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

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Date:   Wed, 16 Aug 2017 09:14:41 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Matt Fleming <matt@...eblueprint.co.uk>
Cc:     Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "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 Wed, Aug 16, 2017 at 5:57 AM, Matt Fleming <matt@...eblueprint.co.uk> wrote:
> On Wed, 16 Aug, at 12:03:22PM, Mark Rutland wrote:
>>
>> I'd expect we'd abort at a higher level, not taking any sample. i.e.
>> we'd have the core overflow handler check in_funny_mm(), and if so, skip
>> the sample, as with the skid case.
>
> FYI, this is my preferred solution for x86 too.

One option for the "funny mm" flag would be literally the condition
current->mm != current->active_mm.  I *think* this gets all the cases
right as long as efi_switch_mm is careful with its ordering and that
the arch switch_mm() code can handle the resulting ordering.  (x86's
can now, I think, or at least will be able to in 4.14 -- not sure
about other arches).

That being said, there's a totally different solution: run EFI
callbacks in a kernel thread.  This has other benefits: we could run
those callbacks in user mode some day, and doing *that* in a user
thread seems like a mistake.

--Andy