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

[Breno Leitao <leitao@xxxxxxxxxx>]

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 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@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: qemu-devel@xxxxxxxxxx
---
 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