Medium [CVE-2026-64404] avoid NULL deref of conn in iso_conn_big_sync

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: ISO: avoid NULL deref of conn in iso_conn_big_sync() iso_conn_big_sync() drops the socket lock to call hci_get_route() and then re-acquires it, but dereferences iso_pi(sk)->conn->hcon afterwards without re-checking that conn is still valid.

While the lock is dropped, the connection can be torn down under the same socket lock: iso_disconn_cfm() -> iso_conn_del() -> iso_chan_del() sets iso_pi(sk)->conn to NULL (and the broadcast teardown path can also clear conn->hcon on its own).

When iso_conn_big_sync() re-acquires the lock and reads conn->hcon, conn may be NULL, causing a NULL pointer dereference (hcon is the first member of struct iso_conn).

This path is reached from iso_sock_recvmsg() for a PA-sync broadcast sink socket (BT_SK_DEFER_SETUP | BT_SK_PA_SYNC), so the dropped-lock window can race with connection teardown driven by controller events.

Re-validate iso_pi(sk)->conn and its hcon after re-acquiring the socket lock and bail out if the connection went away, as already done in the sibling iso_sock_rebind_bc(). A race condition exists in the iso_conn_big_sync() function where a socket lock is temporarily released and re-acquired.

During this window, the Bluetooth connection can be unexpectedly terminated.