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Wed, 15 Oct 2025 11:14:03 -0700 (PDT) From: Deepak Gupta Date: Wed, 15 Oct 2025 11:13:41 -0700 Subject: [PATCH v21 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: <20251015-v5_user_cfi_series-v21-9-6a07856e90e7@rivosinc.com> References: <20251015-v5_user_cfi_series-v21-0-6a07856e90e7@rivosinc.com> In-Reply-To: <20251015-v5_user_cfi_series-v21-0-6a07856e90e7@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-Stat-Signature: fqkwbbixbwt1uqp81zx3n98ckgnfqtyu X-Rspamd-Queue-Id: 26D6A2000F X-Rspamd-Server: rspam09 X-HE-Tag: 1760552044-491270 X-HE-Meta: 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 6c0BLQH0 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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