FAILED: patch "[PATCH] gtp: disable BH before calling udp_tunnel_xmit_skb()" failed to apply to 6.12-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 5638504a2aa9e1b9d72af9060df1a160cce2d379
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026050428-flaky-polish-d0e3@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 5638504a2aa9e1b9d72af9060df1a160cce2d379 Mon Sep 17 00:00:00 2001
From: David Carlier <devnexen@gmail.com>
Date: Fri, 17 Apr 2026 06:54:08 +0100
Subject: [PATCH] gtp: disable BH before calling udp_tunnel_xmit_skb()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

gtp_genl_send_echo_req() runs as a generic netlink doit handler in
process context with BH not disabled. It calls udp_tunnel_xmit_skb(),
which eventually invokes iptunnel_xmit() — that uses __this_cpu_inc/dec
on softnet_data.xmit.recursion to track the tunnel xmit recursion level.

Without local_bh_disable(), the task may migrate between
dev_xmit_recursion_inc() and dev_xmit_recursion_dec(), breaking the
per-CPU counter pairing. The result is stale or negative recursion
levels that can later produce false-positive
SKB_DROP_REASON_RECURSION_LIMIT drops on either CPU.

The other udp_tunnel_xmit_skb() call sites in gtp.c are unaffected:
the data path runs under ndo_start_xmit and the echo response handlers
run from the UDP encap rx softirq, both with BH already disabled.

Fix it by disabling BH around the udp_tunnel_xmit_skb() call, mirroring
commit 2cd7e6971fc2 ("sctp: disable BH before calling
udp_tunnel_xmit_skb()").

Fixes: 6f1a9140ecda ("net: add xmit recursion limit to tunnel xmit functions")
Cc: stable@vger.kernel.org
Signed-off-by: David Carlier <devnexen@gmail.com>
Link: https://patch.msgid.link/20260417055408.4667-1-devnexen@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
index 70b9e58b9b78..5150f2e4f66b 100644
--- a/drivers/net/gtp.c
+++ b/drivers/net/gtp.c
@@ -2400,6 +2400,7 @@ static int gtp_genl_send_echo_req(struct sk_buff *skb, struct genl_info *info)
 		return -ENODEV;
 	}
 
+	local_bh_disable();
 	udp_tunnel_xmit_skb(rt, sk, skb_to_send,
 			    fl4.saddr, fl4.daddr,
 			    inet_dscp_to_dsfield(fl4.flowi4_dscp),
@@ -2409,6 +2410,7 @@ static int gtp_genl_send_echo_req(struct sk_buff *skb, struct genl_info *info)
 			    !net_eq(sock_net(sk),
 				    dev_net(gtp->dev)),
 			    false, 0);
+	local_bh_enable();
 	return 0;
 }