x86/head/64: Load segment registers earlier

Make sure segments are properly set up before setting up an IDT and
doing anything that might cause a #VC exception. This is later needed
for early 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/20200907131613.12703-32-joro@8bytes.org

arch/x86/kernel/head_64.S

@@ -166,6 +166,32 @@ SYM_CODE_START(secondary_startup_64)
 	 */
 	lgdt	early_gdt_descr(%rip)
 
+	/* set up data segments */
+	xorl %eax,%eax
+	movl %eax,%ds
+	movl %eax,%ss
+	movl %eax,%es
+
+	/*
+	 * We don't really need to load %fs or %gs, but load them anyway
+	 * to kill any stale realmode selectors.  This allows execution
+	 * under VT hardware.
+	 */
+	movl %eax,%fs
+	movl %eax,%gs
+
+	/* Set up %gs.
+	 *
+	 * The base of %gs always points to fixed_percpu_data. If the
+	 * stack protector canary is enabled, it is located at %gs:40.
+	 * Note that, on SMP, the boot cpu uses init data section until
+	 * the per cpu areas are set up.
+	 */
+	movl	$MSR_GS_BASE,%ecx
+	movl	initial_gs(%rip),%eax
+	movl	initial_gs+4(%rip),%edx
+	wrmsr
+
 	/* Check if nx is implemented */
 	movl	$0x80000001, %eax
 	cpuid
@@ -193,32 +219,6 @@ SYM_CODE_START(secondary_startup_64)
 	pushq $0
 	popfq
 
-	/* set up data segments */
-	xorl %eax,%eax
-	movl %eax,%ds
-	movl %eax,%ss
-	movl %eax,%es
-
-	/*
-	 * We don't really need to load %fs or %gs, but load them anyway
-	 * to kill any stale realmode selectors.  This allows execution
-	 * under VT hardware.
-	 */
-	movl %eax,%fs
-	movl %eax,%gs
-
-	/* Set up %gs.
-	 *
-	 * The base of %gs always points to fixed_percpu_data. If the
-	 * stack protector canary is enabled, it is located at %gs:40.
-	 * Note that, on SMP, the boot cpu uses init data section until
-	 * the per cpu areas are set up.
-	 */
-	movl	$MSR_GS_BASE,%ecx
-	movl	initial_gs(%rip),%eax
-	movl	initial_gs+4(%rip),%edx
-	wrmsr
-
 	/* rsi is pointer to real mode structure with interesting info.
 	   pass it to C */
 	movq	%rsi, %rdi