Re: Device model operation hypercall (DMOP, re qemu depriv)

[Wei Liu <wei.liu2@citrix.com>]

On Mon, Aug 01, 2016 at 12:32:54PM +0100, Ian Jackson wrote:
> Introducing HVMCTL, Jan wrote:
> > A long while back separating out all control kind operations (intended
> > for use by only the control domain or device model) from the currect
> > hvmop hypercall has been discussed. This series aims at finally making
> > this reality (at once allowing to streamline the associated XSM checking).
> 
> I think we need to introduce a new hypercall (which I will call DMOP
> for now) which may augment or replace some of HVMCTL.  Let me explain:
> 
> 
> We would like to be able to deprivilege qemu-in-dom0.  This is
> because qemu has a large attack surface and has a history of security
> bugs.  If we get this right we can easily reduce the impact of `guest
> can take over qemu' bugs to DoS; and perhaps with a bit of effort we
> can eliminate the DoS too.  (qemu stubdom are another way to do this
> but they have their own difficulties.)
> 
> A part of this plan has to be a way for qemu to make hypercalls
> related to the guest it is servicing.  But qemu needs to be _unable_
> to make _other_ hypercalls.
> 
> I see four possible approaches.  In IMO increasing order of
> desirability:
> 
> 1. We could simply patch the dom0 privcmd driver to know exactly which
>    hypercalls are permitted.  This is obviously never going to work
>    because there would have to be a massive table in the kernel, kept
>    in step with Xen.  We could have a kind of pattern matching engine
>    instead, and load the tables from userspace, but that's a daft
>    edifice to be building (even if we reuse BPF or something) and a
>    total pain to maintain.
> 
> 2. We could have some kind of privileged proxy or helper process,
>    which makes the hypercalls on instruction from qemu.  This would be
>    quite complicated and involve a lot of back-and-forth parameter
>    passing.  Like option 1, this arrangement would end up embedding
>    detailed knowledge about which hypercalls are appropriate, and have
>    to understand all of their parameters.
> 
> 3. We could have the dom0 privcmd driver wrap each of qemu's
>    hypercalls in a special "wrap up with different XSM tag" hypercall.
>    Then, we could specify the set of allowable hypercalls with XSM.
>    If we want qemu deprivileged by default, this depends on turning
>    XSM on by default.  But we want qemu depriv ASAP and there are
>    difficulties with XSM by default.  This approach also involves
>    writing a large and hard-to-verify hypercall permission table, in
>    the form of an XSM policy.
> 
> 4. We could invent a new hypercall `DMOP' for hypercalls which device
>    models should be able to use, which always has the target domain in
>    a fixed location in the arguments.  We have the dom0 privcmd driver
>    know about this one hypercall number and the location of the target
>    domid.
> 
> Option 4 has the following advantages:
> 
> * The specification of which hypercalls are authorised to qemu is
>   integrated with the specification of the hypercalls themselves:
>   There is no need to maintain a separate table which can get out of
>   step (or contain security bugs).
> 
> * The changes required to the rest of the system are fairly small.
>   In particular:
> 
> * We need only one small, non-varying, patch to the dom0 kernel.
> 

I think your analysis makes sense.

> 
> Let me flesh out option 4 in more detail:
> 
> 
> We define a new hypercall DMOP.
> 
> Its first argument is always a target domid.  The DMOP hypercall
> number and position of the target domid in the arguments are fixed.
> 
> A DMOP is defined to never put at risk the stability or security of
> the whole system, nor of the domain which calls DMOP.  However, a DMOP
> may have arbitrary effects on the target domid.
> 

I would like to point out that this is non-trivial since we would need
to audit a lot of stuff.

But the requirement to audit interface is not unique to DMOP -- I expect
this is needed for any other approach.

> In the privcmd driver, we provide a new restriction ioctl, which takes
> a domid parameter.  After that restriction ioctl is called, the
> privcmd driver will permit only DMOP hypercalls, and only with the
> specified target domid.
> 

It is phrased like that the guest kernel is supposed to enforce the
policy?  Would it be possible to make Xen do it? I don't think we should
trust DM domain kernel here.

Wei.

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