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

Brian Gerst
2015-06-13T21:30:25Z

On Sat, Jun 13, 2015 at 2:23 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> 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.

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.

--
Brian Gerst