With the recent updates to the Linux kernel, a crucial vulnerability has been resolved to enhance system performance and security. This post discusses the details of the vulnerability, the solution implemented, and the implications for users.

The Vulnerability

A Linux kernel bug, assigned the identifier CVE-2021-46939, was causing the machine to hang when performing suspend and resume testing. This hang resulted from a deadlock situation in the trace_clock_global() function.

The main issue with this vulnerability is that tracing should never block. If it does, it can cause lockups in the system, which can severely impact system performance and stability.

The following code snippet demonstrates the trace_clock_global() deadlock situation

ring_buffer_lock_reserve() {\
  trace_clock_global() {\
    arch_spin_lock() {\
      queued_spin_lock_slowpath() {\
        /* lock taken */\
        (something else gets traced by function graph tracer)\
          ring_buffer_lock_reserve() {\
            trace_clock_global() {\
              arch_spin_lock() {\
                queued_spin_lock_slowpath() {\
                /* DEAD LOCK! */\

The Solution

To resolve this issue, the trace_clock_global() code has been restructured in a way that it no longer blocks. Instead of taking a lock to update the recorded "prev_time," it now just uses the "prev_time" as it is. The new implementation features a trylock for grabbing the lock and updating the "prev_time" accordingly. If the trylock fails, the function will keep trying until it successfully obtains the lock.

This approach ensures that even if two events occur almost simultaneously on different CPUs, the system will no longer deadlock, thereby maintaining the stability and performance.

Restructure trace_clock_global() {\
  Use trylock to grab the lock for updating the prev_time,\
  If(taking-lock fails) try again the next time.\

Original References

For further information on this vulnerability and its resolution, you can refer to the Linux kernel Bugzilla report.

Exploit Details

While this vulnerability is not directly exploited for malicious purposes, it could lead to system instability, which could then be leveraged by attackers in conjunction with other vulnerabilities. The restructured trace_clock_global() function offers a more stable and resilient Linux kernel for users.

Conclusion

CVE-2021-46939 posed significant risks to the Linux kernel's stability and performance, but the implemented solution effectively addresses these issues. With the new trace_clock_global() restructuring, users can now rely on a more stable and secure Linux kernel.

Timeline

Published on: 02/27/2024 19:04:05 UTC
Last modified on: 04/10/2024 19:49:03 UTC