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Re: [Qemu-devel] [RFC] Device sandboxing

From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC] Device sandboxing
Date: Wed, 07 Dec 2011 12:48:16 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110831 Lightning/1.0b2 Thunderbird/3.1.13

On 12/07/2011 12:25 PM, Corey Bryant wrote:
A group of us are starting to work on sandboxing QEMU device emulation
code. We're just getting started investigating various approaches, and
want to engage the community to gather input.

Following are the design points that we are currently considering:

To be perfectly honest, I think prototyping and measuring performance is going to be the only way to figure out the right approach here. Here are some thoughts on the various approaches.

* Decompose QEMU into multiple processes:

* This could be done such that QEMU devices execute in separate
processes based on device type, e.g. all block devices in one
process and all network devices in a second process. Another
alternative is executing a separate process per device.

I don't think that a HIRD of QEMU-replacing daemons is the best approach to this problem. While I appreciate the academic attraction to such a proposal, I think practical experience tells us that this isn't the easiest type of system to get right.

* Decomposition would not only afford a level of security inherent
in process separation, it would also allow development of stricter
sVirt/SELinux policy for the decomposed QEMU processes (e.g. a
block device specific policy). This would enable a true sandbox
with layers of defense.

* Decompose the device emulation process further into an untrusted and
trusted thread:

I think this general approach is the most rationale place to start.

* The untrusted thread would be restricted by seccomp mode 1 and
would contain the device emulation code.

I think the best strategy would allow for a device to run either in the untrusted thread or the trusted thread. This makes performance testing a bit easier and it also makes development a bit more natural.

* The trusted helper thread would run beside the untrusted thread,
enabling the untrusted thread to make syscalls beyond read(),
write(), exit(), and sigreturn().

I assume you mean process, not thread BTW?

* IPC communication mechanisms:

* An IPC mechanism will be required to enable communication between
untrusted and trusted threads.

* An IPC mechanism will also be required to enable communication
between the main QEMU process and device processes.

IPC is easy. We have tons of infrastructure in QEMU for IPC (virtio, QMP, etc.). Please don't reinvent the wheel here.

* The communication mechanisms must provide secure communication,
be low overhead (easy to generate, parse, and validate), and must
play well with sVirt/LSMs.

I don't see how sVirt/LSM fits into this but all of these requirements are also true for the other big untrusted thread that we interact with (the guest itself).

My view is that we should view the untrusted thread as an extension of the guest and that the interfaces between the trusted thread and the untrusted thread views it simply as another machine type that presents a different (simpler) hardware abstraction.

* Some thoughts for IPC mechanisms are Unix sockets, pipes, virtio,
Google Native Client's IMC, and shared memory.

The actual mechanism doesn't really matter I think, but see above comments.

* If seccomp mode 2 support becomes available, decomposition of device
emulation into untrusted/trusted threads may not be necessary. This
could result in improved performance (no IPC overhead between trusted
and untrusted thread) and reduced complexity (no need for trusted
helper thread).

If mode 2 is the Right Answer, then we shouldn't wait for it to become available. We should make it available by pushing it into the kernel.

If we all agree that if mode 2 existed, it's what we would use, then that we have the answer to this discussion and we know what we need to go off and do.

* Execution of QEMU with the sandboxed device support should be an
optional run-time specification.

Ack with a small exception. If we can demonstrate that sandboxing has an acceptable performance overhead, then we should do it unconditionally to reduce our overall test matrix. It's unclear that that's obtainable though.

* We will be focusing on legacy devices first, both for performance and
risk reasons.

Once we settle on a direction, we will develop a proof of concept to
share with the community.

Proof of concepts are the only way to settle on direction. Code speaks louder than anything else.

We appreciate your input.


Ashley Lai
Corey Bryant
Eduardo Otubo
Michael Halcrow
Paul Moore
Richa Marwaha

In the future, I would suggest beginning these type of discussions on the list to start with. Otherwise valuable in information (including discussion and debate on directions) are not available to the greater community at large.

Not a big deal in this case, but I want to be on the record here about this. I would have greatly preferred this whole effort start out on qemu-devel from day one.


Anthony Liguori

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