FAILED: patch "[PATCH] net: erspan: set lltx to avoid sch_direct_xmit deadlock" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-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-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 2d36ec73836e83175deb467fd494c61006107e5d
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107060722-failed-5.15-2d36ec73836e@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

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

From 2d36ec73836e83175deb467fd494c61006107e5d Mon Sep 17 00:00:00 2001
From: Yun Zhou <yun.zhou@windriver.com>
Date: Thu, 8 Oct 2026 17:38:24 +0800
Subject: [PATCH] net: erspan: set lltx to avoid sch_direct_xmit deadlock

The ERSPAN xmit path re-enters the network stack, causing nested
acquisition of _xmit_lock on the underlay device while already holding
the ERSPAN device's _xmit_lock, creating an ABBA deadlock. This happens
for both IPv4 ERSPAN (erspan_xmit() -> ip_tunnel_xmit()) and IPv6 ERSPAN
(ip6erspan_tunnel_xmit() -> ip6_tnl_xmit()):

  sch_direct_xmit [lock erspan] -> erspan_xmit -> ip_tunnel_xmit ->
  ip_output -> __dev_queue_xmit -> sch_direct_xmit [lock underlay]

  sch_direct_xmit [lock erspan] -> ip6erspan_tunnel_xmit -> ip6_tnl_xmit ->
  ip6_output -> __dev_queue_xmit -> sch_direct_xmit [lock underlay]

Set dev->lltx = true so HARD_TX_LOCK() skips the spinlock for ERSPAN.
This is safe as the ERSPAN xmit path has no shared mutable state: o_seqno
is atomic, TX stats are per-CPU u64_stats, dst_cache is per-CPU, and
o_flags is no longer modified in the xmit path since commit 9958e69b9893
("gre: fix ERSPAN o_flags race/corruption in xmit and fill_info").
GRETAP, the sibling device with identical xmit structure, already sets
lltx.

Reported-by: syzbot+9bda1b9fbb7fbdf9b62b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9bda1b9fbb7fbdf9b62b
Fixes: 84e54fe0a5ea ("gre: introduce native tunnel support for ERSPAN")
Fixes: 5a963eb61b7c ("ip6_gre: Add ERSPAN native tunnel support")
Cc: stable@vger.kernel.org # 9958e69b9893 ("gre: fix ERSPAN o_flags race/corruption in xmit and fill_info")
Cc: stable@vger.kernel.org
Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20261008093824.119453-1-yun.zhou@windriver.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 net/ipv4/ip_gre.c  | 1 +
 net/ipv6/ip6_gre.c | 1 +
 2 files changed, 2 insertions(+)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index e4878e9aa6367..149a4e83b6925 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -1367,6 +1367,7 @@ static int erspan_tunnel_init(struct net_device *dev)
 	dev->features		|= GRE_FEATURES;
 	dev->hw_features	|= GRE_FEATURES;
 	dev->priv_flags		|= IFF_LIVE_ADDR_CHANGE;
+	dev->lltx = true;
 	netif_keep_dst(dev);
 
 	return ip_tunnel_init(dev);
diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 04f7c70b7320e..3429543ec77e7 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -1864,6 +1864,7 @@ static int ip6erspan_tap_init(struct net_device *dev)
 		dev->mtu -= 8;
 
 	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE;
+	dev->lltx = true;
 	ip6erspan_tnl_link_config(tunnel, 1);
 
 	netdev_hold(dev, &tunnel->dev_tracker, GFP_KERNEL);
-- 
2.53.0