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Subject[tip:x86/reboot] x86-64, reboot: Be more paranoid in 64-bit reboot =bios
Commit-ID:  9751d7627582fc1cc64625d63bde9528c14f1544
Gitweb: http://git.kernel.org/tip/9751d7627582fc1cc64625d63bde9528c14f1544
Author: H. Peter Anvin <hpa@zytor.com>
AuthorDate: Thu, 21 Jun 2012 10:25:03 -0700
Committer: H. Peter Anvin <hpa@zytor.com>
CommitDate: Thu, 21 Jun 2012 10:25:03 -0700

x86-64, reboot: Be more paranoid in 64-bit reboot=bios

Be a bit more paranoid in the transition back to 16-bit mode. In
particular, in case the kernel is residing above the 4 GiB mark,
switch to the trampoline GDT, and make the jump after turning off
paging a far jump. In theory, none of this should matter, but it is
exactly the kind of things that broken SMM or virtualization software
could trip up on.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/tip-jopx7y6g6dbcx4tpal8q0jlr@git.kernel.org
---
arch/x86/realmode/rm/reboot.S | 8 ++++++--
1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/realmode/rm/reboot.S b/arch/x86/realmode/rm/reboot.S
index 6bf8fea..f932ea6 100644
--- a/arch/x86/realmode/rm/reboot.S
+++ b/arch/x86/realmode/rm/reboot.S
@@ -22,14 +22,18 @@
ENTRY(machine_real_restart_asm)

#ifdef CONFIG_X86_64
+ /* Switch to trampoline GDT as it is guaranteed < 4 GiB */
+ movl $__KERNEL_DS, %eax
+ movl %eax, %ds
+ lgdtl pa_tr_gdt

/* Disable paging to drop us out of long mode */
movl %cr0, %eax
andl $~X86_CR0_PG, %eax
movl %eax, %cr0
- jmp 1f /* "A branch" may be needed here, assume near is OK */
+ ljmpl $__KERNEL32_CS, $pa_machine_real_restart_paging_off

-1:
+GLOBAL(machine_real_restart_paging_off)
xorl %eax, %eax
xorl %edx, %edx
movl $MSR_EFER, %ecx

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