[PATCH net v3] virtio_net: Do not send RSS key if it is not supported

[Breno Leitao <leitao@debian.org>]

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.

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

V2:
  * Moved from creating a valid packet, by rejecting the request
    completely
V3:
  * Got some good feedback from and Xuan Zhuo and Heng Qi, and reworked
    the rejection path.

---
 drivers/net/virtio_net.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index c22d1118a133..c4a21ec51adf 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3807,6 +3807,7 @@ static int virtnet_set_rxfh(struct net_device *dev,
 			    struct netlink_ext_ack *extack)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
+	bool update = false;
 	int i;
 
 	if (rxfh->hfunc != ETH_RSS_HASH_NO_CHANGE &&
@@ -3814,13 +3815,24 @@ static int virtnet_set_rxfh(struct net_device *dev,
 		return -EOPNOTSUPP;
 
 	if (rxfh->indir) {
+		if (!vi->has_rss)
+			return -EOPNOTSUPP;
+
 		for (i = 0; i < vi->rss_indir_table_size; ++i)
 			vi->ctrl->rss.indirection_table[i] = rxfh->indir[i];
+		update = true;
 	}
-	if (rxfh->key)
+
+	if (rxfh->key) {
+		if (!vi->has_rss && !vi->has_rss_hash_report)
+			return -EOPNOTSUPP;
+
 		memcpy(vi->ctrl->rss.key, rxfh->key, vi->rss_key_size);
+		update = true;
+	}
 
-	virtnet_commit_rss_command(vi);
+	if (update)
+		virtnet_commit_rss_command(vi);
 
 	return 0;
 }
@@ -4729,13 +4741,15 @@ static int virtnet_probe(struct virtio_device *vdev)
 	if (virtio_has_feature(vdev, VIRTIO_NET_F_HASH_REPORT))
 		vi->has_rss_hash_report = true;
 
-	if (virtio_has_feature(vdev, VIRTIO_NET_F_RSS))
+	if (virtio_has_feature(vdev, VIRTIO_NET_F_RSS)) {
 		vi->has_rss = true;
 
-	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));
+	}
+
+	if (vi->has_rss || vi->has_rss_hash_report) {
 		vi->rss_key_size =
 			virtio_cread8(vdev, offsetof(struct virtio_net_config, rss_max_key_size));
 
-- 
2.43.0