Re: [PATCH] x86: General protection fault after STR (32 bit systems only)

Andy Lutomirski
2015-06-13T18:23:59Z

On Sat, Jun 13, 2015 at 12:03 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Brian Gerst <brgerst@gmail.com> wrote:
>
>> On Fri, Jun 12, 2015 at 4:36 AM, Ingo Molnar <mingo@kernel.org> wrote:
>> >
>> > * H. Peter Anvin <hpa@zytor.com> wrote:
>> >
>> >> %es is used implicitly by string instructions.
>> >
>> > Ok, so we are probably better off reloading ES as well early, right
>> > when we return from the firmware, just in case something does
>> > a copy before we hit the ES restore in restore_processor_state(),
>> > which is a generic C function?
>> >
>> > Something like the patch below?
>> >
>> > I also added FS/GS/SS reloading to make it complete. If this (or a variant
>> > thereof, it's still totally untested) works then we can remove the segment
>> > save/restore layer in __save/restore_processor_state().
>> >
>> > Thanks,
>> >
>> > Ingo
>> >
>> > ===========>
>> > arch/x86/kernel/acpi/wakeup_32.S | 13 +++++++++++++
>> > 1 file changed, 13 insertions(+)
>> >
>> > diff --git a/arch/x86/kernel/acpi/wakeup_32.S b/arch/x86/kernel/acpi/wakeup_32.S
>> > index 665c6b7d2ea9..1376a7fc21b7 100644
>> > --- a/arch/x86/kernel/acpi/wakeup_32.S
>> > +++ b/arch/x86/kernel/acpi/wakeup_32.S
>> > @@ -61,6 +61,19 @@ ENTRY(wakeup_pmode_return)
>> >
>> >
>> > restore_registers:
>> > + /*
>> > + * In case the BIOS corrupted our segment descriptors,
>> > + * reload them to clear out any shadow descriptor
>> > + * state:
>> > + */
>> > + movl $__USER_DS, %eax
>> > + movl %eax, %ds
>> > + movl %eax, %es
>> > + movl %eax, %fs
>> > + movl %eax, %gs
>> > + movl $__KERNEL_DS, %eax
>> > + movl %eax, %ss
>> > +
>> > movl saved_context_ebp, %ebp
>> > movl saved_context_ebx, %ebx
>> > movl saved_context_esi, %esi
>>
>> If you follow the convoluted flow of the calls in this file, wakeup_pmode_return
>> is the first thing called from the trampoline on resume, and that loads the data
>> segments with __KERNEL_DS. [...]
>
> 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.

--Andy