Re: [Xen-devel] [PATCH v2] passthrough: give XEN_DOMCTL_test_assign_device more sane semantics

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

On 06/23/2017 11:00 AM, Jan Beulich wrote:
> So far callers of the libxc interface passed in a domain ID which was
> then ignored in the hypervisor. Instead, make the hypervisor honor it
> (accepting DOMID_INVALID to obtain original behavior), allowing to
> query whether a device can be assigned to a particular domain.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v2: Alter the semantics to check whether the device can be assigned to
>      the passed in domain.
> TBD: It's not clear to me whether the test-assign XSM hooks are still
>       useful this way.
As long as the only user of this hypercall is the device assignment
path, I would remove the XSM hook for test_assign and use the
assign_device hook for both test and actual.  That way, if XSM is
actually preventing the assignment, you will get the same AVC denials as
if you tried without doing the test first.  The hook should just skip the
two checks relating to (d) if it is null.

If this test hypercall were to be used as part of a query (such as
populating a list of assignable devices by trying all PCI devices listed
via sysfs), then it would make sense to have either a different hook or
a flag in the XSM hook to silence the audit messages since you aren't
actually trying to do the assignment yet.  That change will make the
cause of the "can't assign" error harder to diagnose, however, so unless
it's being used like that I don't think it's a good idea.

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