Re: [Xen-devel] [PATCH v4] xen-pciback: optionally allow interrupt enable flag writes

[Boris Ostrovsky]

On 1/15/20 11:48 AM, Roger Pau Monnà wrote:
> On Wed, Jan 15, 2020 at 02:46:29AM +0100, 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. Additionally, when qemu runs in dom0, it already have direct
> > access to those bits.
> >
> > This is the second iteration of this feature. First was proposed as a
> > direct Xen interface through a new hypercall, but ultimately it was
> > rejected by the maintainer, because of mixing pciback and hypercalls for
> > PCI config space access isn't a good design. Full discussion at [2].
> >
> > [1]: https://invisiblethingslab.com/resources/2011/Software%20Attacks%20on%20Intel%20VT-d.pdf
> > [2]: https://xen.markmail.org/thread/smpgpws4umdzizze
> >
> > [part of the commit message and sysfs handling]
> > Signed-off-by: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>
> > [the rest]
> > Signed-off-by: Marek Marczykowski-GÃrecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> Some minor nits below, but LGTM:
>
> Reviewed-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
>
> > diff --git a/Documentation/ABI/testing/sysfs-driver-pciback b/Documentation/ABI/testing/sysfs-driver-pciback
> > index 6a733bfa37e6..566a11f2c12f 100644
> > --- a/Documentation/ABI/testing/sysfs-driver-pciback
> > +++ b/Documentation/ABI/testing/sysfs-driver-pciback
> > @@ -11,3 +11,16 @@ Description:
> >                   #echo 00:19.0-E0:2:FF > /sys/bus/pci/drivers/pciback/quirks
> >                   will allow the guest to read and write to the configuration
> >                   register 0x0E.
> > +
> > +What:           /sys/bus/pci/drivers/pciback/allow_interrupt_control
> > +Date:           Jan 2020
> > +KernelVersion:  5.5

5.6

I can fix this and the things that Roger mentioned while committing if 
Marek is OK with that.

-boris