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

1 files changed, 6 insertions, 2 deletions

diff --git a/arch/x86/realmode/rm/reboot.S b/arch/x86/realmode/rm/reboot.S
index 6bf8feac5557..f932ea61d1c8 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