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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 , 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 Subject: Re: [PATCH v12 13/28] prctl: arch-agnostic prctl for indirect branch tracking Message-ID: References: <20250314-v5_user_cfi_series-v12-0-e51202b53138@rivosinc.com> <20250314-v5_user_cfi_series-v12-13-e51202b53138@rivosinc.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: On Wed, Apr 09, 2025 at 10:03:05AM +0200, Alexandre Ghiti wrote: >On 14/03/2025 22:39, Deepak Gupta wrote: >>Three architectures (x86, aarch64, riscv) have support for indirect branch >>tracking feature in a very similar fashion. On a very high level, indirect >>branch tracking is a CPU feature where CPU tracks branches which uses >>memory operand to perform control transfer in program. As part of this >>tracking on indirect branches, CPU goes in a state where it expects a >>landing pad instr on target and if not found then CPU raises some fault >>(architecture dependent) >> >>x86 landing pad instr - `ENDBRANCH` >>arch64 landing pad instr - `BTI` >>riscv landing instr - `lpad` >> >>Given that three major arches have support for indirect branch tracking, >>This patch makes `prctl` for indirect branch tracking arch agnostic. >> >>To allow userspace to enable this feature for itself, following prtcls are >>defined: >> - PR_GET_INDIR_BR_LP_STATUS: Gets current configured status for indirect >> branch tracking. >> - PR_SET_INDIR_BR_LP_STATUS: Sets a configuration for indirect branch >> tracking. >> Following status options are allowed >> - PR_INDIR_BR_LP_ENABLE: Enables indirect branch tracking on user >> thread. >> - PR_INDIR_BR_LP_DISABLE; Disables indirect branch tracking on user >> thread. >> - PR_LOCK_INDIR_BR_LP_STATUS: Locks configured status for indirect branch >> tracking for user thread. >> >>Signed-off-by: Deepak Gupta >>Reviewed-by: Mark Brown >>--- >> include/linux/cpu.h | 4 ++++ >> include/uapi/linux/prctl.h | 27 +++++++++++++++++++++++++++ >> kernel/sys.c | 30 ++++++++++++++++++++++++++++++ >> 3 files changed, 61 insertions(+) >> >>diff --git a/include/linux/cpu.h b/include/linux/cpu.h >>index 6a0a8f1c7c90..fb0c394430c6 100644 >>--- a/include/linux/cpu.h >>+++ b/include/linux/cpu.h >>@@ -204,4 +204,8 @@ static inline bool cpu_mitigations_auto_nosmt(void) >> } >> #endif >>+int arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *status); >>+int arch_set_indir_br_lp_status(struct task_struct *t, unsigned long status); >>+int arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long status); >>+ >> #endif /* _LINUX_CPU_H_ */ >>diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h >>index 5c6080680cb2..6cd90460cbad 100644 >>--- a/include/uapi/linux/prctl.h >>+++ b/include/uapi/linux/prctl.h >>@@ -353,4 +353,31 @@ struct prctl_mm_map { >> */ >> #define PR_LOCK_SHADOW_STACK_STATUS 76 >>+/* >>+ * Get the current indirect branch tracking configuration for the current >>+ * thread, this will be the value configured via PR_SET_INDIR_BR_LP_STATUS. >>+ */ >>+#define PR_GET_INDIR_BR_LP_STATUS 77 >>+ >>+/* >>+ * Set the indirect branch tracking configuration. PR_INDIR_BR_LP_ENABLE will >>+ * enable cpu feature for user thread, to track all indirect branches and ensure >>+ * they land on arch defined landing pad instruction. >>+ * x86 - If enabled, an indirect branch must land on `ENDBRANCH` instruction. >>+ * arch64 - If enabled, an indirect branch must land on `BTI` instruction. >>+ * riscv - If enabled, an indirect branch must land on `lpad` instruction. >>+ * PR_INDIR_BR_LP_DISABLE will disable feature for user thread and indirect >>+ * branches will no more be tracked by cpu to land on arch defined landing pad >>+ * instruction. >>+ */ >>+#define PR_SET_INDIR_BR_LP_STATUS 78 >>+# define PR_INDIR_BR_LP_ENABLE (1UL << 0) > > >Are we missing PR_INDIR_BR_LP_DISABLE definition here? PR_SET_INDIR_BR_LP_STATUS with parameter's bit0 clear will disable branch tracking. This is what arm and riscv settled on for shadow stack enable and disable as well. > > >>+ >>+/* >>+ * Prevent further changes to the specified indirect branch tracking >>+ * configuration. All bits may be locked via this call, including >>+ * undefined bits. >>+ */ >>+#define PR_LOCK_INDIR_BR_LP_STATUS 79 >>+ >> #endif /* _LINUX_PRCTL_H */ >>diff --git a/kernel/sys.c b/kernel/sys.c >>index cb366ff8703a..f347f3518d0b 100644 >>--- a/kernel/sys.c >>+++ b/kernel/sys.c >>@@ -2336,6 +2336,21 @@ int __weak arch_lock_shadow_stack_status(struct task_struct *t, unsigned long st >> return -EINVAL; >> } >>+int __weak arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *status) >>+{ >>+ return -EINVAL; >>+} >>+ >>+int __weak arch_set_indir_br_lp_status(struct task_struct *t, unsigned long status) >>+{ >>+ return -EINVAL; >>+} >>+ >>+int __weak arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long status) >>+{ >>+ return -EINVAL; >>+} >>+ >> #define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE) >> #ifdef CONFIG_ANON_VMA_NAME >>@@ -2811,6 +2826,21 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, >> return -EINVAL; >> error = arch_lock_shadow_stack_status(me, arg2); >> break; >>+ case PR_GET_INDIR_BR_LP_STATUS: >>+ if (arg3 || arg4 || arg5) >>+ return -EINVAL; >>+ error = arch_get_indir_br_lp_status(me, (unsigned long __user *)arg2); >>+ break; >>+ case PR_SET_INDIR_BR_LP_STATUS: >>+ if (arg3 || arg4 || arg5) >>+ return -EINVAL; >>+ error = arch_set_indir_br_lp_status(me, arg2); >>+ break; >>+ case PR_LOCK_INDIR_BR_LP_STATUS: >>+ if (arg3 || arg4 || arg5) >>+ return -EINVAL; >>+ error = arch_lock_indir_br_lp_status(me, arg2); >>+ break; >> default: >> trace_task_prctl_unknown(option, arg2, arg3, arg4, arg5); >> error = -EINVAL; >>