Re: [BUG] broken ebba638ae723d8a8fc2f7abce5ec18b688b791d7

From: matthieu castet
Date: Mon Jan 31 2011 - 18:11:25 EST


Kees Cook a écrit :
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. :(

This is weird because the patch only touch first cpu (startup_32 entry) and cpu hotplug go to startup_32_smp.

Here a untested patch that move the stack setup in the common path.


Matthieu


diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index fc293dc..5df3432 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -284,6 +284,12 @@ ENTRY(startup_32_smp)
movl %eax,%gs
#endif /* CONFIG_SMP */
default_entry:
+ /*
+ * Set up an initial stack
+ */
+ movl $(__BOOT_DS),%eax
+ movl %eax,%ss
+ movl $pa(init_thread_union+THREAD_SIZE), %esp

/*
* New page tables may be in 4Mbyte page mode and may