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

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Message-ID: <20170821172425.5axqiwnef5gkaz23@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Aug 2017 19:24:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        joeyli <jlee@...e.com>, Borislav Petkov <bp@...en8.de>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "Neri, Ricardo" <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually
 twiddling with cr3

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.