oss-security - Re: [langsec-discuss] Fwd: Non-upstream patches for bash

server OS

Date: Mon, 29 Sep 2014 09:41:40 -0400
From: Paul Burchard <paulburchard@...il.com>
To: Sven Kieske <s.kieske@...twald.de>
Cc: langsec-discuss@...l.langsec.org, oss-security@...ts.openwall.com
Subject: Re: [langsec-discuss] Fwd: Non-upstream patches for bash

Even for regular languages, which are theoretically decidable, decision
problems are exponentially costly to solve.  The main langsec approach that
does not restrict the type of security questions that can be asked is to
use languages with some kind of bounded recursion.
 On 27/09/14 17:06, Solar Designer wrote:
> Of course, what input is trusted vs. not may be unclear.  Apparently, 20
> years ago bash developers considered all env vars to be trusted input,
> regardless of the names, which is how we got here.

Well, from a scientific point of view, this was already
solved, if I'm interpreting bash correctly.

See page 12 in this paper:
http://langsec.org/ShotgunParsersShmoo.pdf

To quote for the lazy:

'Input sanitization: “you can suppress ‘bad
stuff’ in input+output to make it safe”

Reality: Halting problem. Deal with it.'

This should be true for all turing complete
input languages (which I assume bash is capable of).

So you can not "filter" turing complete input languages
unless you restrict your language so hard that you
in fact create another class of languages, e.g.
just allow regex, which would create a context-free
language[1], which would circumvent whole classes of exploits.

Also cc'ing langsec-list, as they are interested
in getting this stuff fixed in real applications.

[1]https://en.wikipedia.org/wiki/Context-free_language

--
Mit freundlichen Grüßen / Regards

Sven Kieske

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