Re: [PATCH 3/3] random: add interrupt callback to VMBus IRQ handler

From: Stephan Mueller
Date: Mon May 02 2016 - 05:14:36 EST

Am Montag, 2. Mai 2016, 05:00:47 schrieb Jeffrey Walton:

Hi Jeffrey,

> On Mon, May 2, 2016 at 2:26 AM, Theodore Ts'o <tytso@xxxxxxx> wrote:
> > From: Stephan Mueller <smueller@xxxxxxxxxx>
> >
> > The Hyper-V Linux Integration Services use the VMBus implementation for
> > communication with the Hypervisor. VMBus registers its own interrupt
> > handler that completely bypasses the common Linux interrupt handling.
> > This implies that the interrupt entropy collector is not triggered.
> > ...
> Stephan correctly identified the problem of virtualized environments
> in his paper, but there does not appear to be any real defenses in
> place for VM rollback attacks.

The issue the patch addresses is only that on Hyper-V with para-virt drivers,
the /dev/random implementation does not receive interrupts.

The issue of rollback (if you refer to activating an earlier saved image of
the guest) is a real issue the guest cannot do anything about it that is
effective (i.e. the guest can do without the help of the VMM). Note, rollback
is just a special case of a much broader issue of the duplication of the RNG
state by the VMM (be it snapshots, move of a guest to another VMM,
suspend/resume, ...). However, the patch to enable interrupts does not seem to
be related to that issue as interrupts are not re-issued in case of rollbacks,
are they?