[PATCH] x86/asm: 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>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Mika Penttilà <mika.penttila@xxxxxxxxxxxx>
---
 arch/x86/kernel/head_64.S         | 8 ++++++++
 include/asm-generic/vmlinux.lds.h | 4 +++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 22fbf9df61bb..e69742517a54 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -64,6 +64,14 @@ startup_64:
 	 * tables and then reload them.
 	 */
 
+	/*
+	 * Setup stack for verify_cpu(). "-8" because stack_start is defined
+	 * this way, see below. Our best guess is a NULL ptr for stack
+	 * termination heuristics and we don't want to break anything which
+	 * might depend on it (kgdb, ...).
+	 */
+	leaq	(__end_init_task - 8)(%rip), %rsp
+
 	/* Sanitize CPU configuration */
 	call verify_cpu
 
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 339125bb4d2c..6a67ab94b553 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -245,7 +245,9 @@
 
 #define INIT_TASK_DATA(align)						\
 	. = ALIGN(align);						\
-	*(.data..init_task)
+	VMLINUX_SYMBOL(__start_init_task) = .;				\
+	*(.data..init_task)						\
+	VMLINUX_SYMBOL(__end_init_task) = .;
 
 /*
  * Read only Data
-- 
2.7.3