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Fri, 14 Mar 2025 14:39:50 -0700 (PDT) From: Deepak Gupta Date: Fri, 14 Mar 2025 14:39:28 -0700 Subject: [PATCH v12 09/28] riscv mmu: write protect and shadow stack MIME-Version: 1.0 Message-Id: <20250314-v5_user_cfi_series-v12-9-e51202b53138@rivosinc.com> References: <20250314-v5_user_cfi_series-v12-0-e51202b53138@rivosinc.com> In-Reply-To: <20250314-v5_user_cfi_series-v12-0-e51202b53138@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 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, Zong Li , Deepak Gupta X-Mailer: b4 0.14.0 X-Rspam-User: X-Rspamd-Queue-Id: A3DF8100002 X-Rspamd-Server: rspam03 X-Stat-Signature: peo4sdme5qjrbs5r1kk6h5h93t9yjmyk X-HE-Tag: 1741988391-974752 X-HE-Meta: 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 Z+3CduHt GQCFA3pe1i2d3RlCtYA4qehka+p3XLW34pho38lQcZ2czC/NtlB0mrbiHyWc5G4cdDAAnx7IWTmR2cHswWbtjDb2UE/65DSSlHy1MVp8+yt6vrpg= 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