virtio_net: Do not send RSS key if it is not supported

In the Linux kernel, the following vulnerability has been resolved:

virtio_net: Do not send RSS key if it is not supported

There is a bug when setting the RSS options in virtio_net that can break

the whole machine, getting the kernel into an infinite loop.

Running the following command in any QEMU virtual machine with virtionet

will reproduce this problem:

# ethtool -X eth0 hfunc toeplitz

This is how the problem happens:

1) ethtool_set_rxfh() calls virtnet_set_rxfh()

2) virtnet_set_rxfh() calls virtnet_commit_rss_command()

3) virtnet_commit_rss_command() populates 4 entries for the rss

scatter-gather

4) Since the command above does not have a key, then the last

scatter-gatter entry will be zeroed, since rss_key_size == 0.

sg_buf_size = vi->rss_key_size;

5) This buffer is passed to qemu, but qemu is not happy with a buffer

with zero length, and do the following in virtqueue_map_desc() (QEMU

function):

if (!sz) {

virtio_error(vdev, "virtio: zero sized buffers are not allowed");

6) virtio_error() (also QEMU function) set the device as broken

vdev->broken = true;

7) Qemu bails out, and do not repond this crazy kernel.

8) The kernel is waiting for the response to come back (function

virtnet_send_command())

9) The kernel is waiting doing the following :

while (!virtqueue_get_buf(vi->cvq, &tmp) &&

!virtqueue_is_broken(vi->cvq))

cpu_relax();

10) None of the following functions above is true, thus, the kernel

loops here forever. Keeping in mind that virtqueue_is_broken() does

not look at the qemu `vdev->broken`, so, it never realizes that the

vitio is broken at QEMU side.

Fix it by not sending RSS commands if the feature is not available in

the device.