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So, we need to go over the list of virtual > >>>>>> devices and see if SBDF the guest is trying to access is a valid SBDF. > >>>>>> Is this what you mean? > >>>>> No, you need to prevent accesses to registers not explicitly handled > >>>>> by vpci. Ie: do not forward unhandled accesses to > >>>>> vpci_{read,wrie}_hw). > >>>> I see, so those which have no handlers are not passed to the hardware. > >>>> I need to see how to do that > >>> Indeed. Without fixing that passthrough to domUs is completely unsafe, > >>> as you allow domUs full access to registers not explicitly handled by > >>> current vPCI code. > >> Well, my understanding is: we can let the guest access whatever > >> registers it wants with the following exceptions: > >> - "special" registers we already trap in vPCI, e.g. command, BARs > >> - we must not let the guest go out of the configuration space of a > >> specific PCI device, e.g. prevent it from accessing configuration > >> spaces of other devices. > >> The rest accesses seem to be ok to me as we do not really want: > >> - have handlers and emulate all possible registers > >> - we do not want the guest to fail if it accesses a valid register which > >> we do not emulate. > > IMO that's not good from a security PoV. Xen needs to be sure that > > every registers a guest accesses is not going to cause the system to > > malfunction, so Xen needs to keep a list of the registers it's safe > > for a guest to access. > > > > For example we should only expose the PCI capabilities that we know > > are safe for a guest to use, ie: MSI and MSI-X initially. The rest of > > the capabilities should be blocked from guest access, unless we audit > > them and declare safe for a guest to access. > > > > As a reference you might want to look at the approach currently used > > by QEMU in order to do PCI passthrough. A very limited set of PCI > > capabilities known to be safe for untrusted access are exposed to the > > guest, and registers need to be explicitly handled or else access is > > rejected. We need a fairly similar model in vPCI or else none of this > > will be safe for unprivileged access. > I do agree with this. But at the moment we only emulate some of them, > so in the future we will need revisiting the emulation and put many > more registers under Xen's control Indeed. That's my main point - there's still a lot of work to do internally in vPCI in order to be safe for unprivileged guests to use. Thanks, Roger.