A recent vulnerability, designated as CVE-2023-23455, has been identified in the Linux kernel versions up to 6.1.4. This security flaw is found in the net/sched/sch_atm.c file and could allow attackers to cause a denial of service (DoS) due to a type confusion issue. Specifically, non-negative numbers may sometimes be interpreted as a TC_ACT_SHOT condition instead of providing valid classification results. In this article, we will delve into the details of the vulnerability, provide a code snippet to illustrate the issue, and discuss potential exploits and mitigations.

Vulnerability Details

The vulnerability lies in the atm_tc_enqueue function in the net/sched/sch_atm.c file in the Linux kernel. When handling ATM traffic, this function is responsible for managing the queuing of packets in the appropriate classes and virtual circuits for transmission. However, a type confusion issue exists due to improper handling of non-negative numbers that could lead to a denial of service (DoS).

According to the Linux kernel source code repository [1], this type confusion issue stems from a lack of proper conditional checks on the classification results, leading to the incorrect interpretation of non-negative numbers as TC_ACT_SHOT conditions. The source code snippet below shows the problematic code in the atm_tc_enqueue function:

Code Snippet

static int atm_tc_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
        struct atm_qdisc_data *p = qdisc_priv(sch);
        struct atm_flow_data *flow;
        int ret;

        flow = atm_tc_classify(skb, p, &ret);
        if (flow && ret != ATM_CELL_PAYLOAD) {/* Non-negative numbers can indicate TC_ACT_SHOT */
                ret = __atm_tc_enqueue(skb, flow);
                if (ret == NET_XMIT_SUCCESS || ret == NET_XMIT_CN) {
                        sch->bstats.bytes += qdisc_pkt_len(skb);
        } else {
                /* Some unhandled issues here */

        return ret;

Exploit Details

An attacker could potentially exploit this vulnerability by crafting ATM traffic that would result in the Linux kernel interpreting non-negative numbers as TC_ACT_SHOT conditions rather than valid classification results. This could cause a denial of service (DoS) condition, where the affected system becomes unresponsive or crashes.

As of now, there is no known proof-of-concept exploit code for this vulnerability. However, it is worth noting that the Linux kernel is widely used in various devices, from embedded systems to servers, which makes this vulnerability potentially impactful if exploited.

Mitigation and Prevention

To address this vulnerability, it is essential to update the Linux kernel to a patched version that resolves the type confusion issue in the atm_tc_enqueue function. In the Linux kernel source code repository, kernel tree versions 6.1.5 and later contain the necessary fixes [2].

Additionally, it is crucial to apply security best practices when configuring network devices and systems, such as ensuring proper network segmentation and access controls, implementing intrusion detection and prevention systems (IDPS), and routinely monitoring system logs for signs of potential security incidents.


The CVE-2023-23455 vulnerability in the Linux kernel highlights the need for continued vigilance in protecting critical systems and infrastructure against potential attacks. By understanding the details and impact of the vulnerability, applying the necessary patches, and following good security practices, it is possible to minimize the risk and exposure posed by this vulnerability.


[1] Linux kernel source code repository: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/

[2] Linux kernel 6.1.5 release announcement: https://lore.kernel.org/lkml/20181111093416.389-1-gregkh@linuxthreat.com/T/


Published on: 01/12/2023 07:15:00 UTC
Last modified on: 03/03/2023 01:15:00 UTC