[PATCH net] vhost-net: fix TX stall when vhost owns virtio-net header

[<enrico.zanda@arm.com>]

From: Enrico Zanda <enrico.zanda@arm.com>

When vhost owns the virtio-net header, i.e. when
VHOST_NET_F_VIRTIO_NET_HDR is negotiated, sock_hlen is 0,
meaning that no header will be forwarded to the TAP device.

In the current vhost_net_build_xdp() implementation,
when sock_hlen == 0, the gso pointer can point at the start of the
Ethernet frame instead of a virtio-net header.
This results in a wrong interpretation of the destination MAC address
bytes as struct virtio_net_hdr fields.

This can, for some MAC addresses, trigger -EINVAL and return early
before the TX descriptor is completed, which can stall vhost-net TX.

Before 97b2409f28e0, the gso pointer was set to the zeroed padding area,
using it as a synthetic virtio-net header. Restore that behavior.

Fixes: 97b2409f28e0 ("vhost-net: reduce one userspace copy when building XDP buff")
Signed-off-by: Enrico Zanda <enrico.zanda@arm.com>
---
 drivers/vhost/net.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 77b59f49bddb..3e72b9c6af0c 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -731,10 +731,12 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
 		goto err;
 	}
 
-	gso = buf + pad - sock_hlen;
-
-	if (!sock_hlen)
+	if (!sock_hlen) {
 		memset(buf, 0, pad);
+		gso = buf;
+	} else {
+		gso = buf + pad - sock_hlen;
+	}
 
 	if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
 	    vhost16_to_cpu(vq, gso->csum_start) +
-- 
2.43.0