From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D6FD0C433EF for ; Wed, 11 May 2022 08:03:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243267AbiEKIDE (ORCPT ); Wed, 11 May 2022 04:03:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36670 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239763AbiEKIDB (ORCPT ); Wed, 11 May 2022 04:03:01 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id AC22A8B085 for ; Wed, 11 May 2022 01:03:00 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3DEC8106F; Wed, 11 May 2022 01:03:00 -0700 (PDT) Received: from FVFF77S0Q05N (unknown [10.57.3.187]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C8E5C3F73D; Wed, 11 May 2022 01:02:58 -0700 (PDT) Date: Wed, 11 May 2022 09:02:45 +0100 From: Mark Rutland To: Kees Cook Cc: Alexander Popov , linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org, catalin.marinas@arm.com, linux-kernel@vger.kernel.org, luto@kernel.org, will@kernel.org Subject: Re: [PATCH v2 03/13] stackleak: remove redundant check Message-ID: References: <20220427173128.2603085-1-mark.rutland@arm.com> <20220427173128.2603085-4-mark.rutland@arm.com> <202205101958.2A33DE20@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202205101958.2A33DE20@keescook> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 10, 2022 at 08:00:38PM -0700, Kees Cook wrote: > On Tue, May 10, 2022 at 12:46:48PM +0100, Mark Rutland wrote: > > On Sun, May 08, 2022 at 09:17:01PM +0300, Alexander Popov wrote: > > > On 27.04.2022 20:31, Mark Rutland wrote: > > > > In __stackleak_erase() we check that the `erase_low` value derived from > > > > `current->lowest_stack` is above the lowest legitimate stack pointer > > > > value, but this is already enforced by stackleak_track_stack() when > > > > recording the lowest stack value. > > > > > > > > Remove the redundant check. > > > > > > > > There should be no functional change as a result of this patch. > > > > > > Mark, I can't agree here. I think this check is important. > > > The performance profit from dropping it is less than the confidence decrease :) > > > > > > With this check, if the 'lowest_stack' value is corrupted, stackleak doesn't > > > overwrite some wrong kernel memory, but simply clears the whole thread > > > stack, which is safe behavior. > > > > If you feel strongly about it, I can restore the check, but I struggle to > > believe that it's worthwhile. The `lowest_stack` value lives in the > > task_struct, and if you have the power to corrupt that you have the power to do > > much more interesting things. > > > > If we do restore it, I'd like to add a big fat comment explaining the > > rationale (i.e. that it only matter if someone could corrupt > > `current->lowest_stack`, as otherwise that's guarnateed to be within bounds). > > Yeah, let's restore it and add the comment. While I do agree it's likely > that such an corruption would likely mean an attacker had significant > control over kernel memory already, it is not uncommon that an attack > only has a limited index from a given address, etc. Or some manipulation > is possible via weird gadgets, etc. It's unlikely, but not impossible, > and a bounds-check for that value is cheap compared to the rest of the > work happening. :) Fair enough; I can go spin a patch restoring this. I'm somewhat unhappy with silently fixing that up, though -- IMO it'd be better to BUG() or similar in that case. Thanks, Mark.