CVE-2024-35981

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.