[PATCH v2] Bluetooth: L2CAP: Fix ERTM re-init and zero pdu_len infinite loop

[Hyunwoo Kim <imv4bel@gmail.com>]

l2cap_config_req() processes CONFIG_REQ for channels in BT_CONNECTED
state to support L2CAP reconfiguration (e.g. MTU changes). However,
since both CONF_INPUT_DONE and CONF_OUTPUT_DONE are already set from
the initial configuration, the reconfiguration path falls through to
l2cap_ertm_init(), which re-initializes tx_q, srej_q, srej_list, and
retrans_list without freeing the previous allocations and sets
chan->sdu to NULL without freeing the existing skb. This leaks all
previously allocated ERTM resources.

Additionally, l2cap_parse_conf_req() does not validate the minimum
value of remote_mps derived from the RFC max_pdu_size option. A zero
value propagates to l2cap_segment_sdu() where pdu_len becomes zero,
causing the while loop to never terminate since len is never
decremented, exhausting all available memory.

Fix the double-init by skipping l2cap_ertm_init() and
l2cap_chan_ready() when the channel is already in BT_CONNECTED state,
while still allowing the reconfiguration parameters to be updated
through l2cap_parse_conf_req(). Also add a pdu_len zero check in
l2cap_segment_sdu() as a safeguard.

Fixes: 96298f640104 ("Bluetooth: L2CAP: handle l2cap config request during open state")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
Changes in v2:
- Keep BT_CONNECTED in allowed states for l2cap_config_req()
- Skip l2cap_ertm_init() and l2cap_chan_ready() when already connected
- v1: https://lore.kernel.org/all/abwTCkavRurEcEGO@v4bel/
---
 net/bluetooth/l2cap_core.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 5deb6c4f1e41..b80667c263ef 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -2377,6 +2377,9 @@ static int l2cap_segment_sdu(struct l2cap_chan *chan,
 	/* Remote device may have requested smaller PDUs */
 	pdu_len = min_t(size_t, pdu_len, chan->remote_mps);
 
+	if (!pdu_len)
+		return -EINVAL;
+
 	if (len <= pdu_len) {
 		sar = L2CAP_SAR_UNSEGMENTED;
 		sdu_len = 0;
@@ -4312,14 +4315,16 @@ static inline int l2cap_config_req(struct l2cap_conn *conn,
 	if (test_bit(CONF_INPUT_DONE, &chan->conf_state)) {
 		set_default_fcs(chan);
 
-		if (chan->mode == L2CAP_MODE_ERTM ||
-		    chan->mode == L2CAP_MODE_STREAMING)
-			err = l2cap_ertm_init(chan);
+		if (chan->state != BT_CONNECTED) {
+			if (chan->mode == L2CAP_MODE_ERTM ||
+			    chan->mode == L2CAP_MODE_STREAMING)
+				err = l2cap_ertm_init(chan);
 
-		if (err < 0)
-			l2cap_send_disconn_req(chan, -err);
-		else
-			l2cap_chan_ready(chan);
+			if (err < 0)
+				l2cap_send_disconn_req(chan, -err);
+			else
+				l2cap_chan_ready(chan);
+		}
 
 		goto unlock;
 	}
-- 
2.43.0