CVE-2026-68277: drm/dp/mst: fix OOB reads on 2-byte fields in sideband reply parsers

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

drm/dp/mst: fix OOB reads on 2-byte fields in sideband reply parsers

Three sideband reply parsers read 16-bit fields as:

val = (raw->msg[idx] << 8) | (raw->msg[idx+1]);

and check bounds only after the fact. When idx == raw->curlen, raw->msg[idx+1] reads one byte past the received message data into the following struct fields (curchunklen, curchunkidx, curlen).

Affected functions: - drmdpsidebandparseenumpathresourcesack() fullpayloadbwnumber and availpayloadbwnumber fields - drmdpsidebandparseallocatepayloadack() allocatedpbn field - drmdpsidebandparsequerypayloadack() allocatedpbn field

Fix by using a single combined check (idx + 2 > curlen) before each 2-byte read. Since the check is strictly tighter than idx > curlen, no separate step is needed.

[added fixes tag]