Re: [PATCH] Bluetooth: L2CAP: fix race l2cap_sock_cleanup_listen() vs. put_chan

[Pauli Virtanen]

Hi,

ma, 2026-08-03 kello 14:13 +0800, Hillf Danton kirjoitti:
> On Sun,  2 Aug 2026 15:12:28 +0300 Pauli Virtanen wrote:
> > 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.
> > 
> > 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.
> > 
> > Fixes: 0e2c0392b9dc ("Bluetooth: L2CAP: Fix use-after-free in l2cap_sock_new_connection_cb()")
> > Reported-by: syzbot+e6382a2f53f5fc7453ac@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=e6382a2f53f5fc7453ac
> > Signed-off-by: Pauli Virtanen <pav@xxxxxx>
> > ---
> >  include/net/bluetooth/l2cap.h |  5 +++++
> >  net/bluetooth/l2cap_sock.c    | 23 +++++++++++++----------
> >  2 files changed, 18 insertions(+), 10 deletions(-)
> > 
> > diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
> > index ef6ce1c20a4f..3d9a32094347 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 735167f73f31..9540617a0e6c 100644
> > --- a/net/bluetooth/l2cap_sock.c
> > +++ b/net/bluetooth/l2cap_sock.c
> > @@ -1312,7 +1312,12 @@ 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);
> 
> In l2cap_sock_teardown_cb(), sock is only zapped after cleanup including unlink,
> so why do you see a linked and zapped sock in l2cap_sock_cleanup_listen()?

l2cap_sock_cleanup_listen() is not a single critical section.

There is the following race:

   [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 = bt_sk(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)

> > @@ -1516,14 +1521,10 @@ 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
> > @@ -1533,10 +1534,12 @@ 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;
> >  		}
> > -- 
> > 2.55.0