Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752959Ab3FNQOs (ORCPT ); Fri, 14 Jun 2013 12:14:48 -0400 Received: from bedivere.hansenpartnership.com ([66.63.167.143]:34233 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751964Ab3FNQOq (ORCPT ); Fri, 14 Jun 2013 12:14:46 -0400 Message-ID: <1371226483.2726.19.camel@dabdike> Subject: Re: [PATCH] dma-mapping: Add BUG_ON for uninitialized dma_ops From: James Bottomley To: Arnd Bergmann Cc: Marek Szyprowski , Bjorn Helgaas , Michal Simek , "linux-kernel@vger.kernel.org" , Michal Simek , Linux-Arch Date: Fri, 14 Jun 2013 09:14:43 -0700 In-Reply-To: <201306141636.29390.arnd@arndb.de> References: <201306121706.39368.arnd@arndb.de> <1371157181.2261.8.camel@dabdike> <201306141636.29390.arnd@arndb.de> Content-Type: text/plain; charset="ISO-8859-15" X-Mailer: Evolution 3.8.2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2043 Lines: 47 On Fri, 2013-06-14 at 16:36 +0200, Arnd Bergmann wrote: > On Thursday 13 June 2013, James Bottomley wrote: > > On Wed, 2013-06-12 at 17:06 +0200, Arnd Bergmann wrote: > > > On Tuesday 11 June 2013, James Bottomley wrote: > > > > Really, no, it's not a good idea at all. It invites tons of patches > > > > littering the code with BUG_ONs where we might possibly get a NULL > > > > dereference. All it does is add extra instructions to a code path for > > > > no actual benefit. > > > > > > > > If you can answer the question: what more information does the BUG_ON > > > > give you than the NULL deref Oops would not? then it might be > > > > reasonable. > > > > > > The question is if a user can trigger the NULL dereference intentionally, > > > in which case they might get the kernel to jump into a user-provided > > > buffer. > > > > Can you elaborate on how they could do this? If you're thinking they > > could alter the pointer and trigger the jump, then yes, but a BUG_ON > > won't prevent that because the altered pointer won't be NULL. > > The attack that has been demonstrated a couple of times uses an anomymous > mmap to virtual address 0. You fill that page with pointers to a > function in your program. If there is a NULL pointer to some operations > structure and kernel code calls an operation without checking the > ops pointer first, it gets read from the NULL page and the kernel > jumps into user space. This is the MMAP_PAGE_ZERO exploit. The original exploit relied on a leaky personality capability clearing mask and was fixed in 2.6.31 by commit f9fabcb58a6d26d6efde842d1703ac7cfa9427b6 Author: Julien Tinnes Date: Fri Jun 26 20:27:40 2009 +0200 personality: fix PER_CLEAR_ON_SETID So it's not really relevant to 3.x kernels, is it? James -- 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/