linux-kernel - Re: [PATCH 1/2] efi: Make 'efi_enabled' a function to query EFI facilities

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Date:	Wed, 23 Jan 2013 07:52:12 +0000
From:	Matt Fleming <matt@...sole-pimps.org>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	linux-efi@...r.kernel.org, Tim Gardner <rtg.canonical@...il.com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Olof Johansson <olof@...om.net>,
	Tony Luck <tony.luck@...el.com>,
	Steve Langasek <steve.langasek@...onical.com>,
	Colin Ian King <colin.king@...onical.com>,
	David Airlie <airlied@...ux.ie>,
	Corentin Chary <corentincj@...aif.net>,
	Dave Jiang <dave.jiang@...el.com>,
	Peter Jones <pjones@...hat.com>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] efi: Make 'efi_enabled' a function to query EFI
 facilities

On Wed, 2013-01-23 at 04:16 +0000, Ben Hutchings wrote:
> On Mon, 2013-01-21 at 21:12 +0000, Matt Fleming wrote:
> [...]
> > From 92e73f936e40a8c6562e47425d434a4e62d2b8e2 Mon Sep 17 00:00:00 2001
> > From: Matt Fleming <matt.fleming@...el.com>
> > Date: Wed, 14 Nov 2012 09:42:35 +0000
> > Subject: [PATCH] efi: Make 'efi_enabled' a function to query EFI facilities
> > 
> > Originally 'efi_enabled' indicated whether a kernel was booted from
> > EFI firmware. Over time its semantics have changed, and it now
> > indicates whether or not we are booted on an EFI machine with
> > bit-native firmware, e.g. 64-bit kernel with 64-bit firmware.
> > 
> > The immediate motivation for this patch is the bug report at,
> > 
> >     https://bugs.launchpad.net/ubuntu-cdimage/+bug/1040557
> > 
> > which details how running a platform driver on an EFI machine that is
> > designed to run under BIOS can cause the machine to become
> > bricked. Also, the following report,
> > 
> >     https://bugzilla.kernel.org/show_bug.cgi?id=47121
> > 
> > details how running said driver can also cause Machine Check
> > Exceptions. Drivers need a new means of detecting whether they're
> > running on an EFI machine, as sadly the expression,
> > 
> >     if (!efi_enabled)
> > 
> > hasn't been a sufficient condition for quite some time.
> [...]
> 
> This patch maps the old efi_enabled flag to efi_enabled(EFI_BOOT).  Your
> second patch adds a test for efi_enabled(EFI_BOOT) to the samsung-laptop
> driver.  So the samsung-laptop driver could be fixed by adding a check
> for the old flag; it doesn't depend on the addition of new flags at all.
> The changes elsewhere may well be important, but the cited motivation
> here just doesn't make sense.

If we instead check for the old flag then the samsung-laptop driver
would run if booting a 32-bit kernel with 64-bit EFI firmware, or vice
versa, because efi_enabled == 0 in that case.

It's not sufficient to just check the old flag, the samsung-laptop
driver really can't be allowed to run at all if booting from EFI, and
the old flag doesn't provide us with that guarantee.

-- 
Matt Fleming, Intel Open Source Technology Center

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