[PATCH v2] Bluetooth: L2CAP: fix memory leak from repeated configuration of a connected ERTM channel

[Henry Martin <bsdhenrymartin@gmail.com>]

l2cap_config_req() accepts a CONF_REQ on a channel that is already
BT_CONNECTED (the state check explicitly allows it), and
l2cap_config_rsp() has no state check at all. A misbehaving BR/EDR peer
can therefore drive a connected ERTM channel through the configuration
procedure any number of times:

 1. CONF_REQ on the connected channel re-arms CONF_OUTPUT_DONE
    (l2cap_parse_conf_req() sets it on a successful parse);
 2. CONF_RSP(SUCCESS) sets CONF_INPUT_DONE and, because
    CONF_OUTPUT_DONE is set, calls l2cap_ertm_init() once more.

Every re-entry of l2cap_ertm_init() leaks memory and resets the ERTM
state machine on a live channel:

  - chan->sdu = NULL drops a partially reassembled skb (leak);
  - skb_queue_head_init(&chan->tx_q / &chan->srej_q) detaches every
    queued frame from the lists (leak);
  - l2cap_seq_list_init() overwrites chan->srej_list.list /
    chan->retrans_list.list without freeing the previous arrays (leak
    of two kmalloc arrays per round).

Normally l2cap_chan_ready() clears conf_state so the re-entry cannot
happen; but l2cap_chan_ready() returns early when the channel is
already BT_CONNECTED, so a peer that keeps the channel connected
while replaying CONF_REQ/CONF_RSP pairs bypasses that reset entirely.

Drop BT_CONNECTED from the states that accept a CONF_REQ, and have
l2cap_config_rsp() bail out when the channel is connected, keeping the
configuration procedure strictly pre-connection.

This issue was discovered by Tencent CodeBuddy Security.

Fixes: 96298f640104 ("Bluetooth: L2CAP: handle l2cap config request during open state")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
---

v2: Rebase onto current bluetooth-next (the lookup helpers no longer
    take the channel lock, so the config_rsp bail now drops the
    reference with l2cap_chan_put() only, without
    l2cap_chan_unlock()).  Fixes the CI apply failure against
    bluetooth-next HEAD.

---
 net/bluetooth/l2cap_core.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 0000000000000..0000000000000 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -4591,8 +4591,7 @@
 
 	lockdep_assert_held(&chan->conn->lock);
 
-	if (chan->state != BT_CONFIG && chan->state != BT_CONNECT2 &&
-	    chan->state != BT_CONNECTED) {
+	if (chan->state != BT_CONFIG && chan->state != BT_CONNECT2) {
 		cmd_reject_invalid_cid(conn, cmd->ident, chan->scid,
 				       chan->dcid);
 		goto unlock;
@@ -4709,6 +4708,11 @@
 	if (!chan)
 		return 0;
 
+	if (chan->state == BT_CONNECTED) {
+		l2cap_chan_put(chan);
+		return 0;
+	}
+
 	l2cap_chan_lock(chan);
 
 	lockdep_assert_held(&chan->conn->lock);
---
-- 
2.43.7