From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Hansen Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for shadow stack Date: Tue, 10 Jul 2018 16:37:43 -0700 Message-ID: References: <20180710222639.8241-1-yu-cheng.yu@intel.com> <20180710222639.8241-17-yu-cheng.yu@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <20180710222639.8241-17-yu-cheng.yu@intel.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Yu-cheng Yu , x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann , Andy Lutomirski , Balbir Singh , Cyrill Gorcunov , Florian Weimer , "H.J. Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra List-Id: linux-arch.vger.kernel.org On 07/10/2018 03:26 PM, Yu-cheng Yu wrote: > There are three possible shadow stack PTE settings: > > Normal SHSTK PTE: (R/O + DIRTY_HW) > SHSTK PTE COW'ed: (R/O + DIRTY_HW) > SHSTK PTE shared as R/O data: (R/O + DIRTY_SW) > > Update can_follow_write_pte/pmd for the shadow stack. First of all, thanks for the excellent patch headers. It's nice to have that reference every time even though it's repeated. > -static inline bool can_follow_write_pte(pte_t pte, unsigned int flags) > +static inline bool can_follow_write_pte(pte_t pte, unsigned int flags, > + bool shstk) > { > + bool pte_cowed = shstk ? is_shstk_pte(pte):pte_dirty(pte); > + > return pte_write(pte) || > - ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte)); > + ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_cowed); > } Can we just pass the VMA in here? This use is OK-ish, but I generally detest true/false function arguments because you can't tell what they are when they show up without a named variable. But... Why does this even matter? Your own example showed that all shadowstack PTEs have either DIRTY_HW or DIRTY_SW set, and pte_dirty() checks both. That makes this check seem a bit superfluous. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga09.intel.com ([134.134.136.24]:19415 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732302AbeGJXjK (ORCPT ); Tue, 10 Jul 2018 19:39:10 -0400 Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for shadow stack References: <20180710222639.8241-1-yu-cheng.yu@intel.com> <20180710222639.8241-17-yu-cheng.yu@intel.com> From: Dave Hansen Message-ID: Date: Tue, 10 Jul 2018 16:37:43 -0700 MIME-Version: 1.0 In-Reply-To: <20180710222639.8241-17-yu-cheng.yu@intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Yu-cheng Yu , x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann , Andy Lutomirski , Balbir Singh , Cyrill Gorcunov , Florian Weimer , "H.J. Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , "Ravi V. Shankar" , Vedvyas Shanbhogue Message-ID: <20180710233743.VPGdIG6DK4JcCA72lBT4vRRjSyBk2xSKJnhfY6O2WcU@z> On 07/10/2018 03:26 PM, Yu-cheng Yu wrote: > There are three possible shadow stack PTE settings: > > Normal SHSTK PTE: (R/O + DIRTY_HW) > SHSTK PTE COW'ed: (R/O + DIRTY_HW) > SHSTK PTE shared as R/O data: (R/O + DIRTY_SW) > > Update can_follow_write_pte/pmd for the shadow stack. First of all, thanks for the excellent patch headers. It's nice to have that reference every time even though it's repeated. > -static inline bool can_follow_write_pte(pte_t pte, unsigned int flags) > +static inline bool can_follow_write_pte(pte_t pte, unsigned int flags, > + bool shstk) > { > + bool pte_cowed = shstk ? is_shstk_pte(pte):pte_dirty(pte); > + > return pte_write(pte) || > - ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte)); > + ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_cowed); > } Can we just pass the VMA in here? This use is OK-ish, but I generally detest true/false function arguments because you can't tell what they are when they show up without a named variable. But... Why does this even matter? Your own example showed that all shadowstack PTEs have either DIRTY_HW or DIRTY_SW set, and pte_dirty() checks both. That makes this check seem a bit superfluous.