[PATCH net v2] atm: mpoa: keep mpc->dev referenced across mpoad restart

[Shuvam Pandey <shuvampandey1@gmail.com>]

atm: mpoa: keep mpc->dev referenced across mpoad restart

syzbot reported a netdevice refcount warning:

refcount_t: decrement hit 0; leaking memory.
WARNING: lib/refcount.c:31 at refcount_warn_saturate+0x70/0x110
...
dev_put include/linux/netdevice.h:4466 [inline]
mpoad_close+0x1fc/0x3e0 net/atm/mpc.c:889

mpoad_close() drops the reference held in mpc->dev, but the mpoa_client
itself stays alive and keeps the same device pointer.

When mpoad is attached again, atm_mpoa_mpoad_attach() reuses the existing
mpoa_client and its mpc->dev without reacquiring that reference, so the
next close can hit the netdevice refcount warning.

This reference is owned by the mpoa_client/LEC association rather than a
single mpoad open/close cycle. It is acquired when the client gets its
LEC device and is released later from mpoa_event_listener() on
NETDEV_UNREGISTER. Fix the imbalance by removing the dev_put() from
mpoad_close().

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+5ec223ccb83b24ef982f@syzkaller.appspotmail.com
Link: https://groups.google.com/g/syzkaller-bugs/c/qhZ5MJfLBOE/m/UnotmgRdAQAJ
Signed-off-by: Shuvam Pandey <shuvampandey1@gmail.com>
---
Changes in v2:
- drop the atm_mpoa_cleanup() dev_put()/NULL hunk
- add the syzbot warning excerpt
- add a Fixes tag
- clarify that the final dev_put() comes from the notifier path

 net/atm/mpc.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/net/atm/mpc.c b/net/atm/mpc.c
index ce8e9780373b9..90ab8f2889734 100644
--- a/net/atm/mpc.c
+++ b/net/atm/mpc.c
@@ -886,7 +886,6 @@ static void mpoad_close(struct atm_vcc *vcc)
 		struct lec_priv *priv = netdev_priv(mpc->dev);
 		priv->lane2_ops->associate_indicator = NULL;
 		stop_mpc(mpc);
-		dev_put(mpc->dev);
 	}
 
 	mpc->in_ops->destroy_cache(mpc);