Re: [BUG] broken ebba638ae723d8a8fc2f7abce5ec18b688b791d7

From: H. Peter Anvin
Date: Thu Jan 27 2011 - 21:24:40 EST

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
+ * Set up an initial stack
+ */
+ movl $pa(init_thread_union+THREAD_SIZE), %esp

* Clear BSS first so that there are no surprises...

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