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Thu, 24 Apr 2025 00:20:50 -0700 (PDT) From: Deepak Gupta Date: Thu, 24 Apr 2025 00:20:24 -0700 Subject: [PATCH v13 09/28] riscv mmu: write protect and shadow stack MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20250424-v5_user_cfi_series-v13-9-971437de586a@rivosinc.com> References: <20250424-v5_user_cfi_series-v13-0-971437de586a@rivosinc.com> In-Reply-To: <20250424-v5_user_cfi_series-v13-0-971437de586a@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-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 596FF140003 X-Stat-Signature: uycprp8gmd5a9s77emdq5g9tf95pyhuq X-HE-Tag: 1745479252-851321 X-HE-Meta: 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 EiIj1yEX 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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 ccd2fa34afb8..54707686f042 100644 --- a/arch/riscv/include/asm/pgtable.h +++ b/arch/riscv/include/asm/pgtable.h @@ -411,7 +411,7 @@ 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) */ @@ -612,7 +612,15 @@ 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