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

Brian Gerst
2015-06-12T15:48:25Z

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. The better
fix would be to set ds/es to __USER_DS there. Also since we are in
the kernel, fs is fixed at __KERNEL_PERCPU, and gs is either
__KERNEL_STACK_CANARY or user's gs.

--
Brian Gerst