[PATCH net] Bluetooth: ISO: drop ISO_END frames received without prior ISO_START

[David Carlier <devnexen@gmail.com>]

ISO data PDUs carry a packet-boundary flag indicating START, CONT, END
or SINGLE. The ISO_CONT branch of iso_recv() guards against a missing
ISO_START by checking conn->rx_len before touching conn->rx_skb, but
ISO_END does not.

If a peer sends an ISO_END as the first packet on a fresh ISO
connection, conn->rx_skb is still NULL and conn->rx_len is zero, so
skb_put(conn->rx_skb, ...) dereferences NULL and oopses. For BIS,
where receivers sync to a broadcaster without pairing, any broadcaster
on the air can trigger this.

Mirror the ISO_CONT check at the top of ISO_END so a stray end fragment
is logged and dropped instead of crashing the host.

Fixes: ccf74f2390d6 ("Bluetooth: Add BTPROTO_ISO socket type")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 net/bluetooth/iso.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/bluetooth/iso.c b/net/bluetooth/iso.c
index 7cb2864fe872..b971281f0a2b 100644
--- a/net/bluetooth/iso.c
+++ b/net/bluetooth/iso.c
@@ -2593,6 +2593,11 @@ int iso_recv(struct hci_dev *hdev, u16 handle, struct sk_buff *skb, u16 flags)
 		break;
 
 	case ISO_END:
+		if (!conn->rx_len) {
+			BT_ERR("Unexpected end frame (len %d)", skb->len);
+			goto drop;
+		}
+
 		skb_copy_from_linear_data(skb, skb_put(conn->rx_skb, skb->len),
 					  skb->len);
 		conn->rx_len -= skb->len;
-- 
2.53.0