Re: [virtio-dev] Timing out virtio-pci config space access

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 05, 2021 at 05:59:43PM +0530, Srivatsa Vaddagiri wrote:
> * Michael S. Tsirkin <mst@redhat.com> [2021-11-05 03:38:39]:
> 
> > On Thu, Nov 04, 2021 at 10:37:40PM +0530, Srivatsa Vaddagiri wrote:
> > > We are working on a virtio-pci implementation on a Type-1 hypervisor where
> > > backend drivers are hosted in another VM and are considered untrusted. PCI is
> > > the virtio transport used in this case.
> > > 
> > > One issue that crops up is a read/write of config space can potentially block
> > > forever, as the backend is untrusted and could be causing a denial-of-service of
> > > sorts. This causes the vcpu to stall forever. I was wondering if we can timeout
> > > in such case and have the hypervisor break the stall by letting read return
> > > "error" (-1) along with setting DEVICE_NEEDS_RESET in status register. Will that
> > > allow Linux guest driver to gracefully fail its probe? I don't see where Linux
> > > handles DEVICE_NEEDS_RESET currently and also am not sure if returning -1 will
> > > lead to graceful failure of the driver alone (we don't want VM to come down or
> > > panic because of a mis-behaving device). 
> > 
> > DEVICE_NEEDS_RESET isn't handled ATM. the point of it in any case
> > is a recoverable error, with a malicious backend this is
> > not the case.
> > 
> > 
> > Once thing you can do that will work a bit better is implementing
> > surprise-removal in this case.
> 
> My layman understanding of surprise removal is that it requires the PCI
> controller to interrupt OS and convey which device is removed, so that the PCI
> subsystem can mark it "removed"? Is that possible for the generic controller
> ("pci-host-ecam-generic") that virtio pci devices use?

I think so, yes.

> > So hypervisor detects a timeout
> > (presumably it knows what to expect of the device) and then pretends to
> > guest device is gone, unmapping it completely from guest.
> 
> Can you elaborate on what unmapping means? I think the reads should
> return -1 and writes to be dropped in such case - beyond that what would unmap
> entail?
> 
> Thanks
> vatsa

Removing guest access to device so access attempts end up in QEMU.

-- 
MST


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