply other threads:[~2024-03-10 21:58 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-22 19:36 Bindat can exhaust memory when unpacking to vector Petteri Hintsanen
2023-10-23 11:25 ` Eli Zaretskii
2023-10-23 19:36 ` Petteri Hintsanen
2023-10-24 4:34 ` tomas
2023-11-12 19:11 ` Petteri Hintsanen
2023-11-04 7:48 ` Eli Zaretskii
2023-11-04 8:21 ` Stefan Monnier
2024-03-10 21:58 ` Stefan Monnier [this message]
2024-03-19 20:09 ` Petteri Hintsanen
2024-03-19 21:08 ` Stefan Monnier
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