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Mon, 13 Oct 2025 14:56:18 -0700 (PDT) From: Deepak Gupta Date: Mon, 13 Oct 2025 14:56:01 -0700 Subject: [PATCH v20 09/28] riscv/mm: write protect and shadow stack MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20251013-v5_user_cfi_series-v20-9-b9de4be9912e@rivosinc.com> References: <20251013-v5_user_cfi_series-v20-0-b9de4be9912e@rivosinc.com> In-Reply-To: <20251013-v5_user_cfi_series-v20-0-b9de4be9912e@rivosinc.com> To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Lorenzo Stoakes , Paul Walmsley , Palmer Dabbelt , Albert Ou , Conor Dooley , Rob Herring , Krzysztof Kozlowski , Arnd Bergmann , Christian Brauner , Peter Zijlstra , Oleg Nesterov , Eric Biederman , Kees Cook , Jonathan Corbet , Shuah Khan , Jann Horn , Conor Dooley , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , Andreas Hindborg , Alice Ryhl , Trevor Gross , Benno Lossin Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, devicetree@vger.kernel.org, linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, alistair.francis@wdc.com, richard.henderson@linaro.org, jim.shu@sifive.com, andybnac@gmail.com, kito.cheng@sifive.com, charlie@rivosinc.com, atishp@rivosinc.com, evan@rivosinc.com, cleger@rivosinc.com, alexghiti@rivosinc.com, samitolvanen@google.com, broonie@kernel.org, rick.p.edgecombe@intel.com, rust-for-linux@vger.kernel.org, Zong Li , Deepak Gupta X-Mailer: b4 0.13.0 X-Rspam-User: X-Rspamd-Queue-Id: 1568340008 X-Rspamd-Server: rspam02 X-Stat-Signature: wum99swgm6wr4k1niuwsnmdyp98r5iso X-HE-Tag: 1760392579-192980 X-HE-Meta: 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 7dZr96nh 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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: List-Subscribe: List-Unsubscribe: `fork` implements copy on write (COW) by making pages readonly in child and parent both. ptep_set_wrprotect and pte_wrprotect clears _PAGE_WRITE in PTE. Assumption is that page is readable and on fault copy on write happens. To implement COW on shadow stack pages, clearing up W bit makes them XWR = 000. This will result in wrong PTE setting which says no perms but V=1 and PFN field pointing to final page. Instead desired behavior is to turn it into a readable page, take an access (load/store) fault on sspush/sspop (shadow stack) and then perform COW on such pages. This way regular reads would still be allowed and not lead to COW maintaining current behavior of COW on non-shadow stack but writeable memory. On the other hand it doesn't interfere with existing COW for read-write memory. Assumption is always that _PAGE_READ must have been set and thus setting _PAGE_READ is harmless. Reviewed-by: Alexandre Ghiti Reviewed-by: Zong Li Signed-off-by: Deepak Gupta --- arch/riscv/include/asm/pgtable.h | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h index b03e8f85221f..df4a04b64944 100644 --- a/arch/riscv/include/asm/pgtable.h +++ b/arch/riscv/include/asm/pgtable.h @@ -415,7 +415,7 @@ static inline int pte_special(pte_t pte) static inline pte_t pte_wrprotect(pte_t pte) { - return __pte(pte_val(pte) & ~(_PAGE_WRITE)); + return __pte((pte_val(pte) & ~(_PAGE_WRITE)) | (_PAGE_READ)); } /* static inline pte_t pte_mkread(pte_t pte) */ @@ -611,7 +611,15 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm, static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long address, pte_t *ptep) { - atomic_long_and(~(unsigned long)_PAGE_WRITE, (atomic_long_t *)ptep); + pte_t read_pte = READ_ONCE(*ptep); + /* + * ptep_set_wrprotect can be called for shadow stack ranges too. + * shadow stack memory is XWR = 010 and thus clearing _PAGE_WRITE will lead to + * encoding 000b which is wrong encoding with V = 1. This should lead to page fault + * but we dont want this wrong configuration to be set in page tables. + */ + atomic_long_set((atomic_long_t *)ptep, + ((pte_val(read_pte) & ~(unsigned long)_PAGE_WRITE) | _PAGE_READ)); } #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH -- 2.43.0