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

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Date:   Mon, 21 Aug 2017 17:08:50 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        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 21 August 2017 at 16:59, Peter Zijlstra <peterz@...radead.org> 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.