CVE-2021-46939 is a medium severity vulnerability in the Linux kernel related to the tracing subsystem, specifically the trace_clock_global() function. The vulnerability arises from a flawed fix to ring buffer recursion detection that inadvertently introduced a deadlock condition during suspend/resume operations. The root cause is that trace_clock_global() attempts to acquire a spinlock twice recursively, which leads to a hang or system deadlock. This occurs because the tracing infrastructure uses spinlocks to protect shared data structures, and the recursion causes the same lock to be acquired twice on the same CPU, which is not allowed and results in a deadlock. The vulnerability is triggered during kernel tracing activities, particularly when function graph tracing is enabled and the system undergoes suspend/resume cycles. The fix restructures trace_clock_global() to avoid blocking by replacing the blocking spinlock with a trylock mechanism. If the trylock fails, the update to the recorded "prev_time" is skipped and retried later, which is acceptable since the exact order of updates from different CPUs is not critical. This approach prevents the deadlock by ensuring tracing never blocks, maintaining system stability during tracing operations. The vulnerability affects specific Linux kernel versions identified by commit hashes, and no known exploits are reported in the wild. The CVSS 3.1 score is 5.5 (medium), reflecting a local attack vector requiring low privileges and no user interaction, with impact limited to availability (system hangs) but no confidentiality or integrity loss. The issue is categorized under CWE-400 (Uncontrolled Resource Consumption), as the deadlock effectively causes a denial of service by hanging the system during tracing.

For European organizations, the impact of CVE-2021-46939 primarily concerns systems running vulnerable Linux kernel versions with tracing enabled, especially in environments where suspend/resume cycles are common, such as laptops, embedded devices, or servers with power management features. The vulnerability can cause system hangs or denial of service, potentially disrupting critical services or operations. While it does not compromise confidentiality or integrity, availability loss can affect business continuity, particularly in sectors relying on Linux-based infrastructure for production, industrial control, or cloud services. Organizations using tracing for debugging or performance monitoring may be more exposed. The impact is more pronounced in environments with limited ability to quickly reboot or recover from hangs, such as remote or embedded systems. Given the widespread use of Linux across European enterprises, including government, finance, manufacturing, and telecommunications, the vulnerability poses a moderate risk that requires timely patching to avoid operational disruptions.

1. Apply the official Linux kernel patches that restructure trace_clock_global() to use trylock instead of blocking spinlocks. Ensure all systems are updated to kernel versions containing the fix. 2. Disable kernel tracing features, especially function graph tracing, on production systems where tracing is not essential, to reduce exposure. 3. For systems requiring tracing, implement monitoring to detect hangs or deadlocks during suspend/resume cycles and automate recovery procedures. 4. Test suspend/resume functionality thoroughly after patching to confirm the deadlock is resolved. 5. In environments using custom or older kernels, backport the fix or upgrade to a supported kernel version. 6. Limit local user privileges to prevent unauthorized tracing or kernel debugging activities that could trigger the vulnerability. 7. Maintain an inventory of Linux kernel versions in use across the organization to identify vulnerable systems promptly.