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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 , Andreas Korb , Valentin Haudiquet , Deepak Gupta X-Mailer: b4 0.13.0 X-Developer-Signature: v=1; a=ed25519-sha256; t=1765473664; l=2600; i=debug@rivosinc.com; s=20251023; h=from:subject:message-id; bh=m7ME6PGl9KI5AztkhWmEf8JbroLdBQJ+d2uYeaaihZ8=; b=7r7tctPp48JTpLLMCrtR05us6p7SdJXkKkEEO06m1B4TlLQ16GoFfLyU9wxmcdOzT74COct2N jaVecpC/SXpDU0mrIX2zLnqJBo7xwa+24wGsDowi+y2qRTtlpBOvsDN X-Developer-Key: i=debug@rivosinc.com; a=ed25519; pk=O37GQv1thBhZToXyQKdecPDhtWVbEDRQ0RIndijvpjk= X-Endpoint-Received: by B4 Relay for debug@rivosinc.com/20251023 with auth_id=553 X-Original-From: Deepak Gupta Reply-To: debug@rivosinc.com X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: wykq6sd6ibo7df6suup895co7c1rokmb X-Rspamd-Queue-Id: C150A40006 X-HE-Tag: 1765473668-512267 X-HE-Meta: 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 3QXaXvGb jTYxd6QN1tfyjLzg9r2zMIFOkNtxK6oo0hUg0VAqKNtvygh5dvdVW7CnSPts6k/I5Jb39s4JF+bz4lW0i0JIRTGQCjadSpSaMq8bnIh2kfY0NGml17tqLrU/EXL39LZ1Rq4j31dO3nNjdW0rG5kREdlDKyDEPO3ufDJIF 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: From: Deepak Gupta `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 Tested-by: Andreas Korb Tested-by: Valentin Haudiquet 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