CVE-2023-53021 is a high-severity use-after-free vulnerability in the Linux kernel's network scheduler component, specifically within the taprio (time-aware priority) queuing discipline (qdisc). The flaw arises when an invalid TCA_RATE attribute is provided during the installation of a taprio qdisc. In this scenario, the qdisc_create() function attempts to destroy the just-initialized taprio qdisc by calling taprio_destroy(). However, due to the high-resolution timer (hrtimer) used by taprio having already fired, the advance_sched() function invokes __netif_schedule(), which schedules network transmission actions. This leads to net_tx_action() attempting to access a qdisc that has already been destroyed, resulting in a use-after-free condition. The kernel memory sanitizer (KMSAN) detected uninitialized value bugs in spinlock-related functions during this process, indicating memory safety violations. Exploiting this vulnerability could allow an attacker with local privileges and limited user interaction (no UI required) to cause a kernel crash or potentially execute arbitrary code with kernel privileges, impacting confidentiality, integrity, and availability. The vulnerability affects Linux kernel versions around 6.0.0-rc2 and likely other versions using the affected taprio qdisc implementation. The issue was reported by syzbot and fixed by ensuring that the system waits until one CPU services the qdisc before proceeding, preventing use-after-free. The CVSS v3.1 score is 7.8, reflecting high impact and moderate attack complexity requiring local privileges.

For European organizations, this vulnerability poses a significant risk, particularly for those running Linux-based infrastructure, including servers, network appliances, and embedded systems that utilize the taprio qdisc for traffic scheduling. Exploitation could lead to denial of service through kernel crashes, disrupting critical services and network operations. More severe exploitation might allow privilege escalation to kernel level, compromising system confidentiality and integrity, potentially leading to data breaches or persistent backdoors. Organizations in sectors such as telecommunications, cloud service providers, financial services, and critical infrastructure, which rely heavily on Linux networking stacks, could face operational disruptions and security breaches. The requirement for local privileges limits remote exploitation but insider threats or compromised user accounts could leverage this vulnerability. Additionally, the complexity of the vulnerability means that automated exploitation is less likely, but targeted attacks remain a concern.

European organizations should prioritize updating their Linux kernels to versions that include the patch for CVE-2023-53021 as soon as it becomes available. Until patches are applied, organizations should restrict local access to trusted users only and monitor for unusual kernel crashes or system instability that could indicate exploitation attempts. Employing kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR), Control Flow Integrity (CFI), and enabling kernel lockdown modes can reduce exploitation risk. Network administrators should audit the use of taprio qdisc configurations and avoid deploying invalid or malformed TCA_RATE attributes. Additionally, implementing strict access controls and monitoring on systems that handle network scheduling can help detect and prevent misuse. Regularly reviewing kernel logs and using advanced endpoint detection tools capable of identifying kernel-level anomalies will aid in early detection of exploitation attempts.