When the Qemu launched with vhost but without tap vnet_hdr,
vhost tries to copy vnet_hdr from socket iter with size 0
to the page that may contain some trash.
That trash can be interpreted as unpredictable values for
vnet_hdr.
That leads to dropping some packets and in some cases to
stalling vhost routine when the vhost_net tries to process
packets and fails in a loop.

Qemu options:
  -netdev tap,vhost=on,vnet_hdr=off,...

From security point of view, wrong values on field used later
tap's tap_get_user_xdp() and will affect skb gso and options.
Later the header(and data in headroom) should not be used by the stack.
Using custom socket as a backend to vhost_net can reveal some data
in the vnet_hdr, although it would require kernel access to implement.

The issue happens because the value of sock_len in virtqueue is 0.
That value is set at vhost_net_set_features() with
VHOST_NET_F_VIRTIO_NET_HDR, also it's set to zero at device open()
and reset() routine.
So, currently, to trigger the issue, we need to set up qemu with
vhost=on,vnet_hdr=off, or do not configure vhost in the custom program.

Signed-off-by: Andrew Melnychenko <redacted>
---
 drivers/vhost/net.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index f2ed7167c848..57411ac2d08b 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c

@@ -735,6 +735,9 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
 	hdr = buf;
 	gso = &hdr->gso;
 
+	if (!sock_hlen)
+		memset(buf, 0, pad);
+
 	if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
 	    vhost16_to_cpu(vq, gso->csum_start) +
 	    vhost16_to_cpu(vq, gso->csum_offset) + 2 >

-- 
2.43.0