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Fri, 02 May 2025 16:31:07 -0700 (PDT) From: Deepak Gupta Date: Fri, 02 May 2025 16:30:40 -0700 Subject: [PATCH v15 09/27] riscv mmu: write protect and shadow stack MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20250502-v5_user_cfi_series-v15-9-914966471885@rivosinc.com> References: <20250502-v5_user_cfi_series-v15-0-914966471885@rivosinc.com> In-Reply-To: <20250502-v5_user_cfi_series-v15-0-914966471885@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?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross 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: 1E15A1C000D X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: mqiecj8y9po7kwh96k779pw18kzpp8wh X-HE-Tag: 1746228668-394073 X-HE-Meta: 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 faLRksnz 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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 60d4821627d2..4e3431ccf634 100644 --- a/arch/riscv/include/asm/pgtable.h +++ b/arch/riscv/include/asm/pgtable.h @@ -423,7 +423,7 @@ static inline int pte_devmap(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) */ @@ -624,7 +624,15 @@ 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