In the Linux kernel, the following vulnerability has been resolved:
Bluetooth: L2CAP: fix race l2cap_sock_cleanup_listen() vs. put_chan
For L2CAP sockets without owning sk->sk_socket, reading
l2cap_pi(sk)->chan may race against concurrent l2cap_sock_kill() ->
l2cap_sock_put_chan(). This excludes simultaneous proto_ops callbacks,
but access in l2cap_sock_cleanup_listen() has unsafe lockless read.
[Task 1] [Task 2 (hdev->workqueue)]
l2cap_sock_release(parent) l2cap_disconn_cfm
l2cap_sock_cleanup_listen l2cap_conn_del
bt_accept_dequeue l2cap_chan_del
lock_sock(sk) l2cap_sock_teardown_cb
bt_accept_unlink
bt_sk(sk)->parent = NULL
release_sock(sk) —————-> lock_sock(sk)
parent = /* NULL */
lock_sock(sk) chan l2cap_pi(sk)->chan = NULL
l2cap_chan_hold_unless_zero l2cap_put_chan(chan)
kref_get_unless_zero(&chan->ref)
Task 1 may observe NULL which causes null-ptr-deref.
Fix the race by taking lock_sock() in l2cap_sock_kill() to
synchronize with l2cap_sock_cleanup_listen(). hold_unless_zero() is not
needed here, l2cap_pi(sk)->chan owns reference if it is non-NULL.
Clarify code comments vs. locking.
