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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 1/15/20 11:48 AM, Roger Pau Monné wrote:
> On Wed, Jan 15, 2020 at 02:46:29AM +0100, Marek Marczykowski-Górecki 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@invisiblethingslab.com>
>> [the rest]
>> Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> Some minor nits below, but LGTM:
>
> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
>
>>
>> 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