netdev - [PATCH net 1/2] net: gro: fix outer network offset

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Message-ID: <5a0fd9f22844611b806d1cc8bdfbe1504bb33b47.1769771825.git.pabeni@redhat.com>
Date: Fri, 30 Jan 2026 12:17:46 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Simon Horman <horms@...nel.org>,
	Shuah Khan <shuah@...nel.org>,
	Willem de Bruijn <willemb@...gle.com>,
	Richard Gobert <richardbgobert@...il.com>,
	linux-kselftest@...r.kernel.org
Subject: [PATCH net 1/2] net: gro: fix outer network offset

The udp GRO complete stage assumes that all the packets inserted the RX
have the `encapsulation` flag zeroed. Such assumption is not true, as a
few H/W NICs can set such flag when H/W offloading the checksum for
an UDP encapsulated traffic, the tun driver can inject GSO packets with
UDP encapsulation and the problematic layout can also be created via
a veth based setup.

Due to the above, in the problematic scenarios, udp4_gro_complete() uses
the wrong network offset (inner instead of outer) to compute the outer
UDP header pseudo checksum, leading to csum validation errors later on
in packet processing.

Address the issue always clearing the encapsulation flag at GRO completion
time. Such flag will be set again as needed for encapsulated packets by
udp_gro_complete().

Fixes: 5ef31ea5d053 ("net: gro: fix udp bad offset in socket lookup by adding {inner_}network_offset to napi_gro_cb")
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 net/core/gro.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/gro.c b/net/core/gro.c
index 76f9c3712422..482fa7d7f598 100644
--- a/net/core/gro.c
+++ b/net/core/gro.c
@@ -265,6 +265,8 @@ static void gro_complete(struct gro_node *gro, struct sk_buff *skb)
 		goto out;
 	}

+	/* NICs can feed encapsulated packets into GRO */
+	skb->encapsulation = 0;
 	rcu_read_lock();
 	list_for_each_entry_rcu(ptype, head, list) {
 		if (ptype->type != type || !ptype->callbacks.gro_complete)
-- 
2.52.0