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

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Date:	Mon, 02 May 2016 11:14:25 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	noloader@...il.com
Cc:	Theodore Ts'o <tytso@....edu>, linux-kernel@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>, andi@...stfloor.org,
	Sandy Harris <sandyinchina@...il.com>,
	cryptography@...edaemon.net, jsd@...n.com, hpa@...or.com,
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH 3/3] random: add interrupt callback to VMBus IRQ handler

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@....edu> wrote:
> > From: Stephan Mueller <smueller@...onox.de>
> > 
> > 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?

Ciao
Stephan