From 9d7bbff945f79c62fa503bdf7a59bd0f9d2a4017 Mon Sep 17 00:00:00 2001
From: Guillaume Nault <g.nault@alphalink.fr>
Date: Thu, 12 Apr 2018 20:50:34 +0200
Subject: [PATCH 4/5] l2tp: hold reference on tunnels printed in pppol2tp proc
 file
Git-commit: 0e0c3fee3a59a387aeecc4fca6f3a2e9615a5443
References: git-fixes
Patch-mainline: v4.17-rc2

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>
Signed-off-by: Denis Kirjanov <denis.kirjanov@suse.com>
---
 net/l2tp/l2tp_ppp.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 7080b4bbf1c3..48f83aa7d504 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -1642,16 +1642,19 @@ 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);
 	}
 }
 
@@ -1700,7 +1703,14 @@ 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)
-- 
2.16.4