x86/shstk: Wire in shadow stack interface

The kernel now has the main shadow stack functionality to support
applications. Wire in the WRSS and shadow stack enable/disable functions
into the existing shadow stack API skeleton.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-38-rick.p.edgecombe%40intel.com

arch/x86/kernel/shstk.c

@@ -502,9 +502,17 @@ long shstk_prctl(struct task_struct *task, int option, unsigned long features)
 		return -EINVAL;
 
 	if (option == ARCH_SHSTK_DISABLE) {
+		if (features & ARCH_SHSTK_WRSS)
+			return wrss_control(false);
+		if (features & ARCH_SHSTK_SHSTK)
+			return shstk_disable();
 		return -EINVAL;
 	}
 
 	/* Handle ARCH_SHSTK_ENABLE */
+	if (features & ARCH_SHSTK_SHSTK)
+		return shstk_setup();
+	if (features & ARCH_SHSTK_WRSS)
+		return wrss_control(true);
 	return -EINVAL;
 }