x86/head/64: Switch to initial stack earlier

Make sure there is a stack once the kernel runs from virtual addresses.
At this stage any secondary CPU which boots will have lost its stack
because the kernel switched to a new page-table which does not map the
real-mode stack anymore.

This is needed for handling early #VC exceptions caused by instructions
like CPUID.
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/20200907131613.12703-33-joro@8bytes.org

arch/x86/kernel/head_64.S

@@ -192,6 +192,12 @@ SYM_CODE_START(secondary_startup_64)
 	movl	initial_gs+4(%rip),%edx
 	wrmsr
 
+	/*
+	 * Setup a boot time stack - Any secondary CPU will have lost its stack
+	 * by now because the cr3-switch above unmaps the real-mode stack
+	 */
+	movq initial_stack(%rip), %rsp
+
 	/* Check if nx is implemented */
 	movl	$0x80000001, %eax
 	cpuid
@@ -212,9 +218,6 @@ SYM_CODE_START(secondary_startup_64)
 	/* Make changes effective */
 	movq	%rax, %cr0
 
-	/* Setup a boot time stack */
-	movq initial_stack(%rip), %rsp
-
 	/* zero EFLAGS after setting rsp */
 	pushq $0
 	popfq