CVE-2026-68373: wifi: at76c50x-usb: avoid length underflow in at76_guess_freq()

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

wifi: at76c50x-usb: avoid length underflow in at76guessfreq()

at76guessfreq() checks only that the received frame is at least a bare 802.11 header (24 bytes) before subtracting the fixed management-body offset:

len -= eloff;

For both beacon and probe response frames, eloff is 36. If the frame is shorter than eloff, subtracting it causes the calculated IE length to wrap. The length is eventually passed to cfg80211findelemmatch() as a very large unsigned value, so the element walk runs beyond the RX skb.

This path is reached from at76rxtasklet() while scanning. If the device delivers a truncated beacon or probe response, the oversized IE length causes an out-of-bounds read during scanning.

Skip the IE lookup if the frame does not reach the variable elements, before subtracting eloff.