net: atm: fix /proc/net/atm/lec handling

/proc/net/atm/lec must ensure safety against dev_lec[] changes.

It appears it had dev_put() calls without prior dev_hold(),
leading to imbalance and UAF.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Francois Romieu <romieu@fr.zoreil.com> # Minor atm contributor
Link: https://patch.msgid.link/20250618140844.1686882-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/atm/lec.c

@@ -909,7 +909,6 @@ static void *lec_itf_walk(struct lec_state *state, loff_t *l)
 	v = (dev && netdev_priv(dev)) ?
 		lec_priv_walk(state, l, netdev_priv(dev)) : NULL;
 	if (!v && dev) {
-		dev_put(dev);
 		/* Partial state reset for the next time we get called */
 		dev = NULL;
 	}
@@ -933,6 +932,7 @@ static void *lec_seq_start(struct seq_file *seq, loff_t *pos)
 {
 	struct lec_state *state = seq->private;
 
+	mutex_lock(&lec_mutex);
 	state->itf = 0;
 	state->dev = NULL;
 	state->locked = NULL;
@@ -950,8 +950,9 @@ static void lec_seq_stop(struct seq_file *seq, void *v)
 	if (state->dev) {
 		spin_unlock_irqrestore(&state->locked->lec_arp_lock,
 				       state->flags);
-		dev_put(state->dev);
+		state->dev = NULL;
 	}
+	mutex_unlock(&lec_mutex);
 }
 
 static void *lec_seq_next(struct seq_file *seq, void *v, loff_t *pos)