l2tp: hold reference on tunnels printed in pppol2tp proc file

Use l2tp_tunnel_get_nth() instead of l2tp_tunnel_find_nth(), to be safe
against concurrent tunnel deletion.

Unlike sessions, we can't drop the reference held on tunnels in
pppol2tp_seq_show(). Tunnels are reused across several calls to
pppol2tp_seq_start() when iterating over sessions. These iterations
need the tunnel for accessing the next session. Therefore the only safe
moment for dropping the reference is just before searching for the next
tunnel.

Normally, the last invocation of pppol2tp_next_tunnel() doesn't find
any new tunnel, so it drops the last tunnel without taking any new
reference. However, in case of error, pppol2tp_seq_stop() is called
directly, so we have to drop the reference there.

Fixes: fd558d186df2 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ppp.c

@@ -1551,16 +1551,19 @@ struct pppol2tp_seq_data {
 
 static void pppol2tp_next_tunnel(struct net *net, struct pppol2tp_seq_data *pd)
 {
+	/* Drop reference taken during previous invocation */
+	if (pd->tunnel)
+		l2tp_tunnel_dec_refcount(pd->tunnel);
+
 	for (;;) {
-		pd->tunnel = l2tp_tunnel_find_nth(net, pd->tunnel_idx);
+		pd->tunnel = l2tp_tunnel_get_nth(net, pd->tunnel_idx);
 		pd->tunnel_idx++;
 
-		if (pd->tunnel == NULL)
-			break;
+		/* Only accept L2TPv2 tunnels */
+		if (!pd->tunnel || pd->tunnel->version == 2)
+			return;
 
-		/* Ignore L2TPv3 tunnels */
-		if (pd->tunnel->version < 3)
-			break;
+		l2tp_tunnel_dec_refcount(pd->tunnel);
 	}
 }
 
@@ -1609,7 +1612,14 @@ static void *pppol2tp_seq_next(struct seq_file *m, void *v, loff_t *pos)
 
 static void pppol2tp_seq_stop(struct seq_file *p, void *v)
 {
-	/* nothing to do */
+	struct pppol2tp_seq_data *pd = v;
+
+	if (!pd || pd == SEQ_START_TOKEN)
+		return;
+
+	/* Drop reference taken by last invocation of pppol2tp_next_tunnel() */
+	if (pd->tunnel)
+		l2tp_tunnel_dec_refcount(pd->tunnel);
 }
 
 static void pppol2tp_seq_tunnel_show(struct seq_file *m, void *v)