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

[Deepak Gupta <debug@rivosinc.com>]

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 <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)
>
>
>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;
>>