Re: [PATCH] bluetooth/l2cap: sync sock recv cb and release

[Luiz Augusto von Dentz]

Hi Edward,

On Tue, Jun 11, 2024 at 10:50 AM Edward Adam Davis <eadavis@xxxxxx> wrote:
>
> The problem occurs between the system call to close the sock and hci_rx_work,
> where the former releases the sock and the latter accesses it without lock protection.
>
>            CPU0                       CPU1
>            ----                       ----
>            sock_close                 hci_rx_work
>            l2cap_sock_release         hci_acldata_packet
>            l2cap_sock_kill            l2cap_recv_frame
>            sk_free                    l2cap_conless_channel
>                                       l2cap_sock_recv_cb
>
> If hci_rx_work processes the data that needs to be received before the sock is
> closed, then everything is normal; Otherwise, the work thread may access the
> released sock when receiving data.
>
> Add a chan mutex in the rx callback of the sock to achieve synchronization between
> the sock release and recv cb.
>
> Reported-and-tested-by: syzbot+b7f6f8c9303466e16c8a@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> ---
>  net/bluetooth/l2cap_sock.c | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
> index 6db60946c627..f3e9236293e1 100644
> --- a/net/bluetooth/l2cap_sock.c
> +++ b/net/bluetooth/l2cap_sock.c
> @@ -1413,6 +1413,8 @@ static int l2cap_sock_release(struct socket *sock)
>         l2cap_chan_hold(chan);
>         l2cap_chan_lock(chan);
>
> +       if (refcount_read(&sk->sk_refcnt) == 1)
> +               chan->data = NULL;

Might be a good idea to add some comment on why checking for refcount
== 1 is the right thing to do here, or perhaps we can always assign
chan->data to NULL, instead of that perhaps we could have it done in
l2cap_sock_kill?

>         sock_orphan(sk);
>         l2cap_sock_kill(sk);
>
> @@ -1481,12 +1483,22 @@ static struct l2cap_chan *l2cap_sock_new_connection_cb(struct l2cap_chan *chan)
>
>  static int l2cap_sock_recv_cb(struct l2cap_chan *chan, struct sk_buff *skb)
>  {
> -       struct sock *sk = chan->data;
> -       struct l2cap_pinfo *pi = l2cap_pi(sk);
> +       struct sock *sk;
> +       struct l2cap_pinfo *pi;
>         int err;
>
> -       lock_sock(sk);
> +       l2cap_chan_hold(chan);
> +       l2cap_chan_lock(chan);
> +       sk = chan->data;
> +
> +       if (!sk) {
> +               l2cap_chan_unlock(chan);
> +               l2cap_chan_put(chan);
> +               return -ENXIO;
> +       }
>
> +       pi = l2cap_pi(sk);
> +       lock_sock(sk);
>         if (chan->mode == L2CAP_MODE_ERTM && !list_empty(&pi->rx_busy)) {
>                 err = -ENOMEM;
>                 goto done;
> @@ -1535,6 +1547,8 @@ static int l2cap_sock_recv_cb(struct l2cap_chan *chan, struct sk_buff *skb)
>
>  done:
>         release_sock(sk);
> +       l2cap_chan_unlock(chan);
> +       l2cap_chan_put(chan);
>
>         return err;
>  }
> --
> 2.43.0
>


-- 
Luiz Augusto von Dentz