CVE-2026-68279: drm/dp/mst: fix OOB reads in remote DPCD/I2C sideband reply parsers

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

drm/dp/mst: fix OOB reads in remote DPCD/I2C sideband reply parsers

drmdpsidebandparseremotedpcdread() reads numbytes from the raw message and then unconditionally does:

memcpy(bytes, &raw->msg[idx], numbytes);

without checking that idx + numbytes <= raw->curlen. raw->msg[] is 256 bytes; if a malicious or misbehaving MST hub sets numbytes larger than the remaining payload, the memcpy reads past the received data into whatever follows in raw->msg[].

drmdpsidebandparseremotei2creadack() has the same flaw (noted with a / TODO check / comment since the code was introduced).

Fix both functions by using a single combined check (idx + numbytes > curlen) before each memcpy. Since numbytes is u8, it is always >= 0, so this strictly subsumes the simpler idx > curlen form and no separate step is needed.

[added missing fixes tag]