[PATCH net v5] net: erspan: set lltx to avoid sch_direct_xmit deadlock

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

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
Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
---
Note for stable maintainers:
This patch depends on commit 9958e69b9893 ("gre: fix ERSPAN o_flags
race/corruption in xmit and fill_info"), which moves the o_flags
read-modify-write in erspan_xmit() / ip6erspan_tunnel_xmit() onto a
local copy.

v5:
  - describe both the IPv4 and IPv6 ERSPAN xmit paths in the commit message
  - drop the inline comments, keep only dev->lltx = true

v4:
  - refine commit message

v3:
  - add fix for IPv6

v2:
  - change subject prefix to [PATCH net]

 net/ipv4/ip_gre.c  | 2 ++
 net/ipv6/ip6_gre.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 82309efd417e..38a24a659fcc 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 8ebda0b6a78b..82957e4fedd2 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -1871,6 +1871,7 @@ 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);
-- 
2.43.0