linux-kernel - Re: [PATCH] intel_iommu: Disable vfio and kvm interrupt assignment when unsafe

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Date:	Thu, 7 Feb 2013 18:27:01 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Gleb Natapov <gleb@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Don Zickus <dzickus@...hat.com>,
	Prarit Bhargava <prarit@...hat.com>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH] intel_iommu: Disable vfio and kvm interrupt assignment
 when unsafe

On Thu, Feb 07, 2013 at 08:29:42AM -0800, Andy Lutomirski wrote:
> On Thu, Feb 7, 2013 at 3:33 AM, Joerg Roedel <joro@...tes.org> wrote:
> > On Wed, Feb 06, 2013 at 07:08:24PM -0800, Andy Lutomirski wrote:
> >> -     if (x2apic_present)
> >> -             WARN(1, KERN_WARNING
> >> -                     "Failed to enable irq remapping.  You are vulnerable to irq-injection attacks.\n");
> >> -
> >> +     irq_remapping_is_secure = 0;
> >>       return -1;
> >>  }
> >
> > Why do you remove this warning? It seems unrelated to the rest of the
> > patch.
> 
> The idea is that setting irq_remapping_is_secure = 0 makes you (much
> less) vulnerable to irq-injection attacks: you're vulnerable to
> malicious hardware but not to attack via vfio or kvm, because those
> paths are disabled.
> 
> I'd have no problem leaving the warning in and letting whoever manages
> to trigger it and get annoyed fix it.  FWIW, it's actually likely to
> be interesting if the warning hits.

Hmm, looking into the intel_irq_remapping.c version in the tip tree
makes me wonder even more.

First, I wonder why the warning only hits when an x2apic is present. The
function is not x2apic-specific and the vulnerability also exists in
xapic mode. So that dependency can be removed.

Second, I think that it should be a pr_warn instead of a full WARN. When
IRQ remapping could not be enabled it's most likely because of the BIOS
or the hardware. So a message in the kernel log will do and the
backtrace provides no additional value.

Same is true for the warning in the function iommu_set_irq_remapping():

       if (sts & DMA_GSTS_CFIS)
                WARN(1, KERN_WARNING
                        "Compatibility-format IRQs enabled despite intr remapping;\n"
                        "you are vulnerable to IRQ injection.\n");

>From what I can see this condition depends only on the hardware too. So
a simple pr_warn() provides the same amount of information.


Regards,

	Joerg



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