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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: <56D89CEE.9070406@zytor.com>
Date: Thu, 3 Mar 2016 12:22:06 -0800
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On 03/03/16 08:29, Borislav Petkov wrote:
> On Thu, Mar 03, 2016 at 07:26:06AM -0800, H. Peter Anvin wrote:
>> Why -8?
> 
>         GLOBAL(stack_start)
>         .quad  init_thread_union+THREAD_SIZE-8
> 					    ^^^
> 
> But I don't see why it needed the -8 then. It came with a conglomerate
> dump in 2002:
> 
> commit af53c7a2c81399b805b6d4eff887401a5e50feef
> Author: Andi Kleen <ak@muc.de>
> Date:   Fri Apr 19 20:23:17 2002 -0700
> 
>     [PATCH] x86-64 architecture specific sync for 2.5.8
> 
> 
> -       /* Setup the first kernel stack (this instruction is modified by smpboot) */
> -       .byte 0x48, 0xb8        /* movq *init_rsp,%rax */ 
> -init_rsp:
> -       .quad init_thread_union+THREAD_SIZE
> -       movq    %rax, %rsp
> 
> ...
> 
> -
> -       /* SMP bootup changes this */   
> +       /* SMP bootup changes these two */      
>         .globl  initial_code
>  initial_code:
>         .quad   x86_64_start_kernel
> +       .globl init_rsp
> +init_rsp:
> +       .quad  init_thread_union+THREAD_SIZE-8
> +
> ---
> 
> But since we decrement first and then copy to stack ptr when we push, I
> don't see why we need the -8.
> 
> Do you have a better clue?
> 

The only thing I can think of is that the -8 creates a null pointer that
terminates a stack trace.

	-hpa