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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id i11-20020a170902c94b00b00194ac38bc86sm5757193pla.131.2023.01.19.16.52.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Jan 2023 16:52:45 -0800 (PST) Date: Thu, 19 Jan 2023 16:52:45 -0800 From: Kees Cook To: Rick Edgecombe 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 , Weijiang Yang , "Kirill A . Shutemov" , John Allen , kcc@google.com, eranian@google.com, rppt@kernel.org, jamorris@linux.microsoft.com, dethoma@microsoft.com, akpm@linux-foundation.org, Andrew.Cooper3@citrix.com, christina.schimpe@intel.com, Yu-cheng Yu , Christoph Hellwig Subject: Re: [PATCH v5 08/39] x86/mm: Remove _PAGE_DIRTY from kernel RO pages Message-ID: <202301191652.57B10DA48@keescook> References: <20230119212317.8324-1-rick.p.edgecombe@intel.com> <20230119212317.8324-9-rick.p.edgecombe@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230119212317.8324-9-rick.p.edgecombe@intel.com> X-Rspam-User: X-Rspamd-Queue-Id: 4E803C0009 X-Rspamd-Server: rspam01 X-Stat-Signature: txmo9ce9ynequr5gophecsm4ws6n61i6 X-HE-Tag: 1674175967-420854 X-HE-Meta: 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 Tcgwmy99 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Jan 19, 2023 at 01:22:46PM -0800, Rick Edgecombe wrote: > From: Yu-cheng Yu > > New processors that support Shadow Stack regard Write=0,Dirty=1 PTEs as > shadow stack pages. > > In normal cases, it can be helpful to create Write=1 PTEs as also Dirty=1 > if HW dirty tracking is not needed, because if the Dirty bit is not already > set the CPU has to set Dirty=1 when the memory gets written to. This > creates additional work for the CPU. So traditional wisdom was to simply > set the Dirty bit whenever you didn't care about it. However, it was never > really very helpful for read-only kernel memory. > > When CR4.CET=1 and IA32_S_CET.SH_STK_EN=1, some instructions can write to > such supervisor memory. The kernel does not set IA32_S_CET.SH_STK_EN, so > avoiding kernel Write=0,Dirty=1 memory is not strictly needed for any > functional reason. But having Write=0,Dirty=1 kernel memory doesn't have > any functional benefit either, so to reduce ambiguity between shadow stack > and regular Write=0 pages, remove Dirty=1 from any kernel Write=0 PTEs. > > Tested-by: Pengfei Xu > Tested-by: John Allen > Signed-off-by: Yu-cheng Yu Reviewed-by: Kees Cook -- Kees Cook