From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with ESMTP id 2FBB86B009D for ; Wed, 3 Jun 2009 11:08:16 -0400 (EDT) Date: Wed, 3 Jun 2009 08:07:25 -0700 (PDT) From: Linus Torvalds Subject: Re: Security fix for remapping of page 0 (was [PATCH] Change ZERO_SIZE_PTR to point at unmapped space) In-Reply-To: Message-ID: References: <20090530192829.GK6535@oblivion.subreption.com> <20090530230022.GO6535@oblivion.subreption.com> <20090531022158.GA9033@oblivion.subreption.com> <20090602203405.GC6701@oblivion.subreption.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Christoph Lameter Cc: "Larry H." , linux-mm@kvack.org, Alan Cox , Rik van Riel , linux-kernel@vger.kernel.org, pageexec@freemail.hu List-ID: On Wed, 3 Jun 2009, Christoph Lameter wrote: > > Ok. So what we need to do is stop this toying around with remapping of > page 0. The following patch contains a fix and a test program that > demonstrates the issue. No, we _need_ to be able to map to address zero. It may not be very common, but things like vm86 require it - vm86 mode always starts at virtual address zero. For similar reasons, some other emulation environments will want it too, simply because they want to emulate another environment that has an address space starting at 0, and don't want to add a base to all address calculations. There are historically even some crazy optimizing compilers that decided that they need to be able to optimize accesses of a pointer across a NULL pointer check, so that they can turn code like if (!ptr) return; val = ptr->member; into doing the load early. In order to support that optimization, they have a runtime that always maps some garbage at virtual address zero. (I don't remember who did this, but my dim memory wants to say it was some HP-UX compiler. Scheduling loads early can be a big deal on especially in-order machines with nonblocking cache accesses). The point being that we do need to support mmap at zero. Not necessarily universally, but it can't be some fixed "we don't allow that". Linus -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org