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

[Peter Zijlstra]

On Mon, Aug 21, 2017 at 06:56:01AM -0700, Andy Lutomirski wrote:
> There are two ways this could be a problem.  One is that u privileged
> user apps shouldn't be able to read from EFI memory.  The other is
> that, if EFI were to have IO memory mapped at a "user" address, perf
> could end up reading it.

So assuming the efi_switch_mm() case from the calling thread context, I
don't see how we can avoid it at all.

Suppose we have a free running PEBS counter (PEBS puts samples in DS
buffer and only raises PMI when 'full'). This can easily cover the
entire efi_switch_mm() and back swizzle, and then we have 'userspace'
samples that don't correspond to actual userspace.

EFI (pretending to be userspace) is a giant trainwreck.