Re: [PATCH] virtio_net: Do not send RSS key if it is not supported

On Thu, 21 Mar 2024 09:54:30 -0700, Breno Leitao leitao@debian.org wrote:

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;

if (vi->has_rss || vi->has_rss_hash_report) { vi->rss_indir_table_size = virtio_cread16(vdev, offsetof(struct virtio_net_config, rss_max_indirection_table_length)); vi->rss_key_size = virtio_cread8(vdev, offsetof(struct virtio_net_config, rss_max_key_size));

vi->rss_hash_types_supported = virtio_cread32(vdev, offsetof(struct virtio_net_config, supported_hash_types)); vi->rss_hash_types_supported &= ~(VIRTIO_NET_RSS_HASH_TYPE_IP_EX | VIRTIO_NET_RSS_HASH_TYPE_TCP_EX | VIRTIO_NET_RSS_HASH_TYPE_UDP_EX);

dev->hw_features |= NETIF_F_RXHASH; }

vi->rss_key_size is initiated here, I wonder if there is something wrong?

Thanks.

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 the key scatter-gatter key if it is not set.

Fixes: c7114b1249fa ("drivers/net/virtio_net: Added basic RSS support.") Signed-off-by: Breno Leitao leitao@debian.org Cc: stable@vger.kernel.org Cc: qemu-devel@nongnu.org

---

drivers/net/virtio_net.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index d7ce4a1011ea..5a7700b103f8 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -3041,11 +3041,16 @@ static int virtnet_set_ringparam(struct net_device *dev, static bool virtnet_commit_rss_command(struct virtnet_info *vi) { struct net_device *dev = vi->dev;

• int has_key = vi->rss_key_size; struct scatterlist sgs[4]; unsigned int sg_buf_size;

• int nents = 3;

• if (has_key)

• nents += 1;
  
  

  /* prepare sgs */

• sg_init_table(sgs, 4);

• sg_init_table(sgs, nents);

  sg_buf_size = offsetof(struct virtio_net_ctrl_rss, indirection_table); sg_set_buf(&sgs[0], &vi->ctrl->rss, sg_buf_size);

@@ -3057,8 +3062,13 @@ static bool virtnet_commit_rss_command(struct virtnet_info *vi) - offsetof(struct virtio_net_ctrl_rss, max_tx_vq); sg_set_buf(&sgs[2], &vi->ctrl->rss.max_tx_vq, sg_buf_size);

• sg_buf_size = vi->rss_key_size;
• sg_set_buf(&sgs[3], vi->ctrl->rss.key, sg_buf_size);

• if (has_key) {

• /* Only populate if key is available, otherwise
  

•  * populating a buffer with zero size breaks virtio
  

•  */
  

• sg_buf_size = vi->rss_key_size;
  

• sg_set_buf(&sgs[3], vi->ctrl->rss.key, sg_buf_size);
  

• }

  if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_MQ, vi->has_rss ? VIRTIO_NET_CTRL_MQ_RSS_CONFIG

-- 2.43.0