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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id f9-20020a170902684900b0019488090884sm9626469pln.273.2023.01.19.16.58.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Jan 2023 16:58:53 -0800 (PST) Date: Thu, 19 Jan 2023 16:58:53 -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 Subject: Re: [PATCH v5 12/39] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW Message-ID: <202301191658.A599D620B@keescook> References: <20230119212317.8324-1-rick.p.edgecombe@intel.com> <20230119212317.8324-13-rick.p.edgecombe@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230119212317.8324-13-rick.p.edgecombe@intel.com> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 7BF5518000C X-Stat-Signature: hne6gra6mt3ctpyocjf3w833z57fajqg X-Rspam-User: X-HE-Tag: 1674176335-917923 X-HE-Meta: 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 /tSQu6tR 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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 01:22:50PM -0800, Rick Edgecombe wrote: > From: Yu-cheng Yu > > When shadow stack is in use, Write=0,Dirty=1 PTE are preserved for > shadow stack. Copy-on-write PTEs then have Write=0,Cow=1. > > When a PTE goes from Write=1,Dirty=1 to Write=0,Cow=1, it could > become a transient shadow stack PTE in two cases: > > 1. Some processors can start a write but end up seeing a Write=0 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. > > 2. 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. > > In the case of pmdp_set_wrprotect(), for nopmd configs the ->pmd operated > on does not exist and the logic would need to be different. Although the > extra functionality will normally be optimized out when user shadow > stacks are not configured, also exclude it in the preprocessor stage so > that it will still compile. User shadow stack is not supported there by > Linux anyway. Leave the cpu_feature_enabled() check so that the > functionality also gets disabled based on runtime detection of the > feature. > > Similarly, compile it out in ptep_set_wrprotect() due to a clang warning > on i386. Like above, the code path should get optimized out on i386 > since shadow stack is not supported on 32 bit kernels, but this makes > the compiler happy. > > Dave Hansen, Jann Horn, Andy Lutomirski, and Peter Zijlstra provided many > insights to the issue. Jann Horn provided the cmpxchg solution. > > Tested-by: Pengfei Xu > Tested-by: John Allen > Signed-off-by: Yu-cheng Yu Reviewed-by: Kees Cook -- Kees Cook