linux-kernel - Re: [RFC PATCH] x86: Make sure verify

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Message-ID: <56D73349.3020609@zytor.com>
Date:	Wed, 2 Mar 2016 10:39:05 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Brian Gerst <brgerst@...il.com>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [RFC PATCH] x86: Make sure verify_cpu has a good stack

On 03/02/16 10:15, Borislav Petkov wrote:
> On Wed, Mar 02, 2016 at 09:53:28AM -0800, H. Peter Anvin wrote:
>> Please explain why we can't use rip-relative addressing in some form...
> 
> We *can* do almost what Brian suggested:
> 
>         movq    stack_start(%rip), %rsp
>         subq    $__START_KERNEL_map, %rsp
> 
> But we still have to subtract __START_KERNEL_map.
> 

Well, we definitely should use %rip-relative addressing if we can.

However, even so I believe this breaks if the kernel is loaded anywhere
but its default load address.  I think we need to do something like:

	movq	stack_start(%rip), %rax
	leaq	__START_KERNEL_map(%rip), %rdx
	subq	%rdx, %rax
	movq	%rax, %rsp

The use of temporary registers avoids clobbering a valid stack pointer
for even a single instruction if we are given one.

	-hpa