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) <--------------------- release_sock(sk)
sock_set_flag(sk, SOCK_ZAPPED)
l2cap_sock_close_cb
l2cap_sock_kill(sk)
l2cap_sock_put_chan
chan = READ l2cap_pi(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.
Fixes: 6fef032af009 ("Bluetooth: L2CAP: Fix use-after-free in l2cap_sock_new_connection_cb()")
Reported-by: syzbot+e6382a2f53f5fc7453ac@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e6382a2f53f5fc7453ac
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index ef6ce1c..3d9a320 100644
--- a/include/net/bluetooth/l2cap.h
+++ b/include/net/bluetooth/l2cap.h
@@ -699,7 +699,12 @@ struct l2cap_rx_busy {
struct l2cap_pinfo {
struct bt_sock bt;
+
+ /* With owning sk_socket chan may be read without lock, other access
+ * should hold lock_sock.
+ */
struct l2cap_chan *chan;
+
struct list_head rx_busy;
};
diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index 8bf35bc..1194c37 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c
@@ -1344,7 +1344,12 @@ static void l2cap_sock_kill(struct sock *sk)
BT_DBG("sk %p state %s", sk, state_to_string(sk->sk_state));
+ /* Take lock to synchronize against access without owning sk->sk_socket,
+ * eg. in l2cap_sock_cleanup_listen(). proto_ops etc. don't need lock.
+ */
+ lock_sock(sk);
l2cap_sock_put_chan(sk);
+ release_sock(sk);
/* Kill poor orphan */
sock_set_flag(sk, SOCK_DEAD);
@@ -1548,14 +1553,10 @@ static void l2cap_sock_cleanup_listen(struct sock *parent)
* establish sk_lock -> conn->lock and invert the established
* conn->lock -> chan->lock -> sk_lock order (lockdep deadlock).
*
- * Instead, briefly take the child sk lock to fetch and pin its chan.
- * l2cap_conn_del() reaches the chan free only via
- * l2cap_chan_del() -> l2cap_sock_teardown_cb(), which itself takes
- * the child sk lock; holding it across l2cap_chan_hold_unless_zero()
- * therefore guarantees the chan cannot be freed while we read and
- * pin it (hold_unless_zero() additionally skips a chan already past
- * its last reference). We then drop the sk lock before taking
- * chan->lock, so sk and chan locks are never held together.
+ * Instead, briefly take the child sk lock to synchronize vs.
+ * l2cap_sock_kill that puts l2cap_pi(sk)->chan. We then drop the sk
+ * lock before taking chan->lock, so sk and chan locks are never held
+ * together.
*
* Since we cannot call l2cap_chan_close() without conn->lock,
* schedule l2cap_chan_timeout to close the channel; it already
@@ -1565,10 +1566,12 @@ static void l2cap_sock_cleanup_listen(struct sock *parent)
struct l2cap_chan *chan;
lock_sock_nested(sk, L2CAP_NESTING_NORMAL);
- chan = l2cap_chan_hold_unless_zero(l2cap_pi(sk)->chan);
+ chan = l2cap_pi(sk)->chan;
+ if (chan)
+ l2cap_chan_hold(chan);
release_sock(sk);
if (!chan) {
- /* l2cap_conn_del() already tearing this child down */
+ /* Already torn down */
sock_put(sk);
continue;
}
