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Subject[PATCH 3.2 63/95] l2tp: fix duplicate session creation
3.2.91-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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

commit dbdbc73b44782e22b3b4b6e8b51e7a3d245f3086 upstream.

l2tp_session_create() relies on its caller for checking for duplicate
sessions. This is racy since a session can be concurrently inserted
after the caller's verification.

Fix this by letting l2tp_session_create() verify sessions uniqueness
upon insertion. Callers need to be adapted to check for
l2tp_session_create()'s return code instead of calling
l2tp_session_find().

pppol2tp_connect() is a bit special because it has to work on existing
sessions (if they're not connected) or to create a new session if none
is found. When acting on a preexisting session, a reference must be
held or it could go away on us. So we have to use l2tp_session_get()
instead of l2tp_session_find() and drop the reference before exiting.

Fixes: d9e31d17ceba ("l2tp: Add L2TP ethernet pseudowire support")
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>
[bwh: Backported to 3.2: add 'pos' parameter to
hlist_for_each_entry{,_rcu}() calls]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
net/l2tp/l2tp_core.c | 70 +++++++++++++++++++++++++++++++++++++++-------------
net/l2tp/l2tp_eth.c | 10 ++------
net/l2tp/l2tp_ppp.c | 60 ++++++++++++++++++++++----------------------
3 files changed, 84 insertions(+), 56 deletions(-)

--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -324,6 +324,49 @@ struct l2tp_session *l2tp_session_find_b
}
EXPORT_SYMBOL_GPL(l2tp_session_find_by_ifname);

+static int l2tp_session_add_to_tunnel(struct l2tp_tunnel *tunnel,
+ struct l2tp_session *session)
+{
+ struct l2tp_session *session_walk;
+ struct hlist_head *g_head;
+ struct hlist_head *head;
+ struct l2tp_net *pn;
+ struct hlist_node *walk;
+
+ head = l2tp_session_id_hash(tunnel, session->session_id);
+
+ write_lock_bh(&tunnel->hlist_lock);
+ hlist_for_each_entry(session_walk, walk, head, hlist)
+ if (session_walk->session_id == session->session_id)
+ goto exist;
+
+ if (tunnel->version == L2TP_HDR_VER_3) {
+ pn = l2tp_pernet(tunnel->l2tp_net);
+ g_head = l2tp_session_id_hash_2(l2tp_pernet(tunnel->l2tp_net),
+ session->session_id);
+
+ spin_lock_bh(&pn->l2tp_session_hlist_lock);
+ hlist_for_each_entry(session_walk, walk, g_head, global_hlist)
+ if (session_walk->session_id == session->session_id)
+ goto exist_glob;
+
+ hlist_add_head_rcu(&session->global_hlist, g_head);
+ spin_unlock_bh(&pn->l2tp_session_hlist_lock);
+ }
+
+ hlist_add_head(&session->hlist, head);
+ write_unlock_bh(&tunnel->hlist_lock);
+
+ return 0;
+
+exist_glob:
+ spin_unlock_bh(&pn->l2tp_session_hlist_lock);
+exist:
+ write_unlock_bh(&tunnel->hlist_lock);
+
+ return -EEXIST;
+}
+
/* Lookup a tunnel by id
*/
struct l2tp_tunnel *l2tp_tunnel_find(struct net *net, u32 tunnel_id)
@@ -1616,6 +1659,7 @@ static void l2tp_session_set_header_len(
struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunnel, u32 session_id, u32 peer_session_id, struct l2tp_session_cfg *cfg)
{
struct l2tp_session *session;
+ int err;

session = kzalloc(sizeof(struct l2tp_session) + priv_size, GFP_KERNEL);
if (session != NULL) {
@@ -1662,6 +1706,13 @@ struct l2tp_session *l2tp_session_create

l2tp_session_set_header_len(session, tunnel->version);

+ err = l2tp_session_add_to_tunnel(tunnel, session);
+ if (err) {
+ kfree(session);
+
+ return ERR_PTR(err);
+ }
+
/* Bump the reference count. The session context is deleted
* only when this drops to zero.
*/
@@ -1671,28 +1722,14 @@ struct l2tp_session *l2tp_session_create
/* Ensure tunnel socket isn't deleted */
sock_hold(tunnel->sock);

- /* Add session to the tunnel's hash list */
- write_lock_bh(&tunnel->hlist_lock);
- hlist_add_head(&session->hlist,
- l2tp_session_id_hash(tunnel, session_id));
- write_unlock_bh(&tunnel->hlist_lock);
-
- /* And to the global session list if L2TPv3 */
- if (tunnel->version != L2TP_HDR_VER_2) {
- struct l2tp_net *pn = l2tp_pernet(tunnel->l2tp_net);
-
- spin_lock_bh(&pn->l2tp_session_hlist_lock);
- hlist_add_head_rcu(&session->global_hlist,
- l2tp_session_id_hash_2(pn, session_id));
- spin_unlock_bh(&pn->l2tp_session_hlist_lock);
- }
-
/* Ignore management session in session count value */
if (session->session_id != 0)
atomic_inc(&l2tp_session_count);
+
+ return session;
}

