Re: [Qemu-devel] [RFC PATCH] vfio/pci: Add ioeventfd support

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 9 Feb 2018 15:05:11 +0800
Peter Xu <peterx@redhat.com> wrote:

> On Tue, Feb 06, 2018 at 05:08:14PM -0700, Alex Williamson wrote:
> 
> [...]
> 
> > +long vfio_pci_ioeventfd(struct vfio_pci_device *vdev, loff_t offset,
> > +			uint64_t data, int count, int fd)
> > +{
> > +	struct pci_dev *pdev = vdev->pdev;
> > +	loff_t pos = offset & VFIO_PCI_OFFSET_MASK;
> > +	int ret, bar = VFIO_PCI_OFFSET_TO_INDEX(offset);
> > +	struct vfio_pci_ioeventfd *ioeventfd;
> > +	int (*handler)(void *, void *);
> > +	unsigned long val;
> > +
> > +	/* Only support ioeventfds into BARs */
> > +	if (bar > VFIO_PCI_BAR5_REGION_INDEX)
> > +		return -EINVAL;
> > +
> > +	if (pos + count > pci_resource_len(pdev, bar))
> > +		return -EINVAL;
> > +
> > +	/* Disallow ioeventfds working around MSI-X table writes */
> > +	if (bar == vdev->msix_bar &&
> > +	    !(pos + count <= vdev->msix_offset ||
> > +	      pos >= vdev->msix_offset + vdev->msix_size))
> > +		return -EINVAL;
> > +
> > +	switch (count) {
> > +	case 1:
> > +		handler = &vfio_pci_ioeventfd_handler8;
> > +		val = data;
> > +		break;
> > +	case 2:
> > +		handler = &vfio_pci_ioeventfd_handler16;
> > +		val = le16_to_cpu(data);
> > +		break;
> > +	case 4:
> > +		handler = &vfio_pci_ioeventfd_handler32;
> > +		val = le32_to_cpu(data);
> > +		break;
> > +#ifdef iowrite64
> > +	case 8:
> > +		handler = &vfio_pci_ioeventfd_handler64;
> > +		val = le64_to_cpu(data);
> > +		break;
> > +#endif
> > +	default:
> > +		return -EINVAL;
> > +	}
> > +
> > +	ret = vfio_pci_setup_barmap(vdev, bar);
> > +	if (ret)
> > +		return ret;
> > +
> > +	mutex_lock(&vdev->ioeventfds_lock);
> > +
> > +	list_for_each_entry(ioeventfd, &vdev->ioeventfds_list, next) {
> > +		if (ioeventfd->pos == pos && ioeventfd->bar == bar &&
> > +		    ioeventfd->data == data && ioeventfd->count == count) {
> > +			if (fd == -1) {
> > +				vfio_virqfd_disable(&ioeventfd->virqfd);
> > +				list_del(&ioeventfd->next);
> > +				kfree(ioeventfd);
> > +				ret = 0;
> > +			} else
> > +				ret = -EEXIST;
> > +
> > +			goto out_unlock;
> > +		}
> > +	}
> > +
> > +	if (fd < 0) {
> > +		ret = -ENODEV;
> > +		goto out_unlock;
> > +	}
> > +
> > +	ioeventfd = kzalloc(sizeof(*ioeventfd), GFP_KERNEL);
> > +	if (!ioeventfd) {
> > +		ret = -ENOMEM;
> > +		goto out_unlock;
> > +	}
> > +
> > +	ioeventfd->pos = pos;
> > +	ioeventfd->bar = bar;
> > +	ioeventfd->data = data;
> > +	ioeventfd->count = count;
> > +
> > +	ret = vfio_virqfd_enable(vdev->barmap[ioeventfd->bar] + ioeventfd->pos,
> > +				 handler, NULL, (void *)val,
> > +				 &ioeventfd->virqfd, fd);
> > +	if (ret) {
> > +		kfree(ioeventfd);
> > +		goto out_unlock;
> > +	}
> > +
> > +	list_add(&ioeventfd->next, &vdev->ioeventfds_list);  
> 
> Is there a limit on how many ioeventfds that can be created?
> 
> IIUC we'll create this eventfd "automatically" if a MMIO addr/data
> triggered continuously for N=10 times, then would it be safer we have
> a limitation on maximum eventfds?  Or not sure whether a malicious
> guest can consume the host memory by sending:
> 
> - addr1/data1, 10 times
> - addr2/data2, 10 times
> - ...
> 
> To create unlimited ioeventfds?  Thanks,

Good question, it is somewhat exploitable in the guest the way it's
written, however a user process does have an open file limit and each
eventfd consumes a file handle, so unless someone is running QEMU with
unlimited file handles, there is a built-in limit.  Two problems remain
though:

First, is it still a bad idea that a user within a guest can target
this device page to consume all of the QEMU process' open file handles,
even if ultimately they're only harming themselves?  What would a
reasonable cap of file descriptors for this purpose be?  How would we
know which are actively used and which could be expired?  Currently
only 2 are registered, the MSI-ACK address and some unknown secondary
one that's low frequency, but enough to trigger the algorithm here (and
doesn't seem harmful to let it get enabled).  We could therefore
arbitrarily pick 5 as an upper limit here, maybe with a warning if the
code hits that limit.

Second, is there still an exploit in the proposed vfio interface that a
user could re-use a single file descriptor for multiple vfio
ioeventfds.  I don't know.  I thought about looking to see whether a
file descriptor is re-used, but then I wondered if that might actually
be kind of a neat and potentially useful interface that a single
eventfd could trigger multiple device writes.  It looks like KVM
arbitrarily sets a limit of 1000 iobus devices, where each ioeventfd
would be such a device.  Perhaps we take the same approach and pick an
arbitrary high upper limit per vfio device.  What's your opinion?
Thanks,

Alex