CVE-2024-35929 is a vulnerability identified in the Linux kernel related to the Read-Copy-Update (RCU) subsystem, specifically within the RCU No-CB (no callback) mechanism. The issue arises in kernels configured with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and CONFIG_RCU_LAZY=y, which enable lazy RCU callback processing on all CPUs without dedicated callback threads. The vulnerability manifests as WARN_ON_ONCE() warnings triggered in the functions rcu_nocb_bypass_lock() and rcu_nocb_wait_contended() under certain concurrent execution scenarios. These warnings indicate potential race conditions or improper lock handling when the nocb_bypass_lock is contended and the CPU executing the code does not match the expected CPU context. The problem can be reproduced by executing "echo 3 > /proc/sys/vm/drop_caches", which triggers cache dropping and RCU lazy shrink scans. The root cause is improper use of rcu_nocb_flush_bypass() in lazy_rcu_shrink_scan(), which has been corrected by replacing it with rcu_nocb_try_flush_bypass() to safely handle bypass queue flushing without causing WARN_ON_ONCE() conditions. Although the vulnerability primarily causes kernel warnings, it indicates underlying concurrency and synchronization issues in the RCU subsystem that could potentially lead to kernel instability or deadlocks under specific workloads. No known exploits are reported in the wild, and the vulnerability affects Linux kernel versions built with the specified configuration flags. This vulnerability is technical and low-level, impacting kernel internal synchronization mechanisms rather than user-facing features.

For European organizations, the impact of CVE-2024-35929 depends on their use of Linux systems with kernels configured with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and CONFIG_RCU_LAZY=y. Such configurations are more common in specialized or performance-tuned environments rather than default distributions. The vulnerability could lead to kernel warnings and potentially unstable behavior or deadlocks in systems under heavy memory management operations, such as cache dropping or slab shrinking. This may cause service interruptions, degraded performance, or system crashes in critical infrastructure, cloud services, or data centers relying on affected Linux kernels. While no direct exploitation or privilege escalation is known, the underlying concurrency flaw could be leveraged in complex attack chains or cause denial of service conditions. European organizations operating high-availability Linux servers, especially in sectors like finance, telecommunications, or government, could face operational risks if unpatched. However, typical desktop or standard server deployments with default kernel configurations are less likely to be affected. The absence of known exploits reduces immediate risk, but the vulnerability highlights the need for vigilance in kernel updates and configuration management.

European organizations should take the following specific mitigation steps: 1) Identify Linux systems running kernels with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and CONFIG_RCU_LAZY=y enabled, focusing on performance-tuned or custom kernels. 2) Apply the latest Linux kernel patches that address CVE-2024-35929, ensuring the replacement of rcu_nocb_flush_bypass() with rcu_nocb_try_flush_bypass() in lazy_rcu_shrink_scan(). 3) For systems where immediate patching is not feasible, avoid triggering conditions such as "echo 3 > /proc/sys/vm/drop_caches" in production environments to reduce the risk of encountering the WARN_ON_ONCE() condition. 4) Monitor kernel logs for WARN_ON_ONCE() messages related to rcu_nocb_bypass_lock or rcu_nocb_wait_contended to detect potential occurrences of the issue. 5) Review and test kernel configurations to assess the necessity of CONFIG_RCU_NOCB_CPU_DEFAULT_ALL and CONFIG_RCU_LAZY options, disabling them if not required to reduce attack surface. 6) Incorporate kernel updates into regular patch management cycles and validate stability post-update in staging environments before production deployment. 7) Engage with Linux distribution vendors or maintainers to ensure timely receipt of patches and advisories related to this vulnerability.