- return session;
+ return ERR_PTR(-ENOMEM);
}
EXPORT_SYMBOL_GPL(l2tp_session_create);

--- a/net/l2tp/l2tp_eth.c
+++ b/net/l2tp/l2tp_eth.c
@@ -200,12 +200,6 @@ static int l2tp_eth_create(struct net *n
goto out;
}

- session = l2tp_session_find(net, tunnel, session_id);
- if (session) {
- rc = -EEXIST;
- goto out;
- }
-
if (cfg->ifname) {
dev = dev_get_by_name(net, cfg->ifname);
if (dev) {
@@ -219,8 +213,8 @@ static int l2tp_eth_create(struct net *n

session = l2tp_session_create(sizeof(*spriv), tunnel, session_id,
peer_session_id, cfg);
- if (!session) {
- rc = -ENOMEM;
+ if (IS_ERR(session)) {
+ rc = PTR_ERR(session);
goto out;
}

--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -641,6 +641,7 @@ static int pppol2tp_connect(struct socke
int error = 0;
u32 tunnel_id, peer_tunnel_id;
u32 session_id, peer_session_id;
+ bool drop_refcnt = false;
int ver = 2;
int fd;

@@ -721,36 +722,36 @@ static int pppol2tp_connect(struct socke
tunnel->peer_tunnel_id = sp3->pppol2tp.d_tunnel;
}

- /* Create session if it doesn't already exist. We handle the
- * case where a session was previously created by the netlink
- * interface by checking that the session doesn't already have
- * a socket and its tunnel socket are what we expect. If any
- * of those checks fail, return EEXIST to the caller.
- */
- session = l2tp_session_find(sock_net(sk), tunnel, session_id);
- if (session == NULL) {
- /* Default MTU must allow space for UDP/L2TP/PPP
- * headers.
+ session = l2tp_session_get(sock_net(sk), tunnel, session_id, false);
+ if (session) {
+ drop_refcnt = true;
+ ps = l2tp_session_priv(session);
+
+ /* Using a pre-existing session is fine as long as it hasn't
+ * been connected yet.
*/
- cfg.mtu = cfg.mru = 1500 - PPPOL2TP_HEADER_OVERHEAD;
+ if (ps->sock) {
+ error = -EEXIST;
+ goto end;
+ }

- /* Allocate and initialize a new session context. */
- session = l2tp_session_create(sizeof(struct pppol2tp_session),
- tunnel, session_id,
- peer_session_id, &cfg);
- if (session == NULL) {
- error = -ENOMEM;
+ /* consistency checks */
+ if (ps->tunnel_sock != tunnel->sock) {
+ error = -EEXIST;
goto end;
}
} else {
- ps = l2tp_session_priv(session);
- error = -EEXIST;
- if (ps->sock != NULL)
- goto end;
+ /* Default MTU must allow space for UDP/L2TP/PPP headers */
+ cfg.mtu = 1500 - PPPOL2TP_HEADER_OVERHEAD;
+ cfg.mru = cfg.mtu;

- /* consistency checks */
- if (ps->tunnel_sock != tunnel->sock)
+ session = l2tp_session_create(sizeof(struct pppol2tp_session),
+ tunnel, session_id,
+ peer_session_id, &cfg);
+ if (IS_ERR(session)) {
+ error = PTR_ERR(session);
goto end;
+ }
}

/* Associate session with its PPPoL2TP socket */
@@ -815,6 +816,8 @@ out_no_ppp:
"%s: created\n", session->name);

end:
+ if (drop_refcnt)
+ l2tp_session_dec_refcount(session);
release_sock(sk);

return error;
@@ -842,12 +845,6 @@ static int pppol2tp_session_create(struc
if (tunnel->sock == NULL)
goto out;

- /* Check that this session doesn't already exist */
- error = -EEXIST;
- session = l2tp_session_find(net, tunnel, session_id);
- if (session != NULL)
- goto out;
-
/* Default MTU values. */
if (cfg->mtu == 0)
cfg->mtu = 1500 - PPPOL2TP_HEADER_OVERHEAD;
@@ -855,12 +852,13 @@ static int pppol2tp_session_create(struc
cfg->mru = cfg->mtu;

/* Allocate and initialize a new session context. */
- error = -ENOMEM;
session = l2tp_session_create(sizeof(struct pppol2tp_session),
tunnel, session_id,
peer_session_id, cfg);
- if (session == NULL)
+ if (IS_ERR(session)) {
+ error = PTR_ERR(session);
goto out;
+ }

ps = l2tp_session_priv(session);
ps->tunnel_sock = tunnel->sock;
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