The Linux kernel team has recently addressed a vulnerability in the "ALSA: us122l" module within the Linux kernel. This vulnerability could cause a soft lockup situation in certain scenarios, leading to system instability and potential denial of service (DoS) attack. This post will detail the vulnerability and how to apply the fix to mitigate its effects.

Vulnerability Details

The issue is with the USB disconnect callback process in the Advanced Linux Sound Architecture (ALSA) module. This callback is expected to carry out its operations in a short timeframe, without causing long waiting periods. However, the current implementation uses the "snd_card_free()" function, which can result in long waits as it attempts to close file descriptors (fds).

This long waiting period can block upper layer USB I/O control operations, potentially triggering a soft lockup in the system. To better understand the difference between the two functions and to see why this change resolves the vulnerability, let's take a look at the code and original references.

Original References

1. Linux Kernel Mailing List (LKML) - ALSA: us122l: fix potential soft lockup at disconnect
2. Linux Kernel Git Commit

In the original code, the vulnerable part looks like this

static void snd_us122l_disconnect(struct usb_interface *intf)
{
        ...
        snd_card_free(us122l->chip.card);
        ...
}

The above code directly calls snd_card_free(), which can lead to longer waiting times during USB disconnection.

To fix the vulnerability, the following code changes have been made

static void snd_us122l_disconnect(struct usb_interface *intf)
{
        ...
        snd_card_free_when_closed(us122l->chip.card);
        ...
}

As shown in the updated code snippet, the snd_card_free() function has been replaced with snd_card_free_when_closed(). This new function returns instantly, allowing the release of resources to be carried out asynchronously at the last close operation.

The original code also contained a loop that checked the "us122l->mmap_count" value. This check is unnecessary with the new asynchronous operation, and has been removed from the updated code.

Exploit Details

The exploit for this specific vulnerability has not been publicly disclosed at the time of writing. The main concern with this vulnerability is the potential for an attacker to cause a soft lockup in the target system, leading to instability and denial of service conditions.

Conclusion

To mitigate the CVE-2024-56532 vulnerability, it is essential for Linux kernel maintainers and users to apply the latest updates and patches addressing the issue in the "ALSA: us122l" module. By using snd_card_free_when_closed() in place of snd_card_free(), the risk of a soft lockup during USB disconnection is significantly reduced, effectively mitigating the vulnerability and ensuring system stability.

Timeline

Published on: 12/27/2024 14:15:32 UTC
Last modified on: 01/20/2025 06:22:22 UTC