[PATCH v1 RFC Zisslpcfi 15/20] sslp prctl: arch-agnostic prctl for shadow stack and landing pad instr

[Deepak Gupta]

Three architectures (x86, aarch64, riscv) have announced support for
shadow stack and enforcing requirement of landing pad instructions on
indirect call/jmp. This patch adds arch-agnostic prtcl support to enable
/disable/get/set status of shadow stack and forward control (landing pad)
flow cfi statuses.

New prctls are
      - PR_GET_SHADOW_STACK_STATUS, PR_SET_SHADOW_STACK_STATUS
      - PR_GET_INDIRECT_BR_LP_STATUS, PR_SET_INDIRECT_BR_LP_STATUS

Signed-off-by: Deepak Gupta <debug at rivosinc.com>
---
 include/uapi/linux/prctl.h | 26 +++++++++++++++++++++++++
 kernel/sys.c               | 40 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index a5e06dcbba13..0f401cb2d6d1 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -284,4 +284,30 @@ struct prctl_mm_map {
 #define PR_SET_VMA		0x53564d41
 # define PR_SET_VMA_ANON_NAME		0
 
+/*
+ * get shadow stack status for current thread. Assumes shadow stack is min 4 byte aligned.
+ * Note shadow stack can be 8 byte aligned on 64bit.
+ * Lower 2 bits can give status of locked and enabled/disabled.
+ * size and address range can be obtained via /proc/maps. get_shadow_stack_status will
+ * return base of shadow stack.
+ */
+#define PR_GET_SHADOW_STACK_STATUS      65
+/*
+ * set shadow stack status for current thread (including enabling, disabling or locking)
+ * note that it will only set the status and setup of the shadow stack. Allocating shadow
+ * stack should be done separately using mmap.
+ */
+#define PR_SET_SHADOW_STACK_STATUS      66
+# define PR_SHADOW_STACK_LOCK           (1UL << 0)
+# define PR_SHADOW_STACK_ENABLE         (1UL << 1)
+
+/* get status of requirement of a landing pad instruction for current thread */
+#define PR_GET_INDIRECT_BR_LP_STATUS    67
+/*
+ * set status of requirement of a landing pad instruction for current thread
+ * (including enabling, disabling or locking)
+ */
+#define PR_SET_INDIRECT_BR_LP_STATUS    68
+# define PR_INDIRECT_BR_LP_LOCK         (1UL << 0)
+# define PR_INDIRECT_BR_LP_ENABLE       (1UL << 1)
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 88b31f096fb2..da8c65d474df 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2284,6 +2284,26 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
 	return -EINVAL;
 }
 
+int __weak arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *status)
+{
+	return -EINVAL;
+}
+
+int __weak arch_set_shadow_stack_status(struct task_struct *t, unsigned long __user *status)
+{
+	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;
+}
+
 #define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
 
 #ifdef CONFIG_ANON_VMA_NAME
@@ -2628,6 +2648,26 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 	case PR_SET_VMA:
 		error = prctl_set_vma(arg2, arg3, arg4, arg5);
 		break;
+	case PR_GET_SHADOW_STACK_STATUS:
+		if (arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = arch_get_shadow_stack_status(me, (unsigned long __user *) arg2);
+		break;
+	case PR_SET_SHADOW_STACK_STATUS:
+		if (arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = arch_set_shadow_stack_status(me, (unsigned long __user *) arg2);
+		break;
+	case PR_GET_INDIRECT_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_INDIRECT_BR_LP_STATUS:
+		if (arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = arch_set_indir_br_lp_status(me, (unsigned long __user *) arg2);
+		break;
 	default:
 		error = -EINVAL;
 		break;
-- 
2.25.1