CVE-2024-56547 is a vulnerability in the Linux kernel's Read-Copy-Update (RCU) subsystem, specifically related to the No-CBs (nocb) offloading feature. The issue arises from a missed RCU barrier on deoffloading CPUs, which can lead to a WARN_ON_ONCE() kernel warning and potentially a deadlock scenario. The vulnerability manifests during the execution of rcutorture tests with specific parameters that stress the RCU forward progress and callback handling mechanisms. The root cause involves improper synchronization and callback management when CPUs are deoffloaded (taken offline) and callbacks are enqueued or processed. The kernel's rcu_barrier() function, which is designed to ensure all callbacks have been processed before proceeding, can block indefinitely due to a race condition involving callback counters and mutex locks. This results in a deadlock where the system waits for callbacks on an offline CPU that never completes processing, causing kernel stalls or hangs. The fix involves checking the rdp->nocb_cb_sleep flag before parking the kernel thread to ensure no further rcu_barrier() calls are waiting on the read-copy-update deferred processing (rdp) structure, preventing the deadlock. This vulnerability affects Linux kernel versions identified by the commit hash 1fcb932c8b5ce86219d7dedcd63659351a43291c and likely related versions implementing the nocb RCU offloading feature. While no known exploits are reported in the wild, the issue can cause system instability and denial of service due to kernel deadlocks under specific workloads involving CPU offloading and RCU callback processing.

For European organizations, this vulnerability poses a risk primarily to systems running affected Linux kernel versions with the nocb RCU offloading feature enabled. The impact includes potential system hangs or kernel panics due to deadlocks in the RCU subsystem, which is critical for kernel synchronization and performance. Such instability can lead to denial of service conditions, affecting servers, cloud infrastructure, and embedded systems relying on Linux. Organizations operating high-availability services, data centers, or critical infrastructure that depend on Linux kernel stability may experience outages or degraded performance. The vulnerability does not directly expose confidentiality or integrity breaches but impacts availability, which can disrupt business operations, especially in sectors like finance, telecommunications, manufacturing, and public services. Given the widespread use of Linux in European IT environments, particularly in enterprise servers and cloud platforms, the risk of operational disruption is significant if unpatched kernels are deployed. However, exploitation requires specific conditions involving CPU offloading and RCU torture tests, suggesting that typical workloads may be less affected, but specialized or high-load environments could trigger the issue.

To mitigate CVE-2024-56547, European organizations should: 1) Apply the official Linux kernel patches that address the missed RCU barrier on deoffloading CPUs as soon as they are available from trusted sources or Linux distribution vendors. 2) Avoid running untrusted or experimental workloads that heavily stress the RCU subsystem or involve CPU offloading until patches are applied. 3) Monitor kernel logs for WARN_ON_ONCE() messages related to rcu_nocb_rdp_deoffload or deadlock symptoms to detect potential triggering of this vulnerability. 4) In environments where kernel upgrades are challenging, consider disabling the nocb RCU offloading feature if feasible, as a temporary workaround to prevent deadlocks. 5) Implement robust system monitoring and automated recovery mechanisms to detect and remediate kernel hangs or stalls promptly. 6) Coordinate with Linux distribution maintainers to ensure timely updates and backports for affected kernel versions in enterprise distributions commonly used in Europe. 7) Conduct thorough testing of kernel updates in staging environments that replicate production workloads involving CPU offloading to validate stability before deployment.