Re: [PATCH net v3 2/8] net: virtio: do not use dev->type to find the outer L3 header

From: Michael S. Tsirkin <mst@redhat.com>
Date: 2026-11-06 09:28:06
Also in: virtualization
Reply to: Eric Dumazet, 2026-11-05

On Thu, Nov 05, 2026 at 12:57:09PM +0100, Eric Dumazet wrote:

virtio_net_l3_start() decides from skb->dev->type whether the frame
starts with an Ethernet header. tun decides it from the IFF_TAP and
IFF_TUN flags, and TUNSETLINK can set the type of a down tun device
to any value, without changing the flags.

On an IFF_TAP device set to ARPHRD_NONE, the outer protocol checks
look at the IP header at the network header (ETH_HLEN), and never read
the Ethernet header. eth_type_trans() then sets skb->protocol from an
unchecked h_proto, for instance ETH_P_NSH, and the packet can reach
the skb_under_panic() addressed by the previous patch.

On an IFF_TUN device set to ARPHRD_ETHER, the checks validate an
Ethernet header that tun then treats as the start of the L3 header.

Checking in tun that the device type matches the framing is not
enough: the type is read again later, and a concurrent TUNSETLINK can
change it in between, since tun_get_user() only drops packets of a
down device after these checks.

Add a has_eth argument to virtio_net_hdr_tnl_to_skb(), passed down to
virtio_net_l3_start(), so the device type is no longer used:

- virtio_net always receives Ethernet frames.
- tun_get_user() passes whether the device is IFF_TAP.
- tun_xdp_one() always calls eth_type_trans() on its frames.

Fixes: 288f30435132 ("tun: enable gso over UDP tunnel support.")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/netdev/sashiko-outbox-198573@kernel.org/ (local)
Link: https://lore.kernel.org/netdev/179387332014.488856.9111956413610107951@kernel.org/ (local)
Signed-off-by: Eric Dumazet <edumazet@kernel.org>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

quoted hunk ↗ jump to hunk

---
 drivers/net/tun.c          |  9 +++++++--
 drivers/net/tun_vnet.h     |  5 +++--
 drivers/net/virtio_net.c   |  2 +-
 include/linux/virtio_net.h | 27 +++++++++++++--------------
 4 files changed, 24 insertions(+), 19 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index dcec7311e601..3920b3dff1e8 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c

@@ -1944,7 +1944,8 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		break;
 	}
 
-	if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, &hdr)) {
+	if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, &hdr,
+				    (tun->flags & TUN_TYPE_MASK) == IFF_TAP)) {
 		atomic_long_inc(&tun->rx_frame_errors);
 		err = -EINVAL;
 		goto free_skb;

@@ -2621,7 +2622,11 @@ static int tun_xdp_one(struct tun_struct *tun,
 	tnl_hdr = (struct virtio_net_hdr_v1_hash_tunnel *)gso;
 	skb->dev = tun->dev;
 	skb_set_network_header(skb, ETH_HLEN);
-	if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, tnl_hdr)) {
+	/* Frames always start with an Ethernet header here, as the
+	 * eth_type_trans() call below expects.
+	 */
+	if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, tnl_hdr,
+				    true)) {
 		atomic_long_inc(&tun->rx_frame_errors);
 		kfree_skb(skb);
 		ret = -EINVAL;

diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
index f4c652b1fa44..3364e08dd6e0 100644
--- a/drivers/net/tun_vnet.h
+++ b/drivers/net/tun_vnet.h

@@ -197,9 +197,10 @@ static inline netdev_features_t tun_vnet_hdr_guest_features(int vnet_hdr_sz)
 static inline int
 tun_vnet_hdr_tnl_to_skb(unsigned int flags, netdev_features_t features,
 			struct sk_buff *skb,
-			const struct virtio_net_hdr_v1_hash_tunnel *hdr)
+			const struct virtio_net_hdr_v1_hash_tunnel *hdr,
+			bool has_eth)
 {
-	return virtio_net_hdr_tnl_to_skb(skb, hdr,
+	return virtio_net_hdr_tnl_to_skb(skb, hdr, has_eth,
 				features & NETIF_F_GSO_UDP_TUNNEL,
 				features & NETIF_F_GSO_UDP_TUNNEL_CSUM,
 				tun_vnet_is_little_endian(flags));

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index dd2c41ef6fbe..ed20e76fbc4c 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -2533,7 +2533,7 @@ static void virtnet_receive_done(struct virtnet_info *vi, struct receive_queue *
 
 	skb->dev = dev;
 	skb_set_network_header(skb, ETH_HLEN);
-	if (virtio_net_hdr_tnl_to_skb(skb, &hdr->tnl_hdr, vi->rx_tnl,
+	if (virtio_net_hdr_tnl_to_skb(skb, &hdr->tnl_hdr, true, vi->rx_tnl,
 				      vi->rx_tnl_csum,
 				      virtio_is_little_endian(vi->vdev))) {
 		net_warn_ratelimited("%s: bad gso: type: %x, size: %u, flags %x tunnel %d tnl csum %d\n",

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 357116c2a148..b7b6b542f476 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h

@@ -87,20 +87,18 @@ static inline struct iphdr *virtio_net_ip4_parse(struct sk_buff *skb,
 }
 
 /* Where does the L3 header start, and what protocol is it?
- * Handles Ethernet and VLAN headers; other devices use the
- * network header offset the caller set up.
+ * With @has_eth, the frame starts with an Ethernet header, possibly
+ * followed by VLAN tags. Otherwise the L3 header starts at the network
+ * header offset the caller set up.
+ *
+ * The caller knows how the frame will be parsed later: the device
+ * type does not tell, as tun can change it with TUNSETLINK.
  */
 static inline bool
-virtio_net_l3_start(struct sk_buff *skb, int *nhoff, __be16 *proto)
+virtio_net_l3_start(struct sk_buff *skb, bool has_eth, int *nhoff,
+		    __be16 *proto)
 {
-	/* skb->dev should always be set here already,
-	 * but let's not crash when it is not.
-	 */
-	if (WARN_ON_ONCE(!skb->dev))
-		return false;
-
-	if (skb->dev->type == ARPHRD_ETHER ||
-	    skb->dev->type == ARPHRD_LOOPBACK) {
+	if (has_eth) {
 		__be16 h_proto;
 
 		if (!pskb_may_pull(skb, ETH_HLEN))

@@ -431,7 +429,7 @@ static inline unsigned int virtio_l3min(bool is_ipv6)
  * carries, with UDP following it at @outer_th.
  */
 static inline bool
-virtio_net_tnl_offsets_ok(struct sk_buff *skb, bool outer_isv6,
+virtio_net_tnl_offsets_ok(struct sk_buff *skb, bool has_eth, bool outer_isv6,
 			  unsigned int outer_th)
 {
 	int nhoff;

@@ -439,7 +437,7 @@ virtio_net_tnl_offsets_ok(struct sk_buff *skb, bool outer_isv6,
 	unsigned int thoff;
 	int ip_proto;
 
-	if (!virtio_net_l3_start(skb, &nhoff, &proto))
+	if (!virtio_net_l3_start(skb, has_eth, &nhoff, &proto))
 		return false;
 	if (outer_isv6 != (proto == htons(ETH_P_IPV6)))
 		return false;

@@ -452,6 +450,7 @@ virtio_net_tnl_offsets_ok(struct sk_buff *skb, bool outer_isv6,
 static inline int
 virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 			  const struct virtio_net_hdr_v1_hash_tunnel *vhdr,
+			  bool has_eth,
 			  bool tnl_hdr_negotiated,
 			  bool tnl_csum_negotiated,
 			  bool little_endian)

@@ -505,7 +504,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 	if (ret)
 		return ret;
 
-	if (!virtio_net_tnl_offsets_ok(skb, outer_isv6, outer_th))
+	if (!virtio_net_tnl_offsets_ok(skb, has_eth, outer_isv6, outer_th))
 		return -EINVAL;
 
 	/* In case of USO, the inner protocol is still unknown and

-- 
2.53.0

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