Re: [syzbot] [net?] KASAN: slab-use-after-free Write in l2tp_session_delete

Hillf Danton
2024-06-27T12:06:00Z

On 25/06/2024 14:25, syzbot wrote:
> syzbot found the following issue on:
>
> HEAD commit: 185d72112b95 net: xilinx: axienet: Enable multicast by def..
> git tree: net-next
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1062bd46980000

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git 185d72112b95

--- x/net/l2tp/l2tp_core.c
+++ y/net/l2tp/l2tp_core.c
@@ -1298,8 +1298,10 @@ static void l2tp_tunnel_closeall(struct
list_for_each_safe(pos, tmp, &tunnel->session_list) {
session = list_entry(pos, struct l2tp_session, list);
list_del_init(&session->list);
+ l2tp_session_inc_refcount(session);
spin_unlock_bh(&tunnel->list_lock);
l2tp_session_delete(session);
+ l2tp_session_dec_refcount(session);
spin_lock_bh(&tunnel->list_lock);
}
spin_unlock_bh(&tunnel->list_lock);
--- x/net/l2tp/l2tp_ppp.c
+++ y/net/l2tp/l2tp_ppp.c
@@ -393,14 +393,6 @@ abort:
* Session (and tunnel control) socket create/destroy.
*****************************************************************************/

-static void pppol2tp_put_sk(struct rcu_head *head)
-{
- struct pppol2tp_session *ps;
-
- ps = container_of(head, typeof(*ps), rcu);
- sock_put(ps->__sk);
-}
-
/* Really kill the session socket. (Called from sock_put() if
* refcnt == 0.)
*/
@@ -444,22 +436,8 @@ static int pppol2tp_release(struct socke

session = pppol2tp_sock_to_session(sk);
if (session) {
- struct pppol2tp_session *ps;
-
l2tp_session_delete(session);
-
- ps = l2tp_session_priv(session);
- mutex_lock(&ps->sk_lock);
- ps->__sk = rcu_dereference_protected(ps->sk,
- lockdep_is_held(&ps->sk_lock));
- RCU_INIT_POINTER(ps->sk, NULL);
- mutex_unlock(&ps->sk_lock);
- call_rcu(&ps->rcu, pppol2tp_put_sk);
-
- /* Rely on the sock_put() call at the end of the function for
- * dropping the reference held by pppol2tp_sock_to_session().
- * The last reference will be dropped by pppol2tp_put_sk().
- */
+ l2tp_session_dec_refcount(session);
}

release_sock(sk);
@@ -831,6 +809,7 @@ static int pppol2tp_connect(struct socke
out_no_ppp:
/* This is how we get the session context from the socket. */
sk->sk_user_data = session;
+ l2tp_session_inc_refcount(session);
rcu_assign_pointer(ps->sk, sk);
mutex_unlock(&ps->sk_lock);

--