Exploring the Fix of CVE-2021-46991: Use-After-Free Vulnerability in the Linux Kernel - i40e Client Subtask

A vulnerability labeled CVE-2021-46991 has been identified in the Linux kernel, specifically involving the i40e driver, which is responsible for managing Intel Ethernet Converged Network Adapter. This vulnerability could potentially allow a malicious user to execute unauthorized actions and compromise the system's stability and security. In this long-read post, we will explore the technical details of this vulnerability, the code fix applied, and discuss its potential impact in the world of cybersecurity.

Vulnerability Details

The vulnerability exists in the i40e network driver subsystem of the Linux kernel. The issue has been classified as a use-after-free vulnerability, a type of memory corruption bug that involves accessing a pointer after it has been freed. Accessing a freed pointer could potentially lead to data leakage, denial of service, or remote code execution.

In this specific case, it affects the i40e_client_subtask() function. The issue arises when the call to the i40e_client_del_instance function frees the pf->cinst object. However, the pf->cinst->lan_info member within the freed object is being accessed later in the code, causing the use-after-free vulnerability.

Here's a snippet of the vulnerable code from the Linux kernel, i40e_main.c

static void i40e_client_subtask(struct i40e_pf *pf)
{
	// ...
    if (remove == true) {
        spin_unlock_bh(&pf->instancelist_lock);
        i40e_client_del_instance(itr);
        spin_lock_bh(&pf->instancelist_lock);
    }
    // ...
}

The Fix

To address this vulnerability, a missing return statement has been added after the call to the i40e_client_del_instance() function. This fix ensures that the pf->cinst->lan_info member is no longer accessed after the pf->cinst object has been freed, mitigating the use-after-free issue.

Here's the fixed code snippet from i40e_main.c

static void i40e_client_subtask(struct i40e_pf *pf)
{
	// ...
    if (remove == true) {
        spin_unlock_bh(&pf->instancelist_lock);
        i40e_client_del_instance(itr);
        spin_lock_bh(&pf->instancelist_lock);
        return;
    }
    // ...
}

Original References

- The official CVE record for this vulnerability is available at https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-46991.
- The Linux kernel commit implementing the fix can be found at https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6a708e29d7911af15dbcc574eacea795d52cd3a3.

Exploit Details

At the time of writing this post, there are no known public exploits or proof-of-concept codes detailing how to take advantage of this vulnerability. However, given its nature, attackers might develop exploits targeting this vulnerability to cause denial of service, leak sensitive information, or execute unauthorized code on the affected systems.

Conclusion

The discovery and resolution of a use-after-free vulnerability, such as CVE-2021-46991, are essential in maintaining the security, stability, and integrity of a system. It is crucial to keep software up-to-date and regularly monitor security bulletins for new vulnerabilities and fixes to defend against potential threats.

Timeline

Published on: 02/28/2024 09:15:37 UTC
Last modified on: 12/06/2024 15:27:56 UTC