[PATCH] net: erspan: set lltx to avoid sch_direct_xmit deadlock

[Yun Zhou <yun.zhou@windriver.com>]

erspan_xmit() re-enters the network stack via ip_tunnel_xmit(), causing
nested acquisition of _xmit_lock on the underlay device while already
holding the ERSPAN device's _xmit_lock. Both are ARPHRD_ETHER and share
the same lockdep class, creating an ABBA deadlock:

  sch_direct_xmit [lock erspan] -> erspan_xmit -> ip_tunnel_xmit ->
  ip_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 erspan_xmit() has no shared mutable state: o_seqno is
atomic, stats use atomic_long_inc, and dst_cache is per-CPU. GRETAP,
the sibling device with identical xmit structure, already sets lltx.

Closes: https://syzkaller.appspot.com/bug?extid=9bda1b9fbb7fbdf9b62b
Reported-by: syzbot+9bda1b9fbb7fbdf9b62b@syzkaller.appspotmail.com
Fixes: 84e54fe0a5ea ("gre: introduce native tunnel support for ERSPAN")
Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
---
 net/ipv4/ip_gre.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 3efdfb4ffa21..9fbff16cda1d 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -1363,6 +1363,8 @@ 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;
+	/* Skip TX lock: xmit re-enters stack, risking ABBA with underlay */
+	dev->lltx = true;
 	netif_keep_dst(dev);
 
 	return ip_tunnel_init(dev);
-- 
2.43.0