CVE-2026-98600: Bluetooth: RFCOMM: Reject short EA=0 frames in rfcomm_recv_frame()

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: RFCOMM: Reject short EA=0 frames in rfcommrecvframe()

While rfcommrecvframe() verifies that skb->len is at least sizeof(hdr) + 1 (4 bytes: 3-byte header + 1-byte FCS), an RFCOMM frame with an extended 2-byte length field (!testea(hdr->len)) has a 4-byte header plus a 1-byte FCS (5 bytes minimum, sizeof(hdr) + 2).

When a 4-byte RFCOMM frame with EA == 0 arrives: 1. The initial skb->len < sizeof(hdr) + 1 check passes (4 < 4 is false). 2. Trimming the FCS byte decrements skb->len to 3. 3. If checkfcs() succeeds, skbpull(skb, 4) fails (4 > 3) and returns NULL without advancing skb->data. 4. Because the return value of skbpull() is ignored, the un-pulled 3-byte struct rfcommhdr remains at skb->data and is either queued as application payload via rfcommrecvdata() or parsed as a multiplexer control command via rfcommrecvmcc() on DLCI 0.

Fix this by extending the length check in rfcommrecvframe() to also require skb->len >= sizeof(hdr) + 2 when !testea(hdr->len).