[PATCH v3 net] l2tp: fix tunnel and session refcount leak on seq_file release

[Eric Dumazet <edumazet@google.com>]

In pppol2tp_proc_open() and l2tp_dfs_seq_open(), iteration state
(pd->tunnel and pd->session) is kept in seq_file private data to allow
iteration across multiple read() system calls.

However, if userspace closes /proc/net/pppol2tp or /sys/kernel/debug/l2tp/tunnels
before reading to end-of-file (EOF), any tunnel or session reference stored in
pd->tunnel / pd->session is left un-dropped when seq_file private data is freed.

Fix this by dropping any remaining pd->tunnel and pd->session references in
pppol2tp_proc_release() and l2tp_dfs_seq_release() when closing the file.

Fixes: 0e0c3fee3a59 ("l2tp: hold reference on tunnels printed in pppol2tp proc file")
Fixes: f726214d9b23 ("l2tp: hold reference on tunnels printed in l2tp/tunnels debugfs file")
Reported-by: syzbot+d6fa74e3f19d6ee01e3a@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a760f32.01d0871a.3a0d52.004f.GAE@google.com/T/#u
Assisted-by: Jetski:Gemini-3.1-Pro
Cc: James Chapman <jchapman@katalix.com>
Cc: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
v3: fix CONFIG_PROC_FS=n case (Paolo)
v2: https://lore.kernel.org/netdev/20260810141125.1176545-1-edumazet@google.com/

 net/l2tp/l2tp_debugfs.c |  4 +++
 net/l2tp/l2tp_ppp.c     | 56 ++++++++++++++++++++++++++++++++++++++---
 2 files changed, 57 insertions(+), 3 deletions(-)

diff --git a/net/l2tp/l2tp_debugfs.c b/net/l2tp/l2tp_debugfs.c
index b26986fda9d6cf096519d56032fe3deb5034c7c8..c12d5e1ee550ccc2608b6128cb653ca373280373 100644
--- a/net/l2tp/l2tp_debugfs.c
+++ b/net/l2tp/l2tp_debugfs.c
@@ -306,6 +306,10 @@ static int l2tp_dfs_seq_release(struct inode *inode, struct file *file)
 
 	seq = file->private_data;
 	pd = seq->private;
+	if (pd->session)
+		l2tp_session_put(pd->session);
+	if (pd->tunnel)
+		l2tp_tunnel_put(pd->tunnel);
 	if (pd->net)
 		put_net_track(pd->net, &pd->ns_tracker);
 	kfree(pd);
diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index f8881699e1cad88e6f312e6ca47ae0af94c6137c..ef8fdfaf051d4602691fbd82e164e3b1eb31a86b 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -1597,7 +1597,53 @@ static const struct seq_operations pppol2tp_seq_ops = {
 	.stop		= pppol2tp_seq_stop,
 	.show		= pppol2tp_seq_show,
 };
-#endif /* CONFIG_PROC_FS */
+
+static int pppol2tp_proc_open(struct inode *inode, struct file *file)
+{
+	struct net *net = pde_data(inode);
+	struct pppol2tp_seq_data *pd;
+
+	net = maybe_get_net(net);
+	if (!net)
+		return -ENXIO;
+
+	pd = __seq_open_private(file, &pppol2tp_seq_ops, sizeof(*pd));
+	if (!pd) {
+		put_net(net);
+		return -ENOMEM;
+	}
+
+#ifdef CONFIG_NET_NS
+	pd->p.net = net;
+	netns_tracker_alloc(net, &pd->p.ns_tracker, GFP_KERNEL);
+#endif
+	return 0;
+}
+
+static int pppol2tp_proc_release(struct inode *inode, struct file *file)
+{
+	struct seq_file *seq = file->private_data;
+	struct pppol2tp_seq_data *pd = seq->private;
+
+	if (pd->session)
+		l2tp_session_put(pd->session);
+	if (pd->tunnel)
+		l2tp_tunnel_put(pd->tunnel);
+
+#ifdef CONFIG_NET_NS
+	put_net_track(pd->p.net, &pd->p.ns_tracker);
+#else
+	put_net(&init_net);
+#endif
+	return seq_release_private(inode, file);
+}
+
+static const struct proc_ops pppol2tp_proc_ops = {
+	.proc_open	= pppol2tp_proc_open,
+	.proc_read	= seq_read,
+	.proc_lseek	= seq_lseek,
+	.proc_release	= pppol2tp_proc_release,
+};
 
 /*****************************************************************************
  * Network namespace
@@ -1608,8 +1654,8 @@ static __net_init int pppol2tp_init_net(struct net *net)
 	struct proc_dir_entry *pde;
 	int err = 0;
 
-	pde = proc_create_net("pppol2tp", 0444, net->proc_net,
-			      &pppol2tp_seq_ops, sizeof(struct pppol2tp_seq_data));
+	pde = proc_create_data("pppol2tp", 0444, net->proc_net,
+			       &pppol2tp_proc_ops, net);
 	if (!pde) {
 		err = -ENOMEM;
 		goto out;
@@ -1624,9 +1670,13 @@ static __net_exit void pppol2tp_exit_net(struct net *net)
 	remove_proc_entry("pppol2tp", net->proc_net);
 }
 
+#endif /* CONFIG_PROC_FS */
+
 static struct pernet_operations pppol2tp_net_ops = {
+#ifdef CONFIG_PROC_FS
 	.init = pppol2tp_init_net,
 	.exit = pppol2tp_exit_net,
+#endif
 };
 
 /*****************************************************************************
-- 
2.55.0.679.g6767b8d81c-goog