[PATCH 18/28] prctl: arch-agnostic prtcl for indirect branch tracking

[Deepak Gupta]

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`
aarch64 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@xxxxxxxxxxxx>
---
 include/uapi/linux/prctl.h | 27 +++++++++++++++++++++++++++
 kernel/sys.c               | 30 ++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 3c66ed8f46d8..b7a8212a068e 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -328,4 +328,31 @@ struct prctl_mm_map {
  */
 #define PR_LOCK_SHADOW_STACK_STATUS      73
 
+/*
+ * 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      74
+
+/*
+ * 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      75
+# 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      76
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 96e8a6b5993a..9e2ebf9d9859 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2316,6 +2316,21 @@ 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 __user *status)
+{
+	return -EINVAL;
+}
+
+int __weak arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
+{
+	return -EINVAL;
+}
+
 #define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
 
 #ifdef CONFIG_ANON_VMA_NAME
@@ -2773,6 +2788,21 @@ 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, (unsigned long __user *) arg2);
+		break;
+	case PR_LOCK_INDIR_BR_LP_STATUS:
+		if (arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = arch_lock_indir_br_lp_status(me, (unsigned long __user *) arg2);
+		break;
 	default:
 		error = -EINVAL;
 		break;
-- 
2.43.0