[RFC PATCH] x86: Make sure verify_cpu has a good stack

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>

04633df0c43d ("x86/cpu: Call verify_cpu() after having entered long mode too")
added the call to verify_cpu() for sanitizing CPU configuration.

The latter uses the stack minimally and it can happen that we land in
startup_64() directly from a 64-bit bootloader. Then we want to use our
own, known good stack.

Do that.

APs don't need this as the trampoline sets up a stack for them.

Reported-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
---
 arch/x86/kernel/head_64.S | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 22fbf9df61bb..d60a044c2fdc 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -64,6 +64,10 @@ startup_64:
 	 * tables and then reload them.
 	 */
 
+	/* Setup a stack for verify_cpu */
+	movq    stack_start - __START_KERNEL_map, %rsp
+	subq	$__START_KERNEL_map, %rsp
+
 	/* Sanitize CPU configuration */
 	call verify_cpu
 
-- 
2.3.5