CVE-2021-47461 in Linux
Summary
by MITRE • 05/22/2024
In the Linux kernel, the following vulnerability has been resolved:
userfaultfd: fix a race between writeprotect and exit_mmap()
A race is possible when a process exits, its VMAs are removed by exit_mmap() and at the same time userfaultfd_writeprotect() is called.
The race was detected by KASAN on a development kernel, but it appears to be possible on vanilla kernels as well.
Use mmget_not_zero() to prevent the race as done in other userfaultfd operations.
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Analysis
by VulDB Data Team • 09/24/2025
The vulnerability identified as CVE-2021-47461 represents a critical race condition within the Linux kernel's userfaultfd implementation that could potentially lead to system instability or security implications. This flaw manifests during the process termination phase when the kernel attempts to clean up memory mappings through exit_mmap() while simultaneously processing userfaultfd write protection operations via userfaultfd_writeprotect(). The race condition occurs because the kernel's memory management subsystem does not properly synchronize these concurrent operations, creating a window where memory mappings can become inconsistent or corrupted during the process cleanup sequence. Such a scenario could result in memory corruption that might be exploited to escalate privileges or cause denial of service conditions, making it particularly concerning for systems relying heavily on virtual memory management and userfaultfd functionality.
The technical root cause of this vulnerability stems from inadequate synchronization mechanisms within the kernel's memory management code path. When a process exits, the exit_mmap() function begins removing virtual memory areas while userfaultfd_writeprotect() may be executing concurrently, attempting to protect memory pages from being written to. The absence of proper reference counting mechanisms means that the memory management structure can be accessed or modified by both operations simultaneously, leading to unpredictable behavior. The kernel's memory management subsystem relies on mmget_not_zero() to properly reference the memory management structure, but this protective mechanism was not consistently applied in the specific code path involving the race condition between writeprotect operations and exit_mmap() cleanup. This pattern of insufficient synchronization aligns with common software engineering pitfalls described in CWE-362, which addresses race conditions in concurrent programming environments.
The operational impact of CVE-2021-47461 extends beyond simple system instability, potentially enabling attackers to exploit the memory corruption for privilege escalation or denial of service attacks. Systems that heavily utilize userfaultfd for memory management operations, such as virtualization platforms, containerized environments, or applications that require fine-grained memory control, face increased risk from this vulnerability. The vulnerability's presence in both development and vanilla kernels indicates it affects a broad range of systems, making it particularly dangerous for enterprise environments where kernel stability and security are paramount. Attackers could potentially leverage this race condition to corrupt kernel memory structures, leading to system crashes, data corruption, or in severe cases, arbitrary code execution within kernel space, which would provide complete system compromise.
Mitigation strategies for this vulnerability primarily involve applying the kernel patch that implements the proper use of mmget_not_zero() to prevent concurrent access to memory management structures during process termination. System administrators should prioritize updating their kernel versions to include the fix, which addresses the synchronization issue by ensuring proper reference counting during the race condition scenario. Additionally, monitoring for unusual process termination patterns or memory corruption indicators may help detect potential exploitation attempts. Organizations should also consider implementing kernel lockdown mechanisms and other security hardening measures to reduce the attack surface, though the primary defense remains the kernel update. The fix aligns with ATT&CK technique T1068 by addressing a kernel-level vulnerability that could be leveraged for privilege escalation, making it essential for maintaining system integrity and preventing unauthorized access to privileged kernel operations.