CVE-2024-40966 is a vulnerability identified in the Linux kernel related to the handling of line disciplines (ldiscs) in virtual terminals (ttys). Specifically, the vulnerability arises from the "con_write()" routine, which violates the kernel's "writes cannot sleep" rule. This rule is critical because certain line disciplines rely on non-sleeping write operations to maintain kernel stability and avoid deadlocks. The issue manifests when the N_GSM line discipline is attached to a virtual console, and the gsmld_write() function calls con_write() while holding a spinlock. Since con_write() attempts to acquire the console lock and may sleep, this leads to a BUG triggered by a sleeping function being called from an invalid context, causing kernel instability or crashes. The patch introduces an option for a tty to reject a new line discipline, effectively limiting virtual terminals to only the N_TTY line discipline, which is designed to comply with the non-sleeping write rule. This fix prevents the problematic scenario where con_write() is called in a context that cannot sleep, thus avoiding kernel panics or deadlocks. The vulnerability affects specific Linux kernel versions identified by commit hashes, and no known exploits are currently reported in the wild. No CVSS score has been assigned yet, and the vulnerability was published on July 12, 2024.

For European organizations, the impact of CVE-2024-40966 primarily concerns system stability and availability rather than direct confidentiality or integrity breaches. Linux is widely used across European enterprises, government agencies, and critical infrastructure, especially in servers, embedded systems, and network devices. A kernel panic or deadlock triggered by this vulnerability could lead to unexpected system crashes or reboots, causing service interruptions. This is particularly critical for environments requiring high availability, such as financial institutions, healthcare providers, and industrial control systems. While the vulnerability does not directly enable privilege escalation or remote code execution, the resulting denial of service could be exploited by attackers to disrupt operations or as part of a multi-stage attack. Systems that utilize virtual consoles with the N_GSM line discipline or customized line disciplines are at higher risk. The absence of known exploits reduces immediate threat levels, but the potential for stability issues necessitates prompt attention to avoid operational disruptions.

European organizations should prioritize applying the official Linux kernel patches that address this vulnerability as soon as they become available. Since the fix involves limiting virtual terminals to the N_TTY line discipline and rejecting incompatible ldiscs, system administrators should audit their systems for any use of non-standard line disciplines like N_GSM on virtual consoles. If such configurations are detected, they should be reviewed and adjusted to comply with the patched kernel behavior. Additionally, organizations should implement robust kernel update policies to ensure timely deployment of security patches. For critical systems where immediate patching is not feasible, consider isolating affected systems or limiting access to reduce the risk of triggering the vulnerability. Monitoring kernel logs for BUG messages related to sleeping functions in invalid contexts can help detect attempts to exploit or inadvertently trigger the issue. Finally, testing kernel updates in staging environments before production deployment is recommended to verify compatibility and stability.