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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> Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org 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