linux-kernel - Re: [PATCH] x86: kvm: reset the bootstrap processor when it gets an INIT

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Date:	Mon, 11 Mar 2013 20:51:33 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>
Subject: Re: [PATCH] x86: kvm: reset the bootstrap processor when it gets an
 INIT

On Mon, Mar 11, 2013 at 07:47:03PM +0100, Jan Kiszka wrote:
> On 2013-03-11 19:39, Gleb Natapov wrote:
> > On Mon, Mar 11, 2013 at 07:27:44PM +0100, Jan Kiszka wrote:
> >> On 2013-03-11 19:13, Gleb Natapov wrote:
> >>> On Mon, Mar 11, 2013 at 07:05:48PM +0100, Jan Kiszka wrote:
> >>>> On 2013-03-11 18:41, Gleb Natapov wrote:
> >>>>> On Mon, Mar 11, 2013 at 06:34:03PM +0100, Jan Kiszka wrote:
> >>>>>> On 2013-03-11 18:23, Gleb Natapov wrote:
> >>>>>>> On Mon, Mar 11, 2013 at 04:36:33PM +0100, Jan Kiszka wrote:
> >>>>>>>> On 2013-03-11 15:23, Paolo Bonzini wrote:
> >>>>>>>>> Il 11/03/2013 15:05, Gleb Natapov ha scritto:
> >>>>>>>>>> On Mon, Mar 11, 2013 at 03:01:40PM +0100, Jan Kiszka wrote:
> >>>>>>>>>>>> We are not moving away from mp_state, we are moving away from using
> >>>>>>>>>>>> mp_state for signaling because with nested virt INIT does not always
> >>>>>>>>>>>> change mp_state, not only that it can change mp_state long after signal
> >>>>>>>>>>>> is received after vmx off is done.
> >>>>>>>>>>>
> >>>>>>>>>>> Right.
> >>>>>>>>>>>
> >>>>>>>>>>> BTW, for that to happen, we will also need to influence the INIT level.
> >>>>>>>>>>> Unless I misread the spec, INIT is blocked while in root mode, and if
> >>>>>>>>>>> you deassert INIT before leaving root (vmxoff, vmenter), nothing
> >>>>>>>>>>> actually happens. So what matters is the INIT signal level at the exit
> >>>>>>>>>>> of root mode.
> >>>>>>>>>>>
> >>>>>>>>>> You are talking about INIT# signal received via CPU pin, right? I think
> >>>>>>>>>> INIT send by IPI cannot go away.
> >>>>>>>>>
> >>>>>>>>> Neither can go away.  For INIT sent by IPI, 10.4.7 says:
> >>>>>>>>>
> >>>>>>>>> Only the Pentium and P6 family processors support the INIT-deassert IPI.
> >>>>>>>>> An INIT-disassert IPI has no affect on the state of the APIC, other than
> >>>>>>>>> to reload the arbitration ID register with the value in the APIC ID
> >>>>>>>>> register.
> >>>>>>>>>
> >>>>>>>>> 18.27.1 also says that "In the local APIC, NMI and INIT (except for INIT
> >>>>>>>>> deassert) are always treated as edge triggered interrupts".
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> For INIT#, the ICH9 chipset says that "INIT# is driven low for 16 PCI
> >>>>>>>>> clocks" when a soft reset is requested.  So we can guess that INIT# is
> >>>>>>>>> also edge-triggered.
> >>>>>>>>
> >>>>>>>> Ah, ok. So, virtually, INIT stays asserted until it can be delivered in
> >>>>>>>> form of a reset or a vmexit.
> >>>>>>>>
> >>>>>>> vmexit clears it?
> >>>>>>
> >>>>>> It has to. Otherwise, it would hit the host on vmxoff.
> >>>>>>
> >>>>> Why do you thing this is not happening?
> >>>>>
> >>>>> Look at [1] page 10 "VMX and INIT blocking". Do you think they were
> >>>>> lucky to hit CPU while it was in a root mode?
> >>>>>
> >>>>> [1] http://www.invisiblethingslab.com/resources/2011/Software%20Attacks%20on%20Intel%20VT-d.pdf
> >>>>
> >>>> Interesting. And confusing. If a VMM cannot "consume" INIT events by
> >>>> reentering the guest nor postpone those events up to that point if they
> >>>> arrived in root mode, the whole vmexit-on-INIT thing is practically
> >>>> useless. I wonder what use case Intel had in mind while designing this.
> >>>>
> >>> I actually find it very useful. On INIT vmexit hypervisor may call
> >>> vmxoff and do proper reset. I find it less useful on AMD where you need
> >>> to send self INIT IPI, but then how you can send self SIPI?
> >>
> >> Where's the difference? On Intel, SIPI is also not deliverable until
> >> after vmxoff. So that signal has to come from the INIT sender, just like
> >> on AMD.
> >>
> > On Intel:
> > CPU 1                          CPU 2 in a guest mode
> > send INIT
> > send SIPI
> >                                 INIT vmexit
> >                                 vmxoff
> >                                 reset and start from SIPI vector
> 
> Is SIPI sticky as well, even if the CPU is not in the wait-for-SIPI
> state (but runnable and in vmxon) while receiving it?
> 
That what they seams to be saying:
 However, an INIT and SIPI interrupts sent to a CPU during time when
 it is in a VMX mode are remembered and delivered, perhaps hours later,
 when the CPU exits the VMX mode

Otherwise their exploit will not work.

--
			Gleb.
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