Re: [Xen-devel] [PATCH] vpci: don't allow access to devices not assigned to the domain

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.09.2019 15:58, Roger Pau Monné  wrote:
> On Mon, Sep 02, 2019 at 01:58:07PM +0200, Jan Beulich wrote:
>> On 02.09.2019 13:30, Roger Pau Monne wrote:
>>> Don't allow the hardware domain to access the PCI config space of
>>> devices not assigned to it. Ie: the config space of iommu devices
>>> in use by Xen should not be accessible to the hardware domain.
>>
>> Well, I agree with what you say above, but the code change disallows
>> much more than this. In particular Dom0 (and maybe stub domains too)
>> need to be able to access the config space of devices assigned to
>> guests, e.g. for qemu to control MSI and/or MSI-X.
> 
> Right, I was overlooking the fact that a domain using vPCI itself
> should be able to handle passthrough backends for other domains.
> 
> I think the condition should instead check if the device is assigned
> to dom_xen, and don't allow domains access to devices assigned to
> dom_xen.

Even that goes too far imo: We deliberately allow read access to
r/o devices, in order to avoid anomalies in bus enumeration in
Dom0. And I'd very much hope write attempts already honor the
pseg->ro_map bit for a device.

Jan

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