[PATCH v6 34/76] x86/head/64: Switch to initial stack earlier

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

Make sure there is a stack once the kernel runs from virual 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@xxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200724160336.5435-34-joro@xxxxxxxxxx
---
 arch/x86/kernel/head_64.S | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 057c7bd3eeb6..a5e1939d1dc9 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -200,6 +200,12 @@ 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
@@ -220,9 +226,6 @@ 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
-- 
2.28.0