CVE-2021-46921 is a concurrency vulnerability in the Linux kernel's locking mechanism, specifically within the queued_write_lock_slowpath() function of the queued read-write lock (qrwlock) implementation. The flaw arises due to improper ordering of atomic operations during lock acquisition. While the writer thread holds a wait_lock and attempts to acquire the write lock, it uses atomic_cond_read_acquire() followed by an atomic compare-and-exchange (cmpxchg) operation. However, the cmpxchg operation lacks proper acquire semantics, which can cause a speculative read by a concurrent reader thread to observe stale or inconsistent data before the write lock is fully acquired. This leads to an A-B-A problem where the reader sees intermediate states that should be protected by the lock. The vulnerability is exemplified in the epoll subsystem, where a reader performing an atomic exchange (xchg) under a read lock can observe changes that the writer has not yet fully committed, potentially causing data races or inconsistent state observations. The fix involves switching the cmpxchg to use acquire semantics and relaxing the atomic_cond_read to use relaxed semantics, ensuring proper memory ordering and eliminating the speculative read window. This vulnerability affects Linux kernel versions identified by the commit hash b519b56e378ee82caf9b079b04f5db87dedc3251 and similar versions prior to the patch. No known exploits are reported in the wild as of the publication date (February 27, 2024).

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions, especially those using epoll for event notification in multi-threaded or multi-core environments. Potential impacts include data corruption, race conditions, or inconsistent application behavior due to improper synchronization. While the vulnerability does not directly enable privilege escalation or remote code execution, it undermines the integrity of kernel-level synchronization primitives, which could be leveraged in complex attack chains or cause system instability. Critical infrastructure, cloud service providers, and enterprises relying on Linux-based servers or embedded systems could experience degraded reliability or subtle data integrity issues. Given the widespread use of Linux in European data centers, telecommunications, and industrial control systems, the vulnerability could affect a broad range of sectors. However, the absence of known exploits and the requirement for specific kernel versions and conditions limit immediate exploitation risk. Nonetheless, unpatched systems remain vulnerable to potential future exploit development.

European organizations should prioritize updating Linux kernel versions to those containing the patch that enforces acquire semantics on the cmpxchg operation within queued_write_lock_slowpath(). Specifically, kernel upgrades incorporating the fix identified by commit b519b56e378ee82caf9b079b04f5db87dedc3251 or later should be deployed promptly. For environments where immediate patching is not feasible, organizations should audit and monitor applications relying heavily on epoll and concurrent locking mechanisms for anomalous behavior or data inconsistencies. Employ kernel live patching solutions where available to minimize downtime. Additionally, rigorous testing of multi-threaded applications on updated kernels can help detect latent issues caused by the vulnerability. Security teams should maintain awareness of any emerging exploit reports and coordinate with Linux distribution vendors for timely security advisories. Finally, implementing strict change management and kernel version control policies will reduce exposure to unpatched vulnerable kernels.