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Re: [Xen-devel] RFC Xen signature verification for kexec

On Tue, Apr 24, 2018 at 10:46:38AM +0100, George Dunlap wrote:
> On Mon, Apr 23, 2018 at 11:33 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >>>> On 23.04.18 at 12:25, <daniel.kiper@xxxxxxxxxx> wrote:
> >> On Mon, Apr 23, 2018 at 12:55:45AM -0600, Jan Beulich wrote:
> >>> >>> On 20.04.18 at 21:12, <eric.devolder@xxxxxxxxxx> wrote:
> >>> > Two options for signature verification in Xen
> >>> >
> >>> > This proposal outlines two options under consideration for enhancing
> >>> > Xen to support signature verification of kexec loaded images. The
> >>> > first option is essentially to mirror Linux signature verification
> >>> > code into Xen. The second option utilizes components from sources
> >>> > other than Linux (for example, libgcrypt rather than linux/crypto).
> >>> >
> >>> > NOTE: An option to utilize dom0 kernel signature verification does not
> >>> > prevent the exploit as user space can invoke the hypercall directly,
> >>> > bypassing dom0.
> >>>
> >>> Not exactly - this option nevertheless exists, albeit is perhaps
> >>> unattractive: No user space component can issue hypercalls
> >>> directly, they always go through the privcmd driver. Hence the
> >>> driver cold snoop the kexec hypercall.
> >>
> >> Hmmm... Is not it a problem from security point of view for us in this
> >> case? It should not if dom0 kernel is signed. It have to be signed here.
> >> Just thinking a loud...
> >
> > I'm afraid I don't understand: If the Dom0 kernel isn't signed (or hasn't
> > been verified), the system is insecure in the first place. No reason to
> > bother measuring the kexec kernel then.
> I think you're both saying the same thing.
> FWIW I wouldn't mind coming up with a hypercall that the privcmd
> driver refuses to pass-through as-is, and having some way for the
> tools to ask the kernel to check the signature.

I have a feeling that I should reformulate the question: How far the Xen
hypervisor trusts the privcmd driver? If the privcmd driver is signed
then at first sight there should not be a problem. However, we can be
more strict and require that (every? Daniel is running away...) hypercall
from privcmd to Xen should be verified somehow. Maybe I am overzealous but
I think that it make sense to discuss this now than later have problems.


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