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Message-ID: <20140501040602.GA24961@openwall.com>
Date: Thu, 1 May 2014 08:06:02 +0400
From: Solar Designer <solar@...nwall.com>
To: Dan Carpenter <dan.carpenter@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
devel@...ica.org, kernel-hardening@...ts.openwall.com,
Kees Cook <keescook@...omium.org>, Dave Jones <davej@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch] lib: check for strcpy() overflows to fixed length buffers
On Wed, Apr 30, 2014 at 06:08:44PM +0300, Dan Carpenter wrote:
> There are sometimes where we know that we are doing an strcpy() into a
> fixed length buffer. In those cases, we could verify that the strcpy()
> doesn't overflow. This patch introduces DEBUG_STRICT_SLOW_STRCPY_CHECKS
> if you want to check for that.
FWIW, I had posted similar macros for userland strcpy() and friends to
the security-audit list (now defunct) in 1998. Someone preserved a copy
here (although the indentation is lost):
http://www.opennet.ru/soft/0432.html
In (weird) use, with proper indentation:
http://www.merit.edu/mail.archives/nanog/2000-02/msg00366.html
https://github.com/tureba/trinoo/blob/master/strfix.h
Personally, I was using this at the time for building known-broken
software like wu-ftpd, where the risk of false positives felt lower than
the risk of buffer overflow bugs being in fact present in the code.
I used gcc's Statement Exprs extension, which is also used in the Linux
kernel a lot:
http://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
So maybe you should, too. (That is, if you want to go ahead with this
approach for code that isn't meant to be as broken as wu-ftpd was.)
This lets us propagate the original return value.
To determine the destination buffer size, I simply used sizeof() and
skipped my added protection in case the size looked like that of a
pointer. Now you have those nice new gcc features instead. :-)
> The downside is that it makes strcpy slower.
I guess the slowdown is mostly from the added strlen(). I avoided it by
using strncat(), so I had truncation instead of detection. It is
unclear which is better.
Other functions I did this for are strcat(), sprintf(), vsprintf().
Alexander
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