Re: [Qemu-devel] [PATCH 1/2] guest agent: add RPC blacklist command-line option

[Michael Roth]

On 12/07/2011 06:12 AM, Dor Laor wrote:
> On 12/07/2011 12:52 PM, Daniel P. Berrange wrote:
> > On Wed, Dec 07, 2011 at 12:34:01PM +0200, Dor Laor wrote:
> > > On 12/07/2011 06:03 AM, Michael Roth wrote:
> > > > This adds a command-line option, -b/--blacklist, that accepts a
> > > > comma-seperated list of RPCs to disable, or prints a list of
> > > > available RPCs if passed "?".
> > > >
> > > > In consequence this also adds general blacklisting and RPC listing
> > > > facilities to the new QMP dispatch/registry facilities, should the
> > > > QMP monitor ever have a need for such a thing.
> > >
> > > Beyond run time disablement, how easy it is to compile out some of
> > > the general commands such as exec/file-handling?
> > >
> > > Security certifications like common criteria usually ask to compile
> > > out anything that might tamper security.
> >
> > I don't think that's really relevant/needed. As discussed on the
> > call yesterday, this is security theatre, because nothing can prevent
> > the host admin from accessing guest RAM or disk data. AFAIK the
> > virtualization related security certifications acknowledge this
> > already& don't make any claims about security of guests against
> > a malicious host admin. In any case, a suitable SELinux policy for
> > the guest agent could prevent arbitrary file/binary access via
> > generic 'exec' / 'file-read' commands, in a manner that is sufficient
> > to satisfy security certications.
>
> I absolutely agree that the hypervisor can tweak the guest in multiple
> ways. Nevertheless there are two reasons I asked it:
>
> 1. Reduce code and noise from security reviewers eyes.
> We were asked to do exactly that for other qemu functionality that
> is included but does not run at all. It's just makes the review
> faster.

Actually removing the code, or compiling it out?

If it's a matter of compiling it out, the best solution I can think of 
is having the QAPI code generators create a #define <rpc> for each RPC, 
then wrapping the implementations inside an #ifdef <rpc>. That way you 
could compile out the code by simply modifying the schema.

That said, I'd really like to avoid having distros get into the habit of 
extensively modifying their guest agent source outside of bug fixes and 
whatnot, I think it'll cause too many problems down the road. From a 
management perspective, if you're running a cloud with multiple distros, 
it'll be really difficult to account for agents that have been modified 
or crippled in various ways.

Perhaps we only need, say, shutdown, for ovirt, and compile out the 
rest, but maybe a customer wants to run their RHEL guest in home-brewed 
environment where they use qemu-ga file read/write to handle a specific 
set of guest activation procedures. Now they need a new agent package.

It's a whole lot of hassle for host/guest admins for the sake of saving 
a security reviewer a bit of investigating that'll lead right back to 
the general operating premise that you have to trust your host 
administrators before any chain of trust can be established.

At least with this interface we can provide some semblance of relief to 
users with specific security concerns, but don't have to work with 
distros to re-package agents when those concerns collide with 
requirements on the host side. We can just check to see if they disabled 
the functionality and request they re-enable due to <reason> by updating 
their configs.

> 2. Every piece of code is a risk for exploit
> Imagine that a bug/leak/use-after-free in the blacklist command or
> the exec command on qemu exists and allows attacked to gain control
> of qemu.

A host can never assume that a guest [agent] can be trusted. qemu-ga 
might've been replaced completely by a malicious guest admin, thus 
circumventing any steps a distro has taken to harden it. Fortunately a 
guest can only affect memory outside it's address space by going through 
the virtio-serial/QMP layer. So we can focus our efforts on hardening 
the transport and json parser layers, and a lot of work has gone into 
that already (placing limits on token size, recursion depth, etc). So 
that's more an issue that needs to be addressed on the qemu side, and is 
independent of any particular RPC implementation on the guest side.

> > Regards,
> > Daniel