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

[Borislav Petkov <bp@alien8.de>]

On Wed, Mar 02, 2016 at 02:32:54PM -0800, H. Peter Anvin wrote:
> I'm trying to think of any reason why we couldn't simply have a symbol
> at the top of the initial stack? Then a simple leaq would suffice;
> this is for the BSP after all.

How about something like this:

---
From: Borislav Petkov <bp@suse.de>
Date: Sun, 28 Feb 2016 21:35:44 +0100
Subject: [PATCH -v2] x86/asm: Make sure verify_cpu() has a good stack
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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@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Mika Penttilä <mika.penttila@nextfour.com>
---
 arch/x86/kernel/head_64.S         | 3 +++
 include/asm-generic/vmlinux.lds.h | 4 +++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 22fbf9df61bb..968d6408b887 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -64,6 +64,9 @@ startup_64:
 	 * tables and then reload them.
 	 */
 
+	/* Setup stack for verify_cpu(). */
+	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 772c784ba763..cba2a26628fc 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -246,7 +246,9 @@
 
 #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
-- 
2.3.5

-- 
Regards/Gruss,
    Boris.

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