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Tue, 29 Apr 2025 17:16:54 -0700 (PDT) From: Deepak Gupta Date: Tue, 29 Apr 2025 17:16:26 -0700 Subject: [PATCH v14 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: <20250429-v5_user_cfi_series-v14-9-5239410d012a@rivosinc.com> References: <20250429-v5_user_cfi_series-v14-0-5239410d012a@rivosinc.com> In-Reply-To: <20250429-v5_user_cfi_series-v14-0-5239410d012a@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-Server: rspam12 X-Rspamd-Queue-Id: 87643C0009 X-Rspam-User: X-Stat-Signature: cqqrqz3xfbm1nmqbk8i4ptco94rsfnoc X-HE-Tag: 1745972215-593001 X-HE-Meta: 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 vEB5LIoU 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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