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

From: Andy Lutomirski <luto@kernel.org>
To: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Andy Lutomirski <luto@kernel.org>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	joeyli <jlee@suse.com>, Borislav Petkov <bp@alien8.de>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"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
Date: Wed, 16 Aug 2017 09:14:41 -0700	[thread overview]
Message-ID: <CALCETrVSHuz=hcAgGxYHBmpiNcP4ECrJh885cR4X17QsJVj8+w@mail.gmail.com> (raw)
In-Reply-To: <20170816125715.GB3384@codeblueprint.co.uk>

On Wed, Aug 16, 2017 at 5:57 AM, Matt Fleming <matt@codeblueprint.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