From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933606AbdDFP73 (ORCPT ); Thu, 6 Apr 2017 11:59:29 -0400 Received: from mail-vk0-f47.google.com ([209.85.213.47]:35497 "EHLO mail-vk0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933478AbdDFP7W (ORCPT ); Thu, 6 Apr 2017 11:59:22 -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: Andy Lutomirski Date: Thu, 6 Apr 2017 08:59:00 -0700 Message-ID: Subject: Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() To: 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 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. --Andy From mboxrd@z Thu Jan 1 00:00:00 1970 From: luto@amacapital.net (Andy Lutomirski) Date: Thu, 6 Apr 2017 08:59:00 -0700 Subject: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() In-Reply-To: References: <1490811363-93944-1-git-send-email-keescook@chromium.org> <1490811363-93944-5-git-send-email-keescook@chromium.org> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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. --Andy From mboxrd@z Thu Jan 1 00:00:00 1970 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: Andy Lutomirski Date: Thu, 6 Apr 2017 08:59:00 -0700 Message-ID: Content-Type: text/plain; charset=UTF-8 Subject: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() To: 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" List-ID: 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. --Andy