From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [PATCH] x86 efi: bugfix interrupt disabling sequence Date: Sat, 21 Sep 2013 09:50:02 +0200 Message-ID: <20130921075002.GB7771@gmail.com> References: <1379525333-4373-1-git-send-email-bemk@redhat.com> <20130920152859.GC21381@console-pimps.org> <1379708486.12705.131.camel@dhcp-128-237.ams.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1379708486.12705.131.camel-0VdLhd/A9PkhetGgFr3ssPXAX3CI6PSWQQ4Iyu8u01E@public.gmane.org> Sender: linux-efi-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Bart Kuivenhoven Cc: Matt Fleming , matt.fleming-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org, hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org, tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org, mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, linux-efi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, jcm-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, msalter-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org List-Id: linux-efi@vger.kernel.org * Bart Kuivenhoven wrote: > On Fri, 2013-09-20 at 16:28 +0100, Matt Fleming wrote: > > On Wed, 18 Sep, at 07:28:53PM, Bart Kuivenhoven wrote: > > > The problem in efi_main was that the idt was cleared before the > > > interrupts were disabled. > > > > > > The UEFI spec states that interrupts aren't used so this shouldn't be > > > too much of a problem. Peripherals however don't necessarily know about > > > this and thus might cause interrupts to happen anyway. Even if > > > ExitBootServices() has been called. > > > > > > This means there is a risk of an interrupt being triggered while the IDT > > > register is nullified and the interrupt bit hasn't been cleared, > > > allowing for a triple fault. > > > > Just to be clear, you haven't witnessed a triple fault, correct? > > > > > This patch fixes this by clearing the interrupt bit before the lidt > > > instruction. > > > > I think we can go even further than this and get rid of all of the IDT > > code in the EFI boot stub. The kernel maintains its own IDT anyway. > > > > Well, isn't it so, that the kernel expects a setup in which interrupts > are disabled before the decompressed image is loaded? > > What we can do is remove the lidt instruction and IDT pointer, but that > still doesn't change anything with regards to the kernels expectations. > > And no, I haven't witnessed a triple fault, this is purely theoretical, > with a very slim chance of it actually happening. That does not mean > that it can't happen though. it would also be very hard to prove that it occured (outside of special debug environments) - spurious, low probability triple faults are as undebuggable as it gets. Thanks, Ingo