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

[Theodore Ts'o]

On Mon, May 02, 2016 at 11:14:25AM +0200, Stephan Mueller wrote:
> 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?

Rollback is just a much broader issue of how can you maintain security
when the VMM is run by the NSA, and can do arbitrary things to mess
with the security of the guest OS (including reading keys straight out
of guest kernel memory, etc.).   Hint: you can't.  :-)

If we are talking about someone who is realistically trying to do
something useful with duplicating VMM state, I'm not aware of anyone
who is actually trying to clone a running VMM in order to launch new
worker nodes.  People will clone disk snapshots to rapidly bring up
rapid nodes, and so making sure we have a way to handle cases where
you can't count on /var/state/random.seed on being useful is
important.  The usual answer is to use something like virtio-rng, but
all of the answers are going to assume that the host system is
trustworthy.

If you are worried about a potential attack where the CIA has cut a
deal with Amazon AWS just as the NSA did with RSADSI and DUAL-EC DRBG,
you might as well go home...

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

[Jeffrey Walton]

On Mon, May 2, 2016 at 2:26 AM, Theodore Ts'o  wrote:
> From: Stephan Mueller 
>
> 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.

Perhpas the following will make interesting reading:

* When Virtual is Harder than Real: Security Challenges in Virtual
Machine Based Computing Environments,
https://www.usenix.org/legacy/event/hotos05/final_papers/full_papers/garfinkel/garfinkel.pdf

* When Good Randomness Goes Bad: Virtual Machine Reset Vulnerabilities
and Hedging Deployed Cryptography,
http://pages.cs.wisc.edu/~rist/papers/sslhedge.pdf

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

[Theodore Ts'o]

From: Stephan Mueller 

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.

This patch adds the interrupt entropy collection callback into the VMBus
interrupt handler function.

Signed-off-by: Stephan Mueller 
Signed-off-by: Stephan Mueller 
Signed-off-by: Theodore Ts'o 
---
 drivers/char/random.c  | 1 +
 drivers/hv/vmbus_drv.c | 3 +++
 2 files changed, 4 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index d5bb3b3..c3f17c9 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1133,6 +1133,7 @@ void add_interrupt_randomness(int irq, int irq_flags)
/* award one bit for the contents of the fast pool */
credit_entropy_bits(r, credit + 1);
 }
+EXPORT_SYMBOL_GPL(add_interrupt_randomness);
 
 #ifdef CONFIG_BLOCK
 void add_disk_randomness(struct gendisk *disk)
diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 64713ff..9af61bb 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -41,6 +41,7 @@
 #include 
 #include 
 #include 
+#include 
 #include "hyperv_vmbus.h"
 
 static struct acpi_device  *hv_acpi_dev;
@@ -801,6 +802,8 @@ static void vmbus_isr(void)
else
tasklet_schedule(hv_context.msg_dpc[cpu]);
}
+
+   add_interrupt_randomness(HYPERVISOR_CALLBACK_VECTOR, 0);
 }
 
 
-- 
2.5.0

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