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Mon, 20 Oct 2025 13:53:53 -0700 (PDT) From: Deepak Gupta Date: Mon, 20 Oct 2025 13:53:38 -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: <20251020-v5_user_cfi_series-v22-9-9ae5510d1c6f@rivosinc.com> References: <20251020-v5_user_cfi_series-v22-0-9ae5510d1c6f@rivosinc.com> In-Reply-To: <20251020-v5_user_cfi_series-v22-0-9ae5510d1c6f@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-Rspamd-Queue-Id: 8060A80008 X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: hj8jmdas8x5yss6pnogdpwpy1ih8pmku X-HE-Tag: 1760993635-767412 X-HE-Meta: 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 F2cFc7MV 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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