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.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.