Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3

[Ard Biesheuvel]

On 21 August 2017 at 16:59, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Mon, Aug 21, 2017 at 08:23:10AM -0700, Andy Lutomirski wrote:
>> > Ah, but only root can create per-cpu events or attach events to kernel
>> > threads (with sensible paranoia levels).
>>
>> But this may not need to be percpu.  If a non root user can trigger, say, an EFI variable read in their own thread context, boom.
>
> I was going by the proposed: "everything EFI in a kthread" model. But
> yes, if that's not done, then you're quite right.
>

How does this work in cases where we need to call into UEFI from
non-process context? Or at least from a context where current != EFI's
kthread. We have EFI pstore code, for instance, that records panic
data. Should we make an exception for those?

I'm happy to have a stab at implementing the EFI kthread, but I'd like
to get some of these details clarified first.