CVE-2021-46939: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-46939 is a vulnerability in the Linux kernel's tracing subsystem that could cause system hangs during suspend/resume testing. The issue was discovered when a fix to the ring buffer recursion detection led to deadlocks in the trace_clock_global() function. This vulnerability affects various Linux kernel versions and was assigned a CVSS v3 score of 5.5 (Moderate) (Red Hat Portal).

The vulnerability occurs when trace_clock_global() attempts to take a spin lock recursively during tracing operations. The issue manifests when the function takes a spin lock and then tries to acquire it again during function graph tracer operations, resulting in a deadlock. The problem specifically appears in the ring buffer recursion detection mechanism, where a single recursion during tracing leads to the lock contention (Kernel Git).

When exploited, this vulnerability can cause a system hang during suspend/resume operations. The primary impact is on system availability, with no direct effects on confidentiality or integrity. The vulnerability has been rated with a high availability impact score, indicating its potential to cause system disruption (Red Hat Portal).

The vulnerability requires local access and low complexity to exploit. It needs low privileges but no user interaction to trigger the issue. The attack vector is local, meaning an attacker would need access to the affected system to exploit this vulnerability (Red Hat Portal).

The fix involves restructuring the trace_clock_global() code to use a trylock mechanism instead of a regular lock when updating the prev_time value. This prevents deadlocks by allowing the function to continue if it fails to acquire the lock, as simultaneous events on different CPUs don't require strict ordering. The fix has been implemented in various kernel versions through security updates (Kernel Git).