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

Deepak Gupta
2025-03-14T22:51:16Z

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;

--
2.34.1