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From: Breno Leitao <leitao@debian.org>
To: hengqi@linux.alibaba.com,
	xuanzhuo@linux.alibaba.com,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Paolo Abeni <pabeni@redhat.com>,
	Andrew Melnychenko <andrew@daynix.com>
Cc: rbc@meta.com,
	riel@surriel.com,
	stable@vger.kernel.org,
	qemu-devel@nongnu.org,
	virtualization@lists.linux.dev (open list:VIRTIO CORE AND NET DRIVERS),
	netdev@vger.kernel.org (open list:NETWORKING DRIVERS),
	linux-kernel@vger.kernel.org (open list)
Subject: [PATCH net v2 2/2] virtio_net: Do not send RSS key if it is not supported
Date: Tue, 26 Mar 2024 08:19:09 -0700
Message-ID: <20240326151911.2155689-2-leitao@debian.org>
In-Reply-To: <20240326151911.2155689-1-leitao@debian.org>
References: <20240326151911.2155689-1-leitao@debian.org>
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MIME-Version: 1.0
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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>
---
 drivers/net/virtio_net.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index c640fdf28fc5..e6b0eaf08ac2 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3809,6 +3809,9 @@ static int virtnet_set_rxfh(struct net_device *dev,
 	struct virtnet_info *vi = netdev_priv(dev);
 	int i;
 
+	if (!vi->has_rss && !vi->has_rss_hash_report)
+		return -EOPNOTSUPP;
+
 	if (rxfh->hfunc != ETH_RSS_HASH_NO_CHANGE &&
 	    rxfh->hfunc != ETH_RSS_HASH_TOP)
 		return -EOPNOTSUPP;
-- 
2.43.0