linux-kernel - Re: [patch] lib: check for strcpy() overflows to fixed length buffers

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/