From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755776AbdDGIei (ORCPT ); Fri, 7 Apr 2017 04:34:38 -0400 Received: from mail-yb0-f194.google.com ([209.85.213.194]:33776 "EHLO mail-yb0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755695AbdDGIee (ORCPT ); Fri, 7 Apr 2017 04:34:34 -0400 MIME-Version: 1.0 In-Reply-To: References: <1490811363-93944-1-git-send-email-keescook@chromium.org> <1490811363-93944-5-git-send-email-keescook@chromium.org> From: Mathias Krause Date: Fri, 7 Apr 2017 10:34:32 +0200 Message-ID: Subject: Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() To: Andy Lutomirski , Kees Cook Cc: Andy Lutomirski , "kernel-hardening@lists.openwall.com" , Mark Rutland , Hoeun Ryu , PaX Team , Emese Revfy , Russell King , X86 ML , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6 April 2017 at 17:59, Andy Lutomirski wrote: > On Wed, Apr 5, 2017 at 5:14 PM, Kees Cook wrote: >> On Wed, Apr 5, 2017 at 4:57 PM, Andy Lutomirski wrote: >>> On Wed, Mar 29, 2017 at 6:41 PM, Kees Cook wrote: >>>> On Wed, Mar 29, 2017 at 3:38 PM, Andy Lutomirski wrote: >>>>> On Wed, Mar 29, 2017 at 11:15 AM, Kees Cook wrote: >>>>>> Based on PaX's x86 pax_{open,close}_kernel() implementation, this >>>>>> allows HAVE_ARCH_RARE_WRITE to work on x86. >>>>>> >>>>>> + >>>>>> +static __always_inline unsigned long __arch_rare_write_begin(void) >>>>>> +{ >>>>>> + unsigned long cr0; >>>>>> + >>>>>> + preempt_disable(); >>>>> >>>>> This looks wrong. DEBUG_LOCKS_WARN_ON(!irqs_disabled()) would work, >>>>> as would local_irq_disable(). There's no way that just disabling >>>>> preemption is enough. >>>>> >>>>> (Also, how does this interact with perf nmis?) >>>> >>>> Do you mean preempt_disable() isn't strong enough here? I'm open to >>>> suggestions. The goal would be to make sure nothing between _begin and >>>> _end would get executed without interruption... >>>> >>> >>> Sorry for the very slow response. >>> >>> preempt_disable() isn't strong enough to prevent interrupts, and an >>> interrupt here would run with WP off, causing unknown havoc. I tend >>> to think that the caller should be responsible for turning off >>> interrupts. >> >> So, something like: >> >> Top-level functions: >> >> static __always_inline rare_write_begin(void) >> { >> preempt_disable(); >> local_irq_disable(); >> barrier(); >> __arch_rare_write_begin(); >> barrier(); >> } > > Looks good, except you don't need preempt_disable(). > local_irq_disable() also disables preemption. You might need to use > local_irq_save(), though, depending on whether any callers already > have IRQs off. Well, doesn't look good to me. NMIs will still be able to interrupt this code and will run with CR0.WP = 0. Shouldn't you instead question yourself why PaX can do it "just" with preempt_disable() instead?! Cheers, Mathias