linux-kernel - Re: [BUG] broken ebba638ae723d8a8fc2f7abce5ec18b688b791d7

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Date:	Fri, 28 Jan 2011 08:58:48 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Kees Cook <kees.cook@...onical.com>,
	matthieu castet <castet.matthieu@...e.fr>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [BUG] broken ebba638ae723d8a8fc2f7abce5ec18b688b791d7

On 01/27/2011 07:38 PM, H. Peter Anvin wrote:
> [Adding Jeremy]
>
> Jeremy, would this break Xen?  As far as I know, Xen is the main user
> of skipping segment register initialization.

Xen doesn't go through head_*.S at all.

    J

>
>     -hpa
>
>
> On 01/27/2011 06:24 PM, H. Peter Anvin wrote:
>> On 01/27/2011 03:00 PM, Kees Cook wrote:
>>>
>>> Yikes, good catch.
>>>
>>> arch/x86/kernel/trampoline_64.S uses:
>>> movw $(trampoline_stack_end - r_base), %sp
>>>
>>> arch/x86/boot/compressed/head_64.S uses:
>>> movl $boot_stack_end, %eax
>>> addl %ebp, %eax
>>> movl %eax, %esp
>>>
>>> what would be safe for arch/x86/kernel/head_32.S ? It uses
>>> "stack_start",
>>> but later after paging set-up. Is the following sane to solve this?
>>>
>>
>> To run it before paging is set up, you can't use stack, start; you have
>> to use a pointer based on physical address. You have two problems with
>> using stack_start: you're using a linear address to access stack_start,
>> and stack_start itself contains a linear address.
>>
>> It's not entirely clear to me why we don't initialize %ss to __BOOT_DS
>> with the other segment registers, but it would make most sense to me:
>>
>> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
>> index fc293dc..c10f9ba 100644
>> --- a/arch/x86/kernel/head_32.S
>> +++ b/arch/x86/kernel/head_32.S
>> @@ -99,7 +99,12 @@ ENTRY(startup_32)
>> movl %eax,%es
>> movl %eax,%fs
>> movl %eax,%gs
>> + movl %eax,%ss
>> 2:
>> +/*
>> + * Set up an initial stack
>> + */
>> + movl $pa(init_thread_union+THREAD_SIZE), %esp
>>
>> /*
>> * Clear BSS first so that there are no surprises...
>>
>> -hpa
>

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