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Date: Mon, 21 Aug 2017 19:24:25 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>, Will Deacon <will.deacon@arm.com>,
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        "Neri, Ricardo" <ricardo.neri@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>
Subject: Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually
 twiddling with cr3
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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.