Re: 64-bit PV and Meltdown

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Tue, Jan 23, 2018 at 11:18:47AM -0500, Brian Marcotte wrote:
> This question is specific to Meltdown.
> 
> XSA-254 states that when using the Xen PTI patches a 64-bit PV guest
> is automatically protected against Meltdown even if the guest hasn't
> been updated with a Meltdown mitigation.
> 
> Can anyone confirm this is the case with a NetBSD guest (64-bit PV)?

Sure.
In fact, Xen 86-64 PV guests have separate user/kernel spaces since
day one (because the kernel also runs unprivileged). Meltdown was only
able to attack the hypervisor, but since the hypervisor has a linear
mapping of the whole physical space (much like we do on bare x86-64),
this turns into an attack against everything.
Removing this linear mapping from the hypervisor is enough
to work around Meltdown against guests (not sure but I think with
the current state you could still get some bits from the hypervisor).

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--