Timer interrupt ax25_ds_timeout() could introduce double locks on
&ax25_list_lock.

ax25_ioctl()
--> ax25_ctl_ioctl()
--> ax25_dama_off()
--> ax25_dev_dama_off()
--> ax25_check_dama_slave()
--> spin_lock(&ax25_list_lock)
<timer interrupt>
   --> ax25_ds_timeout()
   --> spin_lock(&ax25_list_lock)

This flaw was found by an experimental static analysis tool I am
developing for irq-related deadlock.

To prevent the potential deadlock, the patch use spin_lock_bh()
on &ax25_list_lock inside ax25_check_dama_slave().

Fixes: c19c4b9c9acb ("[AX.25]: Optimize AX.25 socket list lock")
Signed-off-by: Chengfeng Ye <redacted>
---
V2: add fixes tag

 net/ax25/ax25_ds_subr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ax25/ax25_ds_subr.c b/net/ax25/ax25_ds_subr.c
index f00e27df3c76..010b11303d32 100644
--- a/net/ax25/ax25_ds_subr.c
+++ b/net/ax25/ax25_ds_subr.c

@@ -156,13 +156,13 @@ static int ax25_check_dama_slave(ax25_dev *ax25_dev)
 	ax25_cb *ax25;
 	int res = 0;
 
-	spin_lock(&ax25_list_lock);
+	spin_lock_bh(&ax25_list_lock);
 	ax25_for_each(ax25, &ax25_list)
 		if (ax25->ax25_dev == ax25_dev && (ax25->condition & AX25_COND_DAMA_MODE) && ax25->state > AX25_STATE_1) {
 			res = 1;
 			break;
 		}
-	spin_unlock(&ax25_list_lock);
+	spin_unlock_bh(&ax25_list_lock);
 
 	return res;
 }

-- 
2.17.1