Re: [PATCH 6/9] virtio_pci: harden MSI-X interrupts

[Thomas Gleixner]

On Mon, Sep 13 2021 at 16:54, Michael S. Tsirkin wrote:

> On Mon, Sep 13, 2021 at 09:38:30PM +0200, Thomas Gleixner wrote:
>> On Mon, Sep 13 2021 at 15:07, Jason Wang wrote:
>> > On Mon, Sep 13, 2021 at 2:50 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>> >> > But doen't "irq is disabled" basically mean "we told the hypervisor
>> >> > to disable the irq"?  What extractly prevents hypervisor from
>> >> > sending the irq even if guest thinks it disabled it?
>> >>
>> >> More generally, can't we for example blow away the
>> >> indir_desc array that we use to keep the ctx pointers?
>> >> Won't that be enough?
>> >
>> > I'm not sure how it is related to the indirect descriptor but an
>> > example is that all the current driver will assume:
>> >
>> > 1) the interrupt won't be raised before virtio_device_ready()
>> > 2) the interrupt won't be raised after reset()
>> 
>> If that assumption exists, then you better keep the interrupt line
>> disabled until virtio_device_ready() has completed
>
> started not completed. device is allowed to send
> config interrupts right after DRIVER_OK status is set by
> virtio_device_ready.

Whatever:

 * Define the exact point from which on the driver is able to handle the
   interrupt and put the enable after that point

 * Define the exact point from which on the driver is unable to handle
   the interrupt and put the disable before that point

The above is blury.

>> and disable it again
>> before reset() is invoked. That's a question of general robustness and
>> not really a question of trusted hypervisors and encrypted guests.
>
> We can do this for some MSIX interrupts, sure. Not for shared interrupts though.

See my reply to the next patch. The problem is the same:

 * Define the exact point from which on the driver is able to handle the
   interrupt and allow the handler to proceed after that point

 * Define the exact point from which on the driver is unable to handle
   the interrupt and ensure that the handler denies to proceed before
   that point

Same story just a different mechanism.

Thanks,

        tglx