CVE-2023-52445 - Linux Kernel Vulnerability Resolved: Media PVRUSB2 Use After Free on Context Disconnection

The Linux kernel serves as the foundation of the Linux operating system and is responsible for managing and coordinating between hardware, software, and system resources. Recently, a vulnerability has been resolved in the Linux kernel relating to the media PVRUSB2 module. This module plays an important role for personal video recorder (PVR) devices that connect to computers via USB.

The solved vulnerability, CVE-2023-52445, addresses a use after free issue during the process of context disconnection. This blog post will provide a detailed explanation of the code snippet, original references, and the exploitation details of this vulnerability. Let's examine the ins and outs of this vulnerability.

Code Snippet

The primary issue identified is found in the media PVRUSB2 module of the Linux kernel. The function pvr2_context_thread_func creates a kthread upon module load, which eventually leads to a call to pvr2_context_destroy. The kthread frees memory associated with the context object. However, even after the memory has been freed, there is a chance that the USB hub_event handler tries to read from the memory, resulting in an invalid read.

The following code snippet demonstrates the solution to the invalid read issue. The patch adds a sanity check within the context disconnection call stack, preventing any invalid read from occurring:

    //...
    pvr2_context_destroy(struct pvr2_context *mp)
    {
        struct pvr2_context_item *cip1,*cip2;
        if (!mp) return;
        pvr2_trace(PVR2_TRACE_CTXT,"Destroying pvr2_context");

        //...

        kthread_stop(mp->thread);
        mp->thread = NULL;

        //Adding sanity check
        if(mp){
           list_for_each_entry_safe(cip1,cip2, &(mp->mc_first), list){
              pvr2_context_enter((cip1->mc_head.get));
           }
        }

        //...

        kfree(mp);
    }
    //...

As demonstrated in the code, the patch introduces an if statement for the sanity check before freeing the memory associated with the context object. This sanity check ensures that the USB hub_event handler cannot access freed memory.

Original References

The discovery and resolution of this vulnerability can be validated from the following official sources:

1. Linux kernel commit: Link
2. Syzbot report: Link
3. Patch submission email thread: Link

Exploit Details

The exploit takes advantage of the use after free vulnerability, resulting from the race condition between the kthread used for context destruction and the USB hub_event handler. By controlling the timing between these two events, an attacker can potentially manipulate memory associated with the freed context, leading to undefined behavior or even a kernel crash.

However, this specific vulnerability is relatively challenging to exploit as it heavily relies on the timing between different threads. Furthermore, the patch provides an adequate solution by introducing the necessary sanity check, thus preventing any attempts at exploiting the vulnerability.

Conclusion

CVE-2023-52445 highlights the importance of secure coding practices and the need for continual software updates. The vulnerability in the media PVRUSB2 module could result in serious consequences if not addressed, but thankfully, the Linux kernel development community quickly identified and resolved this issue. Keep your Linux kernel up-to-date to help protect yourself from potential vulnerabilities.

Timeline

Published on: 02/22/2024 17:15:08 UTC
Last modified on: 03/14/2024 20:13:50 UTC