2017-09-19, 18:43:37 +0200, Guillaume Nault wrote:
> On Tue, Sep 19, 2017 at 03:40:40PM +0200, Sabrina Dubroca wrote:
> > If we try to delete the same tunnel twice, the first delete operation
> > does a lookup (l2tp_tunnel_get), finds the tunnel, calls
> > l2tp_tunnel_delete, which queues it for deletion by
> > l2tp_tunnel_del_work.
> >
> > The second delete operation also finds the tunnel and calls
> > l2tp_tunnel_delete. If the workqueue has already fired and started
> > running l2tp_tunnel_del_work, then l2tp_tunnel_delete will queue the
> > same tunnel a second time, and try to free the socket again.
> >
> > Add a dead flag to prevent firing the workqueue twice. Then we can
> > remove the check of queue_work's result that was meant to prevent that
> > race but doesn't.
> >
> > Also check the flag in the tunnel lookup functions, to avoid returning a
> > tunnel that is already scheduled for destruction.
> >
> > Reproducer:
> >
> > ip l2tp add tunnel tunnel_id 3000 peer_tunnel_id 4000 local 192.168.0.2
> > remote 192.168.0.1 encap udp udp_sport 5000 udp_dport 6000
> > ip l2tp add session name l2tp1 tunnel_id 3000 session_id 1000
> > peer_session_id 2000
> > ip link set l2tp1 up
> > ip l2tp del tunnel tunnel_id 3000
> > ip l2tp del tunnel tunnel_id 3000
> >
> > Fixes: f8ccac0e4493 ("l2tp: put tunnel socket release on a workqueue")
> > Reported-by: Jianlin Shi <ji...@redhat.com>
> > Signed-off-by: Sabrina Dubroca <s...@queasysnail.net>
> > ---
> > v2: as Tom Parkin explained, we can't remove the tunnel from the
> > per-net list from netlink. v2 uses only a dead flag, and adds
> > corresponding checks during lookups
> >
> > net/l2tp/l2tp_core.c | 18 +++++++++---------
> > net/l2tp/l2tp_core.h | 5 ++++-
> > 2 files changed, 13 insertions(+), 10 deletions(-)
> >
> > diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> > index ee485df73ccd..3891f0260f2b 100644
> > --- a/net/l2tp/l2tp_core.c
> > +++ b/net/l2tp/l2tp_core.c
> > @@ -203,7 +203,8 @@ struct l2tp_tunnel *l2tp_tunnel_get(const struct net
> > *net, u32 tunnel_id)
> >
> > rcu_read_lock_bh();
> > list_for_each_entry_rcu(tunnel, &pn->l2tp_tunnel_list, list) {
> > - if (tunnel->tunnel_id == tunnel_id) {
> > + if (tunnel->tunnel_id == tunnel_id &&
> > + !test_bit(0, &tunnel->dead)) {
> > l2tp_tunnel_inc_refcount(tunnel);
> > rcu_read_unlock_bh();
> >
> > @@ -390,7 +391,8 @@ struct l2tp_tunnel *l2tp_tunnel_find(const struct net
> > *net, u32 tunnel_id)
> >
> > rcu_read_lock_bh();
> > list_for_each_entry_rcu(tunnel, &pn->l2tp_tunnel_list, list) {
> > - if (tunnel->tunnel_id == tunnel_id) {
> > + if (tunnel->tunnel_id == tunnel_id &&
> > + !test_bit(0, &tunnel->dead)) {
> > rcu_read_unlock_bh();
> > return tunnel;
> > }
> > @@ -409,7 +411,7 @@ struct l2tp_tunnel *l2tp_tunnel_find_nth(const struct
> > net *net, int nth)
> >
> > rcu_read_lock_bh();
> > list_for_each_entry_rcu(tunnel, &pn->l2tp_tunnel_list, list) {
> > - if (++count > nth) {
> > + if (++count > nth && !test_bit(0, &tunnel->dead)) {
> > rcu_read_unlock_bh();
> > return tunnel;
> > }
> >
> I don't get why you're checking the dead flag in l2tp_tunnel_{get,find}*().
> Since it can be set concurrently right after test_bit(), it doesn't
> protect the caller from getting a tunnel that is being removed by
> l2tp_tunnel_delete().
> Or have I missed something?
You're right.
Then I would try going back to essentially v1, but keeping code to
remove the tunnel from the list in l2tp_tunnel_destruct if it's not
dead yet.
What do you think?
-------- 8< --------
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index ee485df73ccd..63cd1f30ac7d 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1234,6 +1234,23 @@ int l2tp_xmit_skb(struct l2tp_session *session, struct
sk_buff *skb, int hdr_len
}
EXPORT_SYMBOL_GPL(l2tp_xmit_skb);
+static bool __l2tp_tunnel_delete(struct l2tp_tunnel *tunnel)
+{
+ struct l2tp_net *pn = l2tp_pernet(tunnel->l2tp_net);
+ bool ret = false;
+
+ spin_lock_bh(&pn->l2tp_tunnel_list_lock);
+ if (!tunnel->dead) {
+ tunnel->dead = 1;
+ list_del_rcu(&tunnel->list);
+ atomic_dec(&l2tp_tunnel_count);
+ ret = true;
+ }
+ spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
+
+ return ret;
+}
+
/*****************************************************************************
* Tinnel and session create/destroy.
*****************************************************************************/
@@ -1245,7 +1262,6 @@ EXPORT_SYMBOL_GPL(l2tp_xmit_skb);
static void l2tp_tunnel_destruct(struct sock *sk)
{
struct l2tp_tunnel *tunnel = l2tp_tunnel(sk);
- struct l2tp_net *pn;
if (tunnel == NULL)
goto end;
@@ -1270,11 +1286,7 @@ static void l2tp_tunnel_destruct(struct sock *sk)
sk->sk_user_data = NULL;
/* Remove the tunnel struct from the tunnel list */
- pn = l2tp_pernet(tunnel->l2tp_net);
- spin_lock_bh(&pn->l2tp_tunnel_list_lock);
- list_del_rcu(&tunnel->list);
- spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
- atomic_dec(&l2tp_tunnel_count);
+ __l2tp_tunnel_delete(tunnel);
l2tp_tunnel_closeall(tunnel);
@@ -1685,14 +1697,12 @@ EXPORT_SYMBOL_GPL(l2tp_tunnel_create);
/* This function is used by the netlink TUNNEL_DELETE command.
*/
-int l2tp_tunnel_delete(struct l2tp_tunnel *tunnel)
+void l2tp_tunnel_delete(struct l2tp_tunnel *tunnel)
{
- l2tp_tunnel_inc_refcount(tunnel);
- if (false == queue_work(l2tp_wq, &tunnel->del_work)) {
- l2tp_tunnel_dec_refcount(tunnel);
- return 1;
+ if (__l2tp_tunnel_delete(tunnel)) {
+ l2tp_tunnel_inc_refcount(tunnel);
+ queue_work(l2tp_wq, &tunnel->del_work);
}
- return 0;
}
EXPORT_SYMBOL_GPL(l2tp_tunnel_delete);
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index a305e0c5925a..173e68bb8119 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -160,6 +160,8 @@ struct l2tp_tunnel_cfg {
struct l2tp_tunnel {
int magic; /* Should be L2TP_TUNNEL_MAGIC
*/
+ int dead;
+
struct rcu_head rcu;
rwlock_t hlist_lock; /* protect session_hlist */
bool acpt_newsess; /* Indicates whether this
@@ -254,7 +256,7 @@ int l2tp_tunnel_create(struct net *net, int fd, int
version, u32 tunnel_id,
u32 peer_tunnel_id, struct l2tp_tunnel_cfg *cfg,
struct l2tp_tunnel **tunnelp);
void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel);
-int l2tp_tunnel_delete(struct l2tp_tunnel *tunnel);
+void l2tp_tunnel_delete(struct l2tp_tunnel *tunnel);
struct l2tp_session *l2tp_session_create(int priv_size,
struct l2tp_tunnel *tunnel,
u32 session_id, u32 peer_session_id,
--
Sabrina
