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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id 62sm5499904pfg.160.2021.02.04.12.21.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 04 Feb 2021 12:21:01 -0800 (PST) Date: Thu, 4 Feb 2021 12:21:00 -0800 From: Kees Cook To: Yu-cheng Yu Cc: 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 , Borislav Petkov , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H.J. Lu" , Jann Horn , Jonathan Corbet , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V. Shankar" , Vedvyas Shanbhogue , Dave Martin , Weijiang Yang , Pengfei Xu Subject: Re: [PATCH v19 11/25] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW Message-ID: <202102041220.3F62D2775@keescook> References: <20210203225547.32221-1-yu-cheng.yu@intel.com> <20210203225547.32221-12-yu-cheng.yu@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210203225547.32221-12-yu-cheng.yu@intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 03, 2021 at 02:55:33PM -0800, Yu-cheng Yu wrote: > When Shadow Stack is introduced, [R/O + _PAGE_DIRTY] PTE is reserved for > shadow stack. Copy-on-write PTEs have [R/O + _PAGE_COW]. > > When a PTE goes from [R/W + _PAGE_DIRTY] to [R/O + _PAGE_COW], it could > become a transient shadow stack PTE in two cases: > > The first case is that some processors can start a write but end up seeing > a read-only PTE by the time they get to the Dirty bit, creating a transient > shadow stack PTE. However, this will not occur on processors supporting > Shadow Stack, and a TLB flush is not necessary. > > The second case is that when _PAGE_DIRTY is replaced with _PAGE_COW non- > atomically, a transient shadow stack PTE can be created as a result. > Thus, prevent that with cmpxchg. > > Dave Hansen, Jann Horn, Andy Lutomirski, and Peter Zijlstra provided many > insights to the issue. Jann Horn provided the cmpxchg solution. > > Signed-off-by: Yu-cheng Yu Reviewed-by: Kees Cook -- Kees Cook