Re: [PATCH v1] xen-pciback: optionally allow interrupt enable flag writes

[Jan Beulich]

On 03.12.2019 06:41, Marek Marczykowski-GÃrecki  wrote:
> QEMU running in a stubdom needs to be able to set INTX_DISABLE, and the
> MSI(-X) enable flags in the PCI config space. This adds an attribute
> 'allow_interrupt_control' which when set for a PCI device allows writes
> to this flag(s). The toolstack will need to set this for stubdoms.
> When enabled, guest (stubdomain) will be allowed to set relevant enable
> flags, but only one at a time - i.e. it refuses to enable more than one
> of INTx, MSI, MSI-X at a time.
> 
> This functionality is needed only for config space access done by device
> model (stubdomain) serving a HVM with the actual PCI device. It is not
> necessary and unsafe to enable direct access to those bits for PV domain
> with the device attached. For PV domains, there are separate protocol
> messages (XEN_PCI_OP_{enable,disable}_{msi,msix}) for this purpose.
> Those ops in addition to setting enable bits, also configure MSI(-X) in
> dom0 kernel - which is undesirable for PCI passthrough to HVM guests.
> 
> This should not introduce any new security issues since a malicious
> guest (or stubdom) can already generate MSIs through other ways, see
> [1] page 8.

True, albeit this doesn't cover INTX_DISABLE.

> Additionally, when qemu runs in dom0, it already have direct
> access to those bits.

True again, but any bug here (as in: too wide exposure) is a qemu
bug (possibly security issue). The statement also isn't really
correct for de-privileged qemu, I think (but I also think PCI
pass-through doesn't work in that mode at all yet).

On the whole this looks to be an acceptable approach to me. But
I'm not the maintainer, so my opinion may not count much. Some
issues with the implementation itself were already pointed out.

Jan