[PATCH 3.16 145/366] l2tp: hold reference on tunnels in netlink dumps

Ben Hutchings
2018-10-14T15:25:41Z

From: Guillaume Nault <g.nault@alphalink.fr>

commit 5846c131c39b6d0add36ec19dc8650700690f930 upstream.

l2tp_tunnel_find_nth() is unsafe: no reference is held on the returned
tunnel, therefore it can be freed whenever the caller uses it.
This patch defines l2tp_tunnel_get_nth() which works similarly, but
also takes a reference on the returned tunnel. The caller then has to
drop it after it stops using the tunnel.

Convert netlink dumps to make them safe against concurrent tunnel
deletion.

Fixes: 309795f4bec2 ("l2tp: Add netlink control API for L2TP")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
net/l2tp/l2tp_core.c | 20 ++++++++++++++++++++
net/l2tp/l2tp_core.h | 2 ++
net/l2tp/l2tp_netlink.c | 11 ++++++++---
3 files changed, 30 insertions(+), 3 deletions(-)

--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -231,6 +231,26 @@ struct l2tp_tunnel *l2tp_tunnel_get(cons
}
EXPORT_SYMBOL_GPL(l2tp_tunnel_get);

+struct l2tp_tunnel *l2tp_tunnel_get_nth(const struct net *net, int nth)
+{
+ const struct l2tp_net *pn = l2tp_pernet(net);
+ struct l2tp_tunnel *tunnel;
+ int count = 0;
+
+ rcu_read_lock_bh();
+ list_for_each_entry_rcu(tunnel, &pn->l2tp_tunnel_list, list) {
+ if (++count > nth) {
+ l2tp_tunnel_inc_refcount(tunnel);
+ rcu_read_unlock_bh();
+ return tunnel;
+ }
+ }
+ rcu_read_unlock_bh();
+
+ return NULL;
+}
+EXPORT_SYMBOL_GPL(l2tp_tunnel_get_nth);
+
/* Like l2tp_session_find() but takes a reference on the returned session.
* Optionally calls session->ref() too if do_ref is true.
*/
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -227,6 +227,8 @@ static inline void *l2tp_session_priv(st
}

struct l2tp_tunnel *l2tp_tunnel_get(const struct net *net, u32 tunnel_id);
+struct l2tp_tunnel *l2tp_tunnel_get_nth(const struct net *net, int nth);
+
void l2tp_tunnel_free(struct l2tp_tunnel *tunnel);

struct l2tp_session *l2tp_session_get(const struct net *net,
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -395,14 +395,17 @@ static int l2tp_nl_cmd_tunnel_dump(struc
struct net *net = sock_net(skb->sk);

for (;;) {
- tunnel = l2tp_tunnel_find_nth(net, ti);
+ tunnel = l2tp_tunnel_get_nth(net, ti);
if (tunnel == NULL)
goto out;

if (l2tp_nl_tunnel_send(skb, NETLINK_CB(cb->skb).portid,
cb->nlh->nlmsg_seq, NLM_F_MULTI,
- tunnel) <= 0)
+ tunnel) <= 0) {
+ l2tp_tunnel_dec_refcount(tunnel);
goto out;
+ }
+ l2tp_tunnel_dec_refcount(tunnel);

ti++;
}
@@ -746,7 +749,7 @@ static int l2tp_nl_cmd_session_dump(stru

for (;;) {
if (tunnel == NULL) {
- tunnel = l2tp_tunnel_find_nth(net, ti);
+ tunnel = l2tp_tunnel_get_nth(net, ti);
if (tunnel == NULL)
goto out;
}
@@ -754,6 +757,7 @@ static int l2tp_nl_cmd_session_dump(stru
session = l2tp_session_get_nth(tunnel, si, false);
if (session == NULL) {
ti++;
+ l2tp_tunnel_dec_refcount(tunnel);
tunnel = NULL;
si = 0;
continue;
@@ -763,6 +767,7 @@ static int l2tp_nl_cmd_session_dump(stru
cb->nlh->nlmsg_seq, NLM_F_MULTI,
session) <= 0) {
l2tp_session_dec_refcount(session);
+ l2tp_tunnel_dec_refcount(tunnel);
break;
}
l2tp_session_dec_refcount(session);