Date: Wed, 2 Mar 2016 20:50:53 +0100
From: Borislav Petkov <bp@alien8.de>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Brian Gerst <brgerst@gmail.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [RFC PATCH] x86: Make sure verify_cpu has a good stack
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On Wed, Mar 02, 2016 at 10:39:05AM -0800, H. Peter Anvin wrote:
> Well, we definitely should use %rip-relative addressing if we can.

Right you are.

> 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.

Yeah, we should be prudent and make this as sturdy as possible. I did this:

CONFIG_PHYSICAL_START=0x100beef

and it aligned startup_64 up to ffffffff82000000. It seems to boot fine
in kvm. But better safe than sorry.

Thanks.

-- 
Regards/Gruss,
    Boris.

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