[PATCH v2 net] virtio_net: do not allow tunnel csum offload for non GSO packets

[PATCH v2 net] virtio_net: do not allow tunnel csum offload for non GSO packets

[Paolo Abeni]

Fiona reports broken connectivity for virtio net setup using UDP tunnel
inside the guest and NIC with not UDP tunnel TSO support in the host.

Currently the virtio_net driver exposes csum offload for UDP-tunneled,
TCP non GSO packets. Such packet reach the host as CSUM_PARTIAL ones
with the 'encapsulation' flag cleared, as the virtio specification do
not support this specific kind of offload.

HW NICs with UDP tunnel TSO support - and those drivers directly
accessing skb->csum_start/csum_offset - are still capable of computing
the needed csum correctly, but otherwise the packets reach the wire with
bad csum on both the inner and outer transport header.

Address the issue explicitly disabling csum offload for UDP tunneled,
non GSO packets via the ndo_features_check op.

Fixes: 56a06bd40fab ("virtio_net: enable gso over UDP tunnel support.")
Reported-by: Fiona Ebner <f.ebner@proxmox.com>
Closes: https://bugzilla.proxmox.com/show_bug.cgi?id=7627
Tested-by: Fiona Ebner <f.ebner@proxmox.com>
Tested-by: Gabriel Goller <g.goller@proxmox.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
v1 -> v2:
 - deal with to-be-segmented skbs, too.
---
 drivers/net/virtio_net.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index f4adcfee7a80..7d2eeb9b1226 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -6222,6 +6222,19 @@ static void virtnet_free_irq_moder(struct virtnet_info *vi)
 	rtnl_unlock();
 }
 
+static netdev_features_t virtnet_features_check(struct sk_buff *skb,
+						struct net_device *dev,
+						netdev_features_t features)
+{
+	/* Inner csum offload is only available for GSO packets. */
+	if (skb->encapsulation &&
+	    (!skb_is_gso(skb) || netif_needs_gso(skb, features)))
+		return features & ~NETIF_F_CSUM_MASK;
+
+	/* Passthru. */
+	return features;
+}
+
 static const struct net_device_ops virtnet_netdev = {
 	.ndo_open            = virtnet_open,
 	.ndo_stop   	     = virtnet_close,
@@ -6235,7 +6248,7 @@ static const struct net_device_ops virtnet_netdev = {
 	.ndo_bpf		= virtnet_xdp,
 	.ndo_xdp_xmit		= virtnet_xdp_xmit,
 	.ndo_xsk_wakeup         = virtnet_xsk_wakeup,
-	.ndo_features_check	= passthru_features_check,
+	.ndo_features_check	= virtnet_features_check,
 	.ndo_get_phys_port_name	= virtnet_get_phys_port_name,
 	.ndo_set_features	= virtnet_set_features,
 	.ndo_tx_timeout		= virtnet_tx_timeout,
-- 
2.54.0

Re: [PATCH v2 net] virtio_net: do not allow tunnel csum offload for non GSO packets

[Gabriel Goller]

> Fiona reports broken connectivity for virtio net setup using UDP tunnel
> inside the guest and NIC with not UDP tunnel TSO support in the host.
> 
> Currently the virtio_net driver exposes csum offload for UDP-tunneled,
> TCP non GSO packets. Such packet reach the host as CSUM_PARTIAL ones
> with the 'encapsulation' flag cleared, as the virtio specification do
> not support this specific kind of offload.
> 
> HW NICs with UDP tunnel TSO support - and those drivers directly
> accessing skb->csum_start/csum_offset - are still capable of computing
> the needed csum correctly, but otherwise the packets reach the wire with
> bad csum on both the inner and outer transport header.
> 
> Address the issue explicitly disabling csum offload for UDP tunneled,
> non GSO packets via the ndo_features_check op.
> 
> Fixes: 56a06bd40fab ("virtio_net: enable gso over UDP tunnel support.")
> Reported-by: Fiona Ebner <f.ebner@proxmox.com>
> Closes: https://bugzilla.proxmox.com/show_bug.cgi?id=7627
> Tested-by: Fiona Ebner <f.ebner@proxmox.com>
> Tested-by: Gabriel Goller <g.goller@proxmox.com>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Tested it on our testbench, consider:

Reviewed-by: Gabriel Goller <g.goller@proxmox.com>
Tested-by: Gabriel Goller <g.goller@proxmox.com>

-- 
Gabriel Goller <g.goller@proxmox.com>

Re: [PATCH v2 net] virtio_net: do not allow tunnel csum offload for non GSO packets

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu, 11 Jun 2026 18:36:48 +0200 you wrote:
> Fiona reports broken connectivity for virtio net setup using UDP tunnel
> inside the guest and NIC with not UDP tunnel TSO support in the host.
> 
> Currently the virtio_net driver exposes csum offload for UDP-tunneled,
> TCP non GSO packets. Such packet reach the host as CSUM_PARTIAL ones
> with the 'encapsulation' flag cleared, as the virtio specification do
> not support this specific kind of offload.
> 
> [...]

Here is the summary with links:
  - [v2,net] virtio_net: do not allow tunnel csum offload for non GSO packets
    https://git.kernel.org/netdev/net/c/86c51f0f2313

You are awesome, thank you!
-- 
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