shstk: add ability to unlock shadow stack features via ptrace

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>

arch/x86/include/uapi/asm/prctl.h

diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 2109769..ea2e43a 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -24,6 +24,8 @@
 #define ARCH_X86_FEATURE_DISABLE	0x3002
 #define ARCH_X86_FEATURE_LOCK		0x3003
 #define ARCH_X86_FEATURE_ENABLE		0x3004
+/* available only via ptrace(PTRACE_ARCH_PRCTL) */
+#define ARCH_X86_FEATURE_UNLOCK		0x3005
 
 /* x86 feature bits to be used with ARCH_X86_FEATURE arch_prctl()s */
 #define LINUX_X86_FEATURE_IBT		0x00000001

arch/x86/kernel/process_64.c

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index f05fe27..0eff5cf 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -832,6 +832,14 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 		return prctl_map_vdso(&vdso_image_64, arg2);
 #endif
 
+#ifdef CONFIG_X86_SHADOW_STACK
+	case ARCH_X86_FEATURE_UNLOCK:
+		if (task == current)
+			return -EINVAL;
+		task->thread.feat_prctl_locked &= ~arg2;
+		break;
+#endif
+
 	default:
 		ret = -EINVAL;
 		break;