linux-kernel - Re: [BUG] broken ebba638ae723d8a8fc2f7abce5ec18b688b791d7

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Date:	Thu, 27 Jan 2011 18:24:14 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kees Cook <kees.cook@...onical.com>
CC:	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 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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