ranap: reject oversized RANAP NAS-PDU

The connection-oriented RANAP handlers ranap_handle_co_initial_ue() and
ranap_handle_co_dt() copy the attacker-controlled NAS-PDU into a msgb
allocated with a fixed 256 bytes. RANAP NAS-PDU is an unconstrained
OCTET STRING, so the APER decoder accepts an arbitrarily large PDU; when
nas_pdu.size exceeds the msgb tailroom, msgb_put() hits MSGB_ABORT and
osmo_panic()s the process (MSGB_DEBUG is compiled in unconditionally),
which a femtocell (HNB) peer can use to crash osmo-hnbgw / osmo-hnodeb
by sending an InitialUE or DirectTransfer with a NAS-PDU > 256 bytes.

Validate nas_pdu.size against the msgb tailroom and drop the message
gracefully instead of panicking.

This issue has been assigned the CVE candidate identifier
CAN-2026-2051037.

This is a backport from osmo-iuh.git f06967126f486bcb185ccf3d1a8f9bc02c4da1f6

Change-Id: I2090c7acbf861dfc295a82ef32a976ea7742438b
---
M src/sgsn/gprs_ranap.c
1 file changed, 14 insertions(+), 0 deletions(-)