CVE-2026-74649: staging: rtl8723bs: fix missing shared-key auth challenge length check

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

staging: rtl8723bs: fix missing shared-key auth challenge length check

The WEP shared-key authentication handler uses the challenge-text element's attacker-controlled length without checking it against the fixed 128-byte chgtxt buffer.

In OnAuthClient() the length from rtwgetie() - up to 255 - is used to perform memcpy() into the 128-byte pmlmeinfo->chgtxt, so a malicious AP sending a malformed WLANEIDCHALLENGE element can overflow/underfill chgtxt by up to 127 bytes. It is reachable over the air, before association, during shared-key authentication. In the case of an overflow, the driver can write out of bounds. In the case of an underfill, the driver can echo stale buffer memory.

The challenge text is defined to be exactly 128 octets, which is already provided as the WLANAUTHCHALLENGELEN define; require the element to be exactly that length before use.