CVE-2026-68184: cdrom: fix stack out-of-bounds read in CDROMVOLCTRL

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

cdrom: fix stack out-of-bounds read in CDROMVOLCTRL

mmcioctlcdromvolume() first reads the audio control mode page into a 32-byte stack buffer with cgc->buflen set to 24. If the device reports a block descriptor, the function increases cgc->buflen to include that descriptor and reads the page again.

For CDROMVOLCTRL, the function then builds a MODE SELECT parameter list by moving cgc->buffer forward by offset - 8 bytes. This drops the block descriptor from the outgoing payload and leaves a new 8-byte mode parameter header in front of the audio control page. However, cgc->buflen is left unchanged.

With a standard 8-byte block descriptor, cgc->buffer points at buffer + 8 but cgc->buflen remains 32. cdrommodeselect() therefore asks the low level packet path to write 32 bytes from that adjusted pointer, reading 8 bytes past the end of the 32-byte stack buffer.

This is not hit by CDROMVOLREAD, and CDROMVOLCTRL only triggers it on drives that return a non-zero block descriptor length, which helps explain why it has gone unnoticed. The overread is also sent to the device as extra MODE SELECT payload, so it may not produce an obvious local failure.

Reduce cgc->buflen by the same amount as the buffer pointer adjustment so the MODE SELECT transfer covers only the intended parameter list.