CVE-2026-74500: ALSA: usb-audio: fix stack info leak in RME Digiface status

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

ALSA: usb-audio: fix stack info leak in RME Digiface status

sndrmedigifacereadstatus() reads a four-word status block from the device into an uninitialised on-stack le32 buf[4] and, whenever the vendor control-IN transfer does not return a negative error, copies all four words into the caller's status[].

sndusbctlmsg() copies the full requested size back into the caller's buffer regardless of how many bytes the data stage actually delivered:

buf = kmemdup(data, size, GFPKERNEL); err = usbcontrolmsg(dev, pipe, request, requesttype, value, index, buf, size, timeout); memcpy(data, buf, size);

usbcontrolmsg() returns the transferred length on a short control-IN, which is a non-negative value, and writes only that many bytes. The remainder of the copy back is the kmemdup()ed image of the caller's buffer, so a device answering with a short data stage leaves the trailing words of buf[] holding leftover kernel stack. The only guard in the caller is err < 0, so those words are stored into status[].

They then reach user space: sndrmedigifacegetstatusval() selects a 16-bit halfword of status[] per the control's reg/mask, and the eight Digiface status controls together expose the whole 16-byte frame to an unprivileged reader of /dev/snd/controlC.

Zero-initialise the buffer so a short read yields zeros instead of stack residue. This mirrors sndrmegetstatus1(), which already clears its output word before the same kind of vendor read.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>