Subject: CVE-2024-35981: virtio_net: Do not send RSS key if it is not supported

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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 Linux kernel CVE team has assigned CVE-2024-35981 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.18 with commit c7114b1249fa and fixed in 6.1.90 with commit 539a2b995a4e
	Issue introduced in 5.18 with commit c7114b1249fa and fixed in 6.6.29 with commit 43a71c1b4b3a
	Issue introduced in 5.18 with commit c7114b1249fa and fixed in 6.8.7 with commit 28e9a64638cd
	Issue introduced in 5.18 with commit c7114b1249fa and fixed in 6.9 with commit 059a49aa2e25

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-35981
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	drivers/net/virtio_net.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/539a2b995a4ed93125cb0efae0f793b00ab2158b
	https://git.kernel.org/stable/c/43a71c1b4b3a6d4db857b1435d271540279fc7de
	https://git.kernel.org/stable/c/28e9a64638cd16bc1ecac9ff74ffeacb9fb652de
	https://git.kernel.org/stable/c/059a49aa2e25c58f90b50151f109dd3c4cdb3a47