A recent bug, marked as CVE-2024-56533, was discovered and fixed in the Linux kernel ALSA (Advanced Linux Sound Architecture) subsystem, specifically in the usx2y driver. This vulnerability could lead to system hangs (soft lockups) when users unplug a USB sound card, affecting the usability and stability of Linux systems using these devices.

This article will break down the issue in simple language, provide code snippets showing the unsafe and fixed code, offer a practical view of potential exploitation, and link to key references.

What is ALSA usx2y?

ALSA is the main framework Linux uses for audio. The usx2y driver is used for some USB sound devices, allowing your computer to interface with certain external audio hardware.

The Vulnerability: What Went Wrong?

When you unplug a USB sound card, the driver code needs to clean up all the resources it used, like memory and file handles. Earlier, the code called a function named snd_card_free() immediately when a device was disconnected:

// Vulnerable approach (simplified)
static void usX2Y_usb_disconnect(struct usb_interface *intf) {
    struct snd_card *card = usb_get_intfdata(intf);
    usb_set_intfdata(intf, NULL);
    snd_card_free(card); // <-- Potential problem spot!
}

The problem is that snd_card_free() waits for all file descriptors to close. That is, if any program is still using the sound card, the function will block here until all programs close their handles. If some process hangs, or for any reason the close is delayed, this ends up blocking the USB stack. In Linux, this can freeze up the USB subsystem and even cause a "soft lockup," where part of your system stops responding but recovery may be possible without a reboot.

The Fix: Cleanup… When Safe

To fix this, kernel developers simply replaced the snd_card_free() call with snd_card_free_when_closed(). This version marks the sound card as "will be freed later" and instantly returns. The actual cleanup happens asynchronously, when all the programs are finally done using the device.

Here's the fixed code

// Fixed approach
static void usX2Y_usb_disconnect(struct usb_interface *intf) {
    struct snd_card *card = usb_get_intfdata(intf);
    usb_set_intfdata(intf, NULL);
    snd_card_free_when_closed(card); // <-- Returns right away; cleanup is safe
}

This protects the USB subsystem from getting stuck, keeps the system responsive, and does not affect audio apps: they will finish naturally, and resources will be freed in the background.

Could This Be Exploited?

While CVE-2024-56533 probably cannot be used for remote code execution, it can be abused for a local denial-of-service attack. Here’s how:

- A program opens the sound device and holds it open (e.g., cat /dev/snd/* > /dev/null in a loop).

While this handle is open, someone unplugs the USB sound card.

- With the buggy kernel, the system’s USB stack may hang, causing all USB devices to stop working until a reboot, potentially causing data loss or more severe problems.

This means a regular user or a misbehaving program could lock up critical hardware with simple steps—a serious availability flaw for desktops and servers using USB audio.

Code Snippet: DIY Exploit Demo

This simple shell script can be used on a vulnerable system to demonstrate the bug. Do NOT run on production systems!

# Open the sound device (replace with correct device for your system)
cat /dev/snd/pcmCDp > /dev/null &
CAT_PID=$!

# Now unplug your USB sound card while the above is running
# On affected kernel, this may freeze the USB subsystem

kill $CAT_PID

*If you’re patched, hardware will be responsive even after unplugging with open handles.*

Patch and References

Linux kernel commit fixing CVE-2024-56533:
- Git Commit Link

Red Hat Security Notice:
- Red Hat CVE Page

Canonical/Ubuntu Security Notice:
- Ubuntu CVE Page

You use a USB sound card supported by the usx2y driver

- Running a Linux kernel version before the fix’s release (mainline kernel commits around June 2024)
- You frequently plug/unplug USB audio while some applications may have the device open

To fix:
Update to a kernel version incorporating the fix (see your distro’s security updates).

Conclusion

CVE-2024-56533 is a straightforward but critical bug that could freeze your entire USB subsystem if a USB sound card is unplugged while still in use. The fix is simple but essential: always free resources asynchronously when hardware can disappear unexpectedly. If you use USB audio on Linux, take a moment to make sure your system is up to date.


*Content is original and exclusive. For more details or in-depth analysis, consult the Linux kernel patch or your distribution’s security advisories.*

Timeline

Published on: 12/27/2024 14:15:32 UTC
Last modified on: 05/04/2025 09:57:29 UTC