netdev - [PATCH net v2 4/4] udp: prevent local UDP tunnel packets from being GROed

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Date: Tue, 19 Mar 2024 10:31:39 +0100
From: Antoine Tenart <atenart@...nel.org>
To: davem@...emloft.net,
	kuba@...nel.org,
	pabeni@...hat.com,
	edumazet@...gle.com
Cc: Antoine Tenart <atenart@...nel.org>,
	steffen.klassert@...unet.com,
	willemdebruijn.kernel@...il.com,
	netdev@...r.kernel.org,
	Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH net v2 4/4] udp: prevent local UDP tunnel packets from being GROed

GRO has a fundamental issue with UDP tunnel packets as it can't detect
those in a foolproof way and GRO could happen before they reach the
tunnel endpoint. Previous commits have fixed issues when UDP tunnel
packets come from a remote host, but if those packets are issued locally
they could run into checksum issues.

If the inner packet has a partial checksum the information will be lost
in the GRO logic, either in udp4/6_gro_complete or in
udp_gro_complete_segment and packets will have an invalid checksum when
leaving the host.

Prevent local UDP tunnel packets from ever being GROed at the outer UDP
level.

Due to skb->encapsulation being wrongly used in some drivers this is
actually only preventing UDP tunnel packets with a partial checksum to
be GROed (see iptunnel_handle_offloads) but those were also the packets
triggering issues so in practice this should be sufficient.

Fixes: 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.")
Fixes: 36707061d6ba ("udp: allow forwarding of plain (non-fraglisted) UDP GRO packets")
Suggested-by: Paolo Abeni <pabeni@...hat.com>
Signed-off-by: Antoine Tenart <atenart@...nel.org>
Reviewed-by: Willem de Bruijn <willemb@...gle.com>
---

Since v1:
  - Added Reviewed-by tag.

 net/ipv4/udp_offload.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 3263ebcaa3f4..4ea72bd4f6d7 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -559,6 +559,12 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
 	 */
 	NAPI_GRO_CB(skb)->is_flist = 0;
 	if (!sk || !udp_sk(sk)->gro_receive) {
+		/* If the packet was locally encapsulated in a UDP tunnel that
+		 * wasn't detected above, do not GRO.
+		 */
+		if (skb->encapsulation)
+			goto out;
+
 		if (skb->dev->features & NETIF_F_GRO_FRAGLIST)
 			NAPI_GRO_CB(skb)->is_flist = sk ? !udp_test_bit(GRO_ENABLED, sk) : 1;

-- 
2.44.0