Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932751AbbFHWl1 (ORCPT ); Mon, 8 Jun 2015 18:41:27 -0400 Received: from mail-vn0-f47.google.com ([209.85.216.47]:42710 "EHLO mail-vn0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932646AbbFHWlQ (ORCPT ); Mon, 8 Jun 2015 18:41:16 -0400 MIME-Version: 1.0 In-Reply-To: <551C2B86.6080204@gmx.de> References: <1427657309-4344-1-git-send-email-xypron.glpk@gmx.de> <1427657309-4344-4-git-send-email-xypron.glpk@gmx.de> <20150331154554.a0c878277c6d912db39316f9@linux-foundation.org> <551C2B86.6080204@gmx.de> Date: Mon, 8 Jun 2015 15:41:15 -0700 X-Google-Sender-Auth: IyQx9ec4bLdDoqwlOtTD42cKHFg Message-ID: Subject: Re: [PATCH 3/3] sysctl: detect overflows when converting to int From: Kees Cook To: Heinrich Schuchardt Cc: Andrew Morton , Michal Nazarewicz , Ingo Molnar , Steven Rostedt , Peter Zijlstra , Joe Perches , Josh Hunt , Rasmus Villemoes , Rusty Russell , Daniel Walter , David Rientjes , "David S. Miller" , Johannes Weiner , Aaron Tomlin , Prarit Bhargava , Eric B Munson , "Paul E. McKenney" , Sam Ravnborg , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1896 Lines: 49 On Wed, Apr 1, 2015 at 10:31 AM, Heinrich Schuchardt wrote: > On 01.04.2015 00:45, Andrew Morton wrote: >> On Sun, 29 Mar 2015 21:28:29 +0200 Heinrich Schuchardt wrote: >> >>> When converting unsigned long to int overflows may occur. >>> These currently are not detected when writing to the sysctl >>> file system. >>> >>> E.g. on a system where int has 32 bits and long has 64 bits >>> echo 0x800001234 > /proc/sys/kernel/threads-max >>> has the same effect as >>> echo 0x1234 > /proc/sys/kernel/threads-max >>> >>> The patch adds the missing check in do_proc_dointvec_conv. >>> >>> With the patch an overflow will result in an error EINVAL when >>> writing to the the sysctl file system. >> >> hm, why fix this? There's a small risk of breaking >> accidentally-working userspace, but I expect we can live with that. >> >> But how big a problem is this, really? This behaviour is quite >> expected, after all. >> > > The typical user of a Linux system has never read the Kernel code and > possibly has limited programming experience. > Furthermore in Documentation/sysctl/kernel.txt there is no hint that > only 32-bit integers can be used. > So why should this typical user expect that on a 64-bit system > +3000000000 is considered a negative number? > > Now that we know this is a bug why shouldn't we fix it? I think this is worth fixing. It is, from a certain perspective, "unexpected behavior". At the very least we could tie it to the sysctl_writes_strict flag? Anything depending on an overflow to get "correct" results seems extremely unlikely to me. -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/