Re: [PATCH v12 13/28] prctl: arch-agnostic prctl for indirect branch tracking

On Sat, Mar 15, 2025 at 6:51 AM Deepak Gupta debug@rivosinc.com 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 debug@rivosinc.com Reviewed-by: Mark Brown broonie@kernel.org

---

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)

+/*

• • 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;
  
  

LGTM

Reviewed-by: Zong Li zong.li@sifive.com

-- 2.34.1

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