Subject: Re: [RFC PATCH] x86: Make sure verify_cpu has a good stack
To: Borislav Petkov <bp@alien8.de>
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Cc: Brian Gerst <brgerst@gmail.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Tom Lendacky <thomas.lendacky@amd.com>
From: "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <56D7612A.4080107@zytor.com>
Date: Wed, 2 Mar 2016 13:54:50 -0800
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On 03/02/16 13:46, Borislav Petkov wrote:
> On Wed, Mar 02, 2016 at 01:35:09PM -0800, H. Peter Anvin wrote:
>> You're not actually testing anything as the real issue is what happens
>> with a relocating bootloader.
> 
> Hmm, how would that relocation happen so that va - __START_KERNEL_map
> doesn't give pa?
> 
> Or do you mean something else with "relocating bootloader"? Do you know
> of one which does that?
> 

A relocating bootloader is one that doesn't load the kernel at
CONFIG_PHYSICAL_ADDRESS.  The EFI stub is one example.

__START_KERNEL_map is not relocated.  On x86-64 we do relocation by
pointing the page tables at a different address.

So I really think we need this to be a leaq, so we take a nonstandard
load address into consideration.

	-hpa