CVE-2017-6874 in Linux
Summary
by MITRE
Race condition in kernel/ucount.c in the Linux kernel through 4.10.2 allows local users to cause a denial of service (use-after-free and system crash) or possibly have unspecified other impact via crafted system calls that leverage certain decrement behavior that causes incorrect interaction between put_ucounts and get_ucounts.
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Analysis
by VulDB Data Team • 09/06/2020
The vulnerability identified as CVE-2017-6874 represents a critical race condition within the Linux kernel's user accounting subsystem located in kernel/ucount.c. This flaw exists in kernel versions through 4.10.2 and affects the fundamental mechanism that tracks user resource limits and accounting. The race condition stems from improper synchronization between decrement operations that occur during user accounting updates, creating a window where concurrent access patterns can lead to inconsistent state management. The vulnerability manifests when crafted system calls are executed in specific sequences that exploit the interaction between put_ucounts and get_ucounts functions, which are responsible for managing user resource limits and accounting structures.
The technical implementation of this vulnerability exploits a fundamental flaw in the kernel's locking mechanisms and atomic operations within the user accounting system. When multiple threads or processes attempt to simultaneously modify user accounting data structures, the lack of proper synchronization creates a scenario where one operation may complete while another is in progress, leading to memory corruption. The put_ucounts function handles the decrementing of user resource counts while get_ucounts retrieves these counts for validation or reporting purposes. When these operations are not properly serialized, the decrement behavior can cause incorrect interactions that result in memory being freed while still referenced, creating a use-after-free condition. This specific interaction pattern allows for unpredictable behavior that can escalate into system crashes or more severe consequences.
The operational impact of CVE-2017-6874 extends beyond simple denial of service to potentially enable privilege escalation or system compromise. Local attackers with minimal privileges can exploit this vulnerability to cause system instability through repeated exploitation of the race condition, leading to denial of service scenarios that can affect system availability. The use-after-free condition created by this vulnerability provides a potential attack surface for more sophisticated exploitation techniques, as the corrupted memory structures may allow for arbitrary code execution under certain circumstances. The vulnerability's impact is particularly concerning because it operates at the kernel level where any compromise can lead to complete system control, making it a prime target for advanced persistent threats. The race condition's nature means that exploitation can be unreliable but potentially devastating when successful, as it can trigger system crashes or create memory corruption that persists across system operations.
Mitigation strategies for CVE-2017-6874 focus on both immediate patching and operational hardening measures. The most effective solution involves upgrading to a patched kernel version where the race condition has been resolved through proper synchronization mechanisms and atomic operations. System administrators should prioritize patch deployment across all affected systems, particularly those running kernel versions 4.10.2 or earlier. Additional operational mitigations include implementing strict process and resource monitoring to detect anomalous system behavior that may indicate exploitation attempts. The vulnerability aligns with CWE-362, which describes race conditions in concurrent programming, and can be mapped to ATT&CK technique T1068, which covers local privilege escalation through kernel vulnerabilities. Organizations should also consider implementing kernel lockdown features and restricting user privileges to minimize the potential impact of successful exploitation attempts. Regular security audits and kernel integrity checks help ensure that systems remain protected against similar vulnerabilities in the kernel's resource accounting subsystem.