Bluetooth: ISO: null ptr deref in iso_recv() on ISO_END without prior ISO_START

[Alexandru Hossu <hossu.alexandru@gmail.com>]

Hi,

I found a null pointer dereference in iso_recv() in net/bluetooth/iso.c.

At line 2595-2596:

    case ISO_END:
        skb_copy_from_linear_data(skb,
            skb_put(conn->rx_skb, skb->len), skb->len);

conn->rx_skb is passed to skb_put() without a NULL check.

It can be NULL in two cases. First, if ISO_END arrives without a prior
ISO_START frame, conn->rx_skb was never allocated. Second, the ISO_CONT
overflow error path at line 2581-2587 calls kfree_skb(conn->rx_skb) and
sets it to NULL. A subsequent ISO_END hits the same path.

The ISO_CONT case at line 2575 has a guard:

    if (!conn->rx_len) {
        goto drop;
    }

ISO_END has no equivalent protection.

Suggested fix:

    case ISO_END:
        if (!conn->rx_skb)
            goto drop;
        skb_copy_from_linear_data(skb,
            skb_put(conn->rx_skb, skb->len), skb->len);

Attack surface: CIS requires an established ACL connection. BIS requires
the victim to sync to a malicious broadcaster. Not zero-click but reachable
from an adjacent unauthenticated source on CIS paths.

Tested on linux-next commit e98d21c170b0 (2026-05-08).

Alexandru