x86/head/64: Load GDT after switch to virtual addresses

Load the GDT right after switching to virtual addresses to make sure
there is a defined GDT for exception handling.
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20200907131613.12703-31-joro@8bytes.org

arch/x86/kernel/head_64.S

@@ -158,6 +158,14 @@ SYM_CODE_START(secondary_startup_64)
 1:
 	UNWIND_HINT_EMPTY
 
+	/*
+	 * We must switch to a new descriptor in kernel space for the GDT
+	 * because soon the kernel won't have access anymore to the userspace
+	 * addresses where we're currently running on. We have to do that here
+	 * because in 32bit we couldn't load a 64bit linear address.
+	 */
+	lgdt	early_gdt_descr(%rip)
+
 	/* Check if nx is implemented */
 	movl	$0x80000001, %eax
 	cpuid
@@ -185,14 +193,6 @@ SYM_CODE_START(secondary_startup_64)
 	pushq $0
 	popfq
 
-	/*
-	 * We must switch to a new descriptor in kernel space for the GDT
-	 * because soon the kernel won't have access anymore to the userspace
-	 * addresses where we're currently running on. We have to do that here
-	 * because in 32bit we couldn't load a 64bit linear address.
-	 */
-	lgdt	early_gdt_descr(%rip)
-
 	/* set up data segments */
 	xorl %eax,%eax
 	movl %eax,%ds