Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755099Ab0F2TOZ (ORCPT ); Tue, 29 Jun 2010 15:14:25 -0400 Received: from smtp.outflux.net ([198.145.64.163]:50531 "EHLO smtp.outflux.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751810Ab0F2TOY (ORCPT ); Tue, 29 Jun 2010 15:14:24 -0400 Date: Tue, 29 Jun 2010 12:13:06 -0700 From: Kees Cook To: Andrew Morton Cc: Alexey Dobriyan , Oleg Nesterov , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Alexander Viro , KOSAKI Motohiro , Neil Horman , Roland McGrath , Ingo Molnar , Peter Zijlstra , Thomas Gleixner Subject: Re: [PATCH] sanitize task->comm to avoid leaking escape codes Message-ID: <20100629191306.GJ4175@outflux.net> References: <20100623181129.GM5876@outflux.net> <20100623194145.GA19628@redhat.com> <20100623202335.GA4424@x200> <20100628130028.73757a46.akpm@linux-foundation.org> <20100628210342.GW4175@outflux.net> <20100629150952.GF4175@outflux.net> <20100629115956.03c4a0b4.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100629115956.03c4a0b4.akpm@linux-foundation.org> Organization: Canonical X-HELO: www.outflux.net Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3470 Lines: 79 On Tue, Jun 29, 2010 at 11:59:56AM -0700, Andrew Morton wrote: > On Tue, 29 Jun 2010 08:09:52 -0700 > Kees Cook wrote: > > > On Tue, Jun 29, 2010 at 11:45:14AM +0300, Alexey Dobriyan wrote: > > > On Tue, Jun 29, 2010 at 12:03 AM, Kees Cook wrote: > > > > On Mon, Jun 28, 2010 at 01:00:28PM -0700, Andrew Morton wrote: > > > >> Surely it would be better to fix the tools which display this info > > > >> rather than making the kernel tell fibs. > > > > > > > > The strncpy in get_task_comm() is totally wrong -- it's testing the length > > > > of task->comm. > > > > > > It also fills not just any buffer but buffer which is TASK_COMM_LEN byte wide. > > > > > > > Why should get_task_comm not take a destination buffer length argument? > > > > > > If you pass too small, you needlessly truncate output. > > > > If you pass too small a buffer, get_task_comm will happily write all over > > the caller's stack past the end of the buffer if the contents of task->comm > > are large enough: > > > > strncpy(buf, tsk->comm, sizeof(tsk->comm)); > > > > The "n" argument to get_task_comm's use of strncpy is totally wrong -- > > it needs to be the size of the destination, not the size of the source. > > Luckily, everyone using get_task_comm currently uses buffers that are > > sizeof(task->comm). > > It's not "totally wrong" at all. get_task_comm() *requires* that it be Using strncpy with n as the source buffer length is meaningless here (tsk->comm is always null terminated at TASK_COMM_LEN or earlier). > passed a buffer of at least TASK_COMM_LEN bytes. sizeof(tsk->comm) > equals TASK_COMM_LEN and always will do so. We could replace the > sizeof with TASK_COMM_LEN for cosmetic reasons but that's utter > nitpicking. But then, the comment right there says "buf must be at > least sizeof(tsk->comm) in size". That's so simple that even a kernel > developer could understand it? If so, strncpy should just be replaced with strcpy. You're assuming buf will always be at least TASK_COMM_LEN. We know the source buffer size is TASK_COMM_LEN because it's already defined that way. There is nothing in the build or runtime that makes sure that buf is at least TASK_COMM_LEN. > Do we need a runtime check every time to make sure that some developer > didn't misunderstand such a simple thing? Seems pretty pointless - > there are a zillion such runtime checks we could add. It'd be better > to do > > #define get_task_comm(buf, tsk) { \ > BUILD_BUG_ON(sizeof(buf) < TASK_COMM_LEN); \ > __get_task_comm(buf, tsk); \ > } > > and save the runtime bloat. But again, what was special about this > particular programmer error? There are five or six instances of > strcpy(foo, current->comm). Do we need runtime checks there as well?? I can't see how it could be a bad thing. Why not try to do some defensive programming here? It's a trivial fix and your define would block this from ever being a problem. As I said before, either get_task_comm() is considered sensitive or it's not. If it is, I've sent a few patches that might help. If it's not, then code should not be criticised for using it. -Kees -- Kees Cook Ubuntu Security Team -- 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/