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Bluetooth: Fix refcount use-after-free issue

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There is no lock preventing both l2cap_sock_release() and
chan->ops->close() from running at the same time.

If we consider Thread A running l2cap_chan_timeout() and Thread B running
l2cap_sock_release(), expected behavior is:
  A::l2cap_chan_timeout()->l2cap_chan_close()->l2cap_sock_teardown_cb()
  A::l2cap_chan_timeout()->l2cap_sock_close_cb()->l2cap_sock_kill()
  B::l2cap_sock_release()->sock_orphan()
  B::l2cap_sock_release()->l2cap_sock_kill()

where,
sock_orphan() clears "sk->sk_socket" and l2cap_sock_teardown_cb() marks
socket as SOCK_ZAPPED.

In l2cap_sock_kill(), there is an "if-statement" that checks if both
sock_orphan() and sock_teardown() has been run i.e. sk->sk_socket is NULL
and socket is marked as SOCK_ZAPPED. Socket is killed if the condition is
satisfied.

In the race condition, following occurs:
  A::l2cap_chan_timeout()->l2cap_chan_close()->l2cap_sock_teardown_cb()
  B::l2cap_sock_release()->sock_orphan()
  B::l2cap_sock_release()->l2cap_sock_kill()
  A::l2cap_chan_timeout()->l2cap_sock_close_cb()->l2cap_sock_kill()

In this scenario, "if-statement" is true in both B::l2cap_sock_kill() and
A::l2cap_sock_kill() and we hit "refcount: underflow; use-after-free" bug.

Similar condition occurs at other places where teardown/sock_kill is
happening:
  l2cap_disconnect_rsp()->l2cap_chan_del()->l2cap_sock_teardown_cb()
  l2cap_disconnect_rsp()->l2cap_sock_close_cb()->l2cap_sock_kill()

  l2cap_conn_del()->l2cap_chan_del()->l2cap_sock_teardown_cb()
  l2cap_conn_del()->l2cap_sock_close_cb()->l2cap_sock_kill()

  l2cap_disconnect_req()->l2cap_chan_del()->l2cap_sock_teardown_cb()
  l2cap_disconnect_req()->l2cap_sock_close_cb()->l2cap_sock_kill()

  l2cap_sock_cleanup_listen()->l2cap_chan_close()->l2cap_sock_teardown_cb()
  l2cap_sock_cleanup_listen()->l2cap_sock_kill()

Protect teardown/sock_kill and orphan/sock_kill by adding hold_lock on
l2cap channel to ensure that the socket is killed only after marked as
zapped and orphan.

Signed-off-by: Manish Mandlik <mmandlik@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
This commit is contained in:
Manish Mandlik 2020-01-28 10:54:14 -08:00 committed by Marcel Holtmann
parent 151129df2f
commit 6c08fc896b
2 changed files with 28 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

26
net/bluetooth/l2cap_core.c
View File

@ -419,6 +419,9 @@ static void l2cap_chan_timeout(struct work_struct *work)
BT_DBG("chan %p state %s", chan, state_to_string(chan->state));
mutex_lock(&conn->chan_lock);
/* __set_chan_timer() calls l2cap_chan_hold(chan) while scheduling
* this work. No need to call l2cap_chan_hold(chan) here again.
*/
l2cap_chan_lock(chan);
if (chan->state == BT_CONNECTED || chan->state == BT_CONFIG)
@ -431,12 +434,12 @@ static void l2cap_chan_timeout(struct work_struct *work)
l2cap_chan_close(chan, reason);
l2cap_chan_unlock(chan);
chan->ops->close(chan);
mutex_unlock(&conn->chan_lock);
l2cap_chan_unlock(chan);
l2cap_chan_put(chan);
mutex_unlock(&conn->chan_lock);
}
struct l2cap_chan *l2cap_chan_create(void)
@ -1737,9 +1740,9 @@ static void l2cap_conn_del(struct hci_conn *hcon, int err)
l2cap_chan_del(chan, err);
l2cap_chan_unlock(chan);
chan->ops->close(chan);
l2cap_chan_unlock(chan);
l2cap_chan_put(chan);
}
@ -4405,6 +4408,7 @@ static inline int l2cap_disconnect_req(struct l2cap_conn *conn,
return 0;
}
l2cap_chan_hold(chan);
l2cap_chan_lock(chan);
rsp.dcid = cpu_to_le16(chan->scid);
@ -4413,12 +4417,11 @@ static inline int l2cap_disconnect_req(struct l2cap_conn *conn,
chan->ops->set_shutdown(chan);
l2cap_chan_hold(chan);
l2cap_chan_del(chan, ECONNRESET);
l2cap_chan_unlock(chan);
chan->ops->close(chan);
l2cap_chan_unlock(chan);
l2cap_chan_put(chan);
mutex_unlock(&conn->chan_lock);
@ -4450,20 +4453,21 @@ static inline int l2cap_disconnect_rsp(struct l2cap_conn *conn,
return 0;
}
l2cap_chan_hold(chan);
l2cap_chan_lock(chan);
if (chan->state != BT_DISCONN) {
l2cap_chan_unlock(chan);
l2cap_chan_put(chan);
mutex_unlock(&conn->chan_lock);
return 0;
}
l2cap_chan_hold(chan);
l2cap_chan_del(chan, 0);
l2cap_chan_unlock(chan);
chan->ops->close(chan);
l2cap_chan_unlock(chan);
l2cap_chan_put(chan);
mutex_unlock(&conn->chan_lock);

16
net/bluetooth/l2cap_sock.c
View File

@ -1042,7 +1042,7 @@ done:
}
/* Kill socket (only if zapped and orphan)
* Must be called on unlocked socket.
* Must be called on unlocked socket, with l2cap channel lock.
*/
static void l2cap_sock_kill(struct sock *sk)
{
@ -1203,8 +1203,15 @@ static int l2cap_sock_release(struct socket *sock)
err = l2cap_sock_shutdown(sock, 2);
l2cap_chan_hold(l2cap_pi(sk)->chan);
l2cap_chan_lock(l2cap_pi(sk)->chan);
sock_orphan(sk);
l2cap_sock_kill(sk);
l2cap_chan_unlock(l2cap_pi(sk)->chan);
l2cap_chan_put(l2cap_pi(sk)->chan);
return err;
}
@ -1222,12 +1229,15 @@ static void l2cap_sock_cleanup_listen(struct sock *parent)
BT_DBG("child chan %p state %s", chan,
state_to_string(chan->state));
l2cap_chan_hold(chan);
l2cap_chan_lock(chan);
__clear_chan_timer(chan);
l2cap_chan_close(chan, ECONNRESET);
l2cap_chan_unlock(chan);
l2cap_sock_kill(sk);
l2cap_chan_unlock(chan);
l2cap_chan_put(chan);
}
}