CVE-2018-20784 in Linux
Summary
by MITRE
In the Linux kernel before 4.20.2, kernel/sched/fair.c mishandles leaf cfs_rq's, which allows attackers to cause a denial of service (infinite loop in update_blocked_averages) or possibly have unspecified other impact by inducing a high load.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 07/19/2023
The vulnerability identified as CVE-2018-20784 represents a critical flaw in the Linux kernel's Completely Fair Scheduler implementation that affects versions prior to 4.20.2. This issue resides within the kernel/sched/fair.c source file and specifically targets the handling of leaf cfs_rq structures which are fundamental components of the kernel's process scheduling mechanism. The flaw manifests