A vulnerability in the Linux kernel has been recently addressed, specifically involving the Advanced Linux Sound Architecture (ALSA) subsystem and its handling of USB disconnections. The issue, assigned the identifier CVE-2024-56531, stems from the way resources are released in the ALSA caiaq driver during USB device disconnections. This post will dive into the details of the vulnerability, the code changes made to address it, and references to the original sources.
Problem Description
When a USB device is disconnected in the Linux kernel, a callback function is triggered to release the resources associated with the device. This function is expected to be short and non-blocking. However, in the ALSA caiaq driver, the current implementation employs the snd_card_free() function during disconnections. This causes the function to wait for the close of all file descriptors in use before returning, which can lead to longer execution times. Consequently, the longer execution times can block upper-layer USB ioctls and eventually trigger a soft lockup in the system.
Solution
To resolve this issue, the recommended approach is to replace the snd_card_free() function with the snd_card_free_when_closed() function. This alternative implementation returns immediately, and resource release is handled asynchronously by the card device when the last file descriptor is closed. This change prevents the vnode from being delayed by the release of resources during disconnect operations.
Additionally, the code for disconnects and resource release should be separated. The disconnect code should be called during the USB disconnect callback, while the resource release code should be called from the card destructor.
Below is a snippet of the code changes made to address this vulnerability
//Replace snd_card_free() with snd_card_free_when_closed()
- snd_card_free(card);
+ snd_card_free_when_closed(card);
//Separate disconnect and resource release codes
+ static void snd_caiaq_disconnect(struct usb_interface *intf)
+ {
+ ...
+ snd_card_free_when_closed(card);
+ }
+ static void snd_caiaq_card_free(struct snd_card *card)
+ {
+ ...
+ snd_card_free(card);
+ }
Original References
For more information on this vulnerability and the proposed changes, please refer to the following sources:
1. Linux kernel mailing list post discussing the vulnerability
2. ALSA caiaq driver source code
3. Linux kernel source with the fixed vulnerability
4. Official CVE Record for CVE-2024-56531
Exploit Details
At the time of writing, there are no known exploits for this vulnerability. However, it is crucial to update your Linux kernel to the latest version containing the code changes to protect against potential attacks that may take advantage of this issue.
Timeline
Published on: 12/27/2024 14:15:32 UTC
Last modified on: 01/20/2025 06:22:21 UTC