CVE-2024-40935 is a vulnerability identified in the Linux kernel's cachefiles subsystem, specifically related to the handling of cache state transitions in ondemand mode. The issue arises when the kernel marks the cache as CACHEFILES_DEAD, an irreversible state indicating a cache I/O error. In this state, the cachefiles_daemon_write() function returns an -EIO error code, preventing the daemon from successfully passing the copen (cache open) request to the kernel. Consequently, the kernel process waiting for this copen request becomes hung, triggering a hung_task condition. This hang occurs because the daemon cannot proceed with the cache open operation, and the kernel waits indefinitely. The vulnerability is rooted in the lack of flushing all pending requests after the cache is marked as DEAD. The fix involves flushing all requests in ondemand mode immediately after setting CACHEFILES_DEAD to prevent the hung_task scenario. Additionally, the patch relies on reference counting for the requests to avoid use-after-free (UAF) issues, which were addressed in a prior patch. This vulnerability affects specific Linux kernel versions identified by the commit hash c8383054506c77b814489c09877b5db83fd4abf2. There are no known exploits in the wild as of the publication date (July 12, 2024). The vulnerability primarily impacts systems using the cachefiles feature in ondemand mode, which is used to cache data on local storage to improve performance. The issue can lead to denial of service (DoS) conditions due to hung kernel tasks waiting indefinitely, potentially affecting system stability and availability. The vulnerability does not appear to allow privilege escalation or direct data corruption but can degrade system performance and reliability.

For European organizations, the impact of CVE-2024-40935 centers on potential denial of service conditions in Linux systems utilizing the cachefiles subsystem in ondemand mode. Organizations relying on Linux servers for critical infrastructure, web services, or cloud environments may experience system hangs or degraded performance if this vulnerability is triggered. This can disrupt business operations, especially for service providers and enterprises with high availability requirements. The vulnerability does not directly compromise confidentiality or integrity but affects availability, which is critical for sectors such as finance, healthcare, telecommunications, and government services prevalent across Europe. Systems running older or unpatched Linux kernels with cachefiles enabled are at risk. Given the widespread use of Linux in European data centers and enterprise environments, the vulnerability could have a broad impact if exploited or triggered inadvertently. However, the lack of known exploits and the requirement for specific cachefiles configurations somewhat limit the immediate risk. Nonetheless, organizations should prioritize patching to maintain system stability and avoid potential service disruptions.

European organizations should implement the following specific mitigation steps: 1) Identify Linux systems running kernel versions containing the vulnerable commit (c8383054506c77b814489c09877b5db83fd4abf2) and verify if the cachefiles subsystem is enabled and operating in ondemand mode. 2) Apply the official Linux kernel patches that address CVE-2024-40935 as soon as they become available from trusted Linux distributions or kernel maintainers. 3) Temporarily disable the cachefiles feature or switch from ondemand mode to a safer mode if patching cannot be immediately performed, to prevent the hung_task condition. 4) Monitor system logs and kernel messages for signs of hung tasks or cachefiles errors indicating CACHEFILES_DEAD states. 5) Implement robust system monitoring and alerting to detect early signs of kernel hangs or degraded performance related to cachefiles. 6) For critical systems, consider kernel live patching solutions where supported to minimize downtime during patch deployment. 7) Educate system administrators about this vulnerability and the importance of maintaining up-to-date kernel versions and configurations. These measures will help prevent denial of service conditions and maintain system availability.