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Thu, 23 Oct 2025 06:25:56 -0700 (PDT) From: Deepak Gupta Date: Thu, 23 Oct 2025 06:25:38 -0700 Subject: [PATCH v22 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: <20251023-v5_user_cfi_series-v22-9-1d53ce35d8fd@rivosinc.com> References: <20251023-v5_user_cfi_series-v22-0-1d53ce35d8fd@rivosinc.com> In-Reply-To: <20251023-v5_user_cfi_series-v22-0-1d53ce35d8fd@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-Stat-Signature: yt77ghc1imwhsjtyrb5nprwubrfzjtop X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 6E52640007 X-HE-Tag: 1761225958-736621 X-HE-Meta: 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 P8j4pDUC 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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