On Mon, May 22, 2023 at 11:41:51PM +0000, Ying Hsu wrote:
> L2CAP assumes that the locks conn->chan_lock and chan->lock are
> acquired in the order conn->chan_lock, chan->lock to avoid
> potential deadlock.
> For example, l2sock_shutdown acquires these locks in the order:
> mutex_lock(&conn->chan_lock)
> l2cap_chan_lock(chan)
>
> However, l2cap_disconnect_req acquires chan->lock in
> l2cap_get_chan_by_scid first and then acquires conn->chan_lock
> before calling l2cap_chan_del. This means that these locks are
> acquired in unexpected order, which leads to potential deadlock:
> l2cap_chan_lock(c)
> mutex_lock(&conn->chan_lock)
>
> This patch uses __l2cap_get_chan_by_scid to replace
> l2cap_get_chan_by_scid and adjusts the locking order to avoid the
> potential deadlock.
>
> Signed-off-by: Ying Hsu <yinghsu@xxxxxxxxxxxx>
> ---
> This commit has been tested on a Chromebook device.
>
> Changes in v2:
> - Adding the prefix "Bluetooth:" to subject line.
>
> net/bluetooth/l2cap_core.c | 26 ++++++++++++++++++++------
> 1 file changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index 376b523c7b26..8f08192b8fb1 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -4651,8 +4651,16 @@ static inline int l2cap_disconnect_req(struct l2cap_conn *conn,
>
> BT_DBG("scid 0x%4.4x dcid 0x%4.4x", scid, dcid);
>
> - chan = l2cap_get_chan_by_scid(conn, dcid);
> + mutex_lock(&conn->chan_lock);
> + chan = __l2cap_get_chan_by_scid(conn, dcid);
> + if (chan) {
> + chan = l2cap_chan_hold_unless_zero(chan);
> + if (chan)
> + l2cap_chan_lock(chan);
> + }
> +
> if (!chan) {
> + mutex_unlock(&conn->chan_lock);
> cmd_reject_invalid_cid(conn, cmd->ident, dcid, scid);
> return 0;
> }
Hi Ying,
The conditional setting of chan and calling l2cap_chan_lock()
is both non-trivial and repeated. It seems that it ought to be
in a helper.
Something like this (I'm sure a better function name can be chosen):
chan = __l2cap_get_and_lock_chan_by_scid(conn, dcid);
if (!chan) {
...
}
...
