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net/l2tp: reset skb control buffer

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The L2TP stack did not reset the skb control buffer before handing off
the package to the lower netdev.

In a setup with an ath10k radio and batman-adv over an L2TP tunnel
massive fragmentations happen sporadically if the L2TP tunnel is
established over IPv4.

L2TP might reset some of the fields in the IP control buffer, but L2TP
assumes the type of the control buffer to be of an IPv4 packet, while
due to it being Layer 2, this is not a given.

Clear the entire control buffer to avoid such mishaps altogether.

Link: https://github.com/torvalds/linux/commit/d70b592551ff23747e26e74081205babf8dba9b6
Signed-off-by: David Bauer <mail@david-bauer.net>
This commit is contained in:
David Bauer
2026-09-08 00:39:23 +02:00
parent b68374ee50
commit 7114f29fe2
1 changed files with 40 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files
target/linux/generic/backport-6.12/615-v6.18-net-l2tp-reset-skb-control-buffer.patch
+40
View File
@@ -0,0 +1,40 @@
From 6317095a52e769169ca1bd9a2d9100c0feb5e92b Mon Sep 17 00:00:00 2001
From: David Bauer <mail@david-bauer.net>
Date: Mon, 17 Nov 2025 01:46:47 +0100
Subject: [PATCH] net/l2tp: reset skb control buffer
The L2TP stack did not reset the skb control buffer before handing off
the package to the lower netdev.
In a setup with an ath10k radio and batman-adv over an L2TP tunnel
massive fragmentations happen sporadically if the L2TP tunnel is
established over IPv4.
L2TP might reset some of the fields in the IP control buffer, but L2TP
assumes the type of the control buffer to be of an IPv4 packet, while
due to it being Layer 2, this is not a given.
Clear the entire control buffer to avoid such mishaps altogether.
Link: https://github.com/torvalds/linux/commit/d70b592551ff23747e26e74081205babf8dba9b6
Signed-off-by: David Bauer <mail@david-bauer.net>
---
net/l2tp/l2tp_core.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1249,9 +1249,9 @@ static int l2tp_xmit_core(struct l2tp_se
else
l2tp_build_l2tpv3_header(session, __skb_push(skb, session->hdr_len));
- /* Reset skb netfilter state */
- memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
- IPCB(skb)->flags &= ~(IPSKB_XFRM_TUNNEL_SIZE | IPSKB_XFRM_TRANSFORMED | IPSKB_REROUTED);
+ /* Reset control buffer */
+ memset(skb->cb, 0, sizeof(skb->cb));
+
nf_reset_ct(skb);
/* L2TP uses its own lockdep subclass to avoid lockdep splats caused by