[PATCH] x86: Load __USER_DS into DS/ES after resume

Ingo Molnar
2015-06-14T06:56:50Z

* Brian Gerst <brgerst@gmail.com> wrote:

> >> So if wakeup_pmode_return is really the first thing called then the whole
> >> premise of shadow descriptor corruption goes out the window: we reload all
> >> relevant segment registers.
> >
> > True, but it still leaves the fact that we're loading __KERNEL_DS instead of
> > __USER_DS, right? So we end up in the kernel in some context (I have no clue
> > what context) with __KERNEL_DS loaded. It's very easy for us to inadvertently
> > fix it: we could return to userspace by any means whatsoever except SYSEXIT,
> > or we could even return back to some preempted kernel context.
> >
> > I still think we should replace __KERNEL_DS with __USER_DS in
> > wakeup_pmode_return and see if the problem goes away.
>
> I'm pretty sure that's what the problem is. If you look at the sysexit path, it
> never reloads ds/es. It assumes they are still __USER_DS set at sysenter. The
> iret path does restore all the user segments.

Ok, so something like the patch below, right?

Thanks,

Ingo

=====================>
arch/x86/kernel/acpi/wakeup_32.S | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/acpi/wakeup_32.S b/arch/x86/kernel/acpi/wakeup_32.S
index 665c6b7d2ea9..7302bbaea184 100644
--- a/arch/x86/kernel/acpi/wakeup_32.S
+++ b/arch/x86/kernel/acpi/wakeup_32.S
@@ -12,11 +12,13 @@ ENTRY(wakeup_pmode_return)
wakeup_pmode_return:
movw $__KERNEL_DS, %ax
movw %ax, %ss
- movw %ax, %ds
- movw %ax, %es
movw %ax, %fs
movw %ax, %gs

+ movw $__KERNEL_DS, %ax
+ movw %ax, %ds
+ movw %ax, %es
+
# reload the gdt, as we need the full 32 bit address
lidt saved_idt
lldt saved_ldt