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

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Date:	Wed, 2 Mar 2016 23:09:54 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Wed, Mar 02, 2016 at 01:54:50PM -0800, H. Peter Anvin wrote:
> 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.

Hmm, but __START_KERNEL_map is a simple macro:

#define __START_KERNEL_map      _AC(0xffffffff80000000, UL)

Ok, I think you want to do something like this for stack_start too:

        /*
         * Compute the delta between the address I am compiled to run at and the
         * address I am actually running at.
         */
        leaq    _text(%rip), %rbp
        subq    $_text - __START_KERNEL_map, %rbp
	...

in the normal case %rbp is 0, of course.

-- 
Regards/Gruss,
    Boris.

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