linux-kernel - Re: [BUG] broken ebba638ae723d8a8fc2f7abce5ec18b688b791d7

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Date:	Mon, 31 Jan 2011 13:38:47 -0800
From:	Kees Cook <kees.cook@...onical.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	matthieu castet <castet.matthieu@...e.fr>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [BUG] broken ebba638ae723d8a8fc2f7abce5ec18b688b791d7

Hi,

On Thu, Jan 27, 2011 at 06:24:14PM -0800, 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...

This doesn't appear to work for me. While I can boot fine, doing CPU
hotplugging hangs the system. :(

-Kees

-- 
Kees Cook
Ubuntu Security Team
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