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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id u4-20020a17090a518400b00200a85fa777sm10516489pjh.1.2022.10.03.11.20.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Oct 2022 11:20:24 -0700 (PDT) Date: Mon, 3 Oct 2022 11:20:23 -0700 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 , "Ravi V . Shankar" , Weijiang Yang , "Kirill A . Shutemov" , joao.moreira@intel.com, John Allen , kcc@google.com, eranian@google.com, rppt@kernel.org, jamorris@linux.microsoft.com, dethoma@microsoft.com, Yu-cheng Yu Subject: Re: [PATCH v2 15/39] x86/mm: Check Shadow Stack page fault errors Message-ID: <202210031120.5B6D7BBAC@keescook> References: <20220929222936.14584-1-rick.p.edgecombe@intel.com> <20220929222936.14584-16-rick.p.edgecombe@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220929222936.14584-16-rick.p.edgecombe@intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 29, 2022 at 03:29:12PM -0700, Rick Edgecombe wrote: > From: Yu-cheng Yu > > The CPU performs "shadow stack accesses" when it expects to encounter > shadow stack mappings. These accesses can be implicit (via CALL/RET > instructions) or explicit (instructions like WRSS). > > Shadow stacks accesses to shadow-stack mappings can see faults in normal, > valid operation just like regular accesses to regular mappings. Shadow > stacks need some of the same features like delayed allocation, swap and > copy-on-write. The kernel needs to use faults to implement those features. > > The architecture has concepts of both shadow stack reads and shadow stack > writes. Any shadow stack access to non-shadow stack memory will generate > a fault with the shadow stack error code bit set. > > This means that, unlike normal write protection, the fault handler needs > to create a type of memory that can be written to (with instructions that > generate shadow stack writes), even to fulfill a read access. So in the > case of COW memory, the COW needs to take place even with a shadow stack > read. Otherwise the page will be left (shadow stack) writable in > userspace. So to trigger the appropriate behavior, set FAULT_FLAG_WRITE > for shadow stack accesses, even if the access was a shadow stack read. > > Shadow stack accesses can also result in errors, such as when a shadow > stack overflows, or if a shadow stack access occurs to a non-shadow-stack > mapping. Also, generate the errors for invalid shadow stack accesses. > > Signed-off-by: Yu-cheng Yu Reviewed-by: Kees Cook -- Kees Cook