A critical security vulnerability, identified as CVE-2023-39180, has been discovered in the handling of SMB2_READ commands within the kernel ksmbd module of Linux. This flaw can potentially lead to a denial-of-service (DoS) attack, negatively impacting the affected systems; however, exploitation is only possible on systems that have ksmbd enabled. The good news is that authentication is not required to exploit this vulnerability, and in this article, we'll take an in-depth look at the underlying issue, the code snippet responsible, and ways to mitigate this risk.

Exploit Details

The main issue in CVE-2023-39180 stems from the failure of proper memory management. Specifically, the ksmbd module isn't releasing memory once it has outlived its purpose, resulting in a steady growth in memory consumption over time. This issue affects the SMB2 server, as the ksmbd module incorrectly handles SMB2_READ commands.

Here is a code snippet in the ksmbd module where the issue arises

int ksmbd_smb2_read(struct ksmbd_work *work)
{
    // [...]
    unsigned int count;
    // [...]
    count = le32_to_cpu(req->Length);
    if (count > ksmbd_session_rpc_method(work->sess))
    {
        // Issue: memory allocated but not released.
        char *buf = kmalloc(count, GFP_KERNEL);
        ksmbd_debug(SMB, "failed in allocating memory\n");
        rsp->hdr.Status = NT_STATUS_INSUFF_SERVER_RESOURCES;
        rsp->StructureSize = cpu_to_le16(9);
    }
    // [...]
}

In the above example, when the count is higher than the value returned by ksmbd_session_rpc_method(work->sess), the module allocates memory with kmalloc(). However, it fails to free up the memory afterward, leaving it to remain occupied and consequently leading to a memory leak.

The following sources provide further context and details regarding this vulnerability

1. Official CVE Entry: CVE-2023-39180
2. National Vulnerability Database (NVD) Entry: CVE-2023-39180
3. Linux Kernel Mailing List: ksmbd issue discussion

Mitigations and Recommendations

As a Linux user, you're advised to safeguard your system with proper security measures. Here are some recommendations to follow:

1. To begin with, ensure that your Linux distribution is up-to-date. Developers frequently release patches to address known vulnerabilities, and staying updated is key to securing your system.

2. Unless absolutely necessary, disable the ksmbd module on your Linux system to minimize the attack surface and the probability of successful exploitation.

3. Monitor resources usage on your system, specifically memory consumption. This can help you proactively identify potential memory leaks and take necessary actions before a full-blown DoS attack occurs.

4. Finally, as good security practice, restrict network access to only those users and devices that absolutely require it. This will reduce potential attack vectors and minimize the risks of your system being compromised.

Conclusion

The CVE-2023-39180 vulnerability presents a potential risk for Linux users with ksmbd enabled. By improperly handling SMB2_READ commands and leaking memory, attackers can exploit this issue to cause a DoS attack on affected systems. Therefore, it's essential for users to stay updated on the latest patches and follow security best practices to protect their systems from such vulnerabilities.

Timeline

Published on: 11/18/2024 10:15:05 UTC
Last modified on: 11/18/2024 17:11:17 UTC