netdev - [PATCH 1/2] xfrm: Fix inner mode lookup in tunnel mode GSO segmentation

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Message-ID: <20260114121817.1106134-2-steffen.klassert@secunet.com>
Date: Wed, 14 Jan 2026 13:18:08 +0100
From: Steffen Klassert <steffen.klassert@...unet.com>
To: David Miller <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>
CC: Herbert Xu <herbert@...dor.apana.org.au>, Steffen Klassert
	<steffen.klassert@...unet.com>, <netdev@...r.kernel.org>
Subject: [PATCH 1/2] xfrm: Fix inner mode lookup in tunnel mode GSO segmentation

From: Jianbo Liu <jianbol@...dia.com>

Commit 61fafbee6cfe ("xfrm: Determine inner GSO type from packet inner
protocol") attempted to fix GSO segmentation by reading the inner
protocol from XFRM_MODE_SKB_CB(skb)->protocol. This was incorrect
because the field holds the inner L4 protocol (TCP/UDP) instead of the
required tunnel protocol. Also, the memory location (shared by
XFRM_SKB_CB(skb) which could be overwritten by xfrm_replay_overflow())
is prone to corruption. This combination caused the kernel to select
the wrong inner mode and get the wrong address family.

The correct value is in xfrm_offload(skb)->proto, which is set from
the outer tunnel header's protocol field by esp[4|6]_gso_encap(). It
is initialized by xfrm[4|6]_tunnel_encap_add() to either IPPROTO_IPIP
or IPPROTO_IPV6, using xfrm_af2proto() and correctly reflects the
inner packet's address family.

Fixes: 61fafbee6cfe ("xfrm: Determine inner GSO type from packet inner protocol")
Signed-off-by: Jianbo Liu <jianbol@...dia.com>
Reviewed-by: Sabrina Dubroca <sd@...asysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@...unet.com>
---
 net/ipv4/esp4_offload.c | 4 ++--
 net/ipv6/esp6_offload.c | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/esp4_offload.c b/net/ipv4/esp4_offload.c
index 05828d4cb6cd..abd77162f5e7 100644
--- a/net/ipv4/esp4_offload.c
+++ b/net/ipv4/esp4_offload.c
@@ -122,8 +122,8 @@ static struct sk_buff *xfrm4_tunnel_gso_segment(struct xfrm_state *x,
 						struct sk_buff *skb,
 						netdev_features_t features)
 {
-	const struct xfrm_mode *inner_mode = xfrm_ip2inner_mode(x,
-					XFRM_MODE_SKB_CB(skb)->protocol);
+	struct xfrm_offload *xo = xfrm_offload(skb);
+	const struct xfrm_mode *inner_mode = xfrm_ip2inner_mode(x, xo->proto);
 	__be16 type = inner_mode->family == AF_INET6 ? htons(ETH_P_IPV6)
 						     : htons(ETH_P_IP);
 
diff --git a/net/ipv6/esp6_offload.c b/net/ipv6/esp6_offload.c
index 22410243ebe8..22895521a57d 100644
--- a/net/ipv6/esp6_offload.c
+++ b/net/ipv6/esp6_offload.c
@@ -158,8 +158,8 @@ static struct sk_buff *xfrm6_tunnel_gso_segment(struct xfrm_state *x,
 						struct sk_buff *skb,
 						netdev_features_t features)
 {
-	const struct xfrm_mode *inner_mode = xfrm_ip2inner_mode(x,
-					XFRM_MODE_SKB_CB(skb)->protocol);
+	struct xfrm_offload *xo = xfrm_offload(skb);
+	const struct xfrm_mode *inner_mode = xfrm_ip2inner_mode(x, xo->proto);
 	__be16 type = inner_mode->family == AF_INET ? htons(ETH_P_IP)
 						    : htons(ETH_P_IPV6);
 
-- 
2.43.0