CVE-2025-21889 is a vulnerability identified in the Linux kernel's perf subsystem, specifically within the perf_iterate_ctx() function. This function is responsible for traversing an RCU (Read-Copy-Update) list, a synchronization mechanism used in the kernel to allow concurrent read access to data structures without locking. The vulnerability arises because perf_iterate_ctx() lacks the necessary RCU read lock protection during its traversal. This omission leads to lockdep warnings indicating suspicious RCU usage, which can be observed when running perf probe with unshare(1) under the kernel configuration option CONFIG_PROVE_RCU_LIST=y. The root cause is that the RCU read lock protection, which was originally present, was removed in a prior commit (bd2756811766) that rewrote core context handling in the perf subsystem. The absence of rcu_read_lock()/rcu_read_unlock() pairs around the perf_iterate_ctx() call in perf_event_exec() can cause unsafe concurrent access to RCU-protected lists, potentially leading to data races or inconsistent kernel state. The fix involves reintroducing these RCU read lock protections to ensure safe traversal of the RCU list. Although this vulnerability does not have known exploits in the wild, the underlying issue relates to kernel synchronization correctness, which is critical for system stability and security. The vulnerability affects Linux kernel versions including the specified commit bd2756811766 and likely subsequent versions until patched. Since the perf subsystem is widely used for performance monitoring and tracing, this vulnerability could impact many Linux-based systems.

For European organizations, the impact of CVE-2025-21889 primarily concerns the stability and security of Linux-based infrastructure. Many enterprises, cloud providers, and service operators in Europe rely heavily on Linux servers for critical workloads. The lack of proper RCU locking in the perf subsystem could lead to kernel data corruption or race conditions, potentially causing system crashes or unpredictable behavior. While this vulnerability does not directly enable privilege escalation or remote code execution, kernel instability can result in denial of service (DoS) conditions, affecting availability of services. Organizations running performance monitoring tools or custom probes that utilize perf, especially with unshare(1) or under configurations that enable RCU list verification, are at higher risk of encountering these issues. This could disrupt monitoring capabilities or lead to kernel warnings and logs that complicate incident response. Additionally, any kernel instability in production environments can have cascading effects on business continuity, especially in sectors like finance, telecommunications, and critical infrastructure prevalent in Europe. Given the widespread use of Linux in European data centers and cloud environments, the vulnerability's impact is significant in terms of operational reliability.

European organizations should prioritize updating their Linux kernels to versions where this vulnerability is patched, ensuring that the perf subsystem includes the restored RCU read lock protections. Specifically, kernel maintainers and system administrators should apply the fix that reintroduces rcu_read_lock()/rcu_read_unlock() around the perf_iterate_ctx() call in perf_event_exec(). For environments where immediate patching is not feasible, organizations should consider disabling or limiting the use of perf probes, especially those involving unshare(1) or configurations that enable CONFIG_PROVE_RCU_LIST=y, to reduce exposure. Monitoring kernel logs for RCU-related warnings can help detect attempts to trigger the issue or identify unstable kernel states. Additionally, thorough testing of kernel updates in staging environments is recommended to ensure compatibility and stability before deployment. Organizations should also review their kernel configuration options and disable CONFIG_PROVE_RCU_LIST in production if not required, as it is primarily a debugging feature. Finally, maintaining robust backup and recovery procedures will mitigate the impact of potential kernel crashes resulting from this vulnerability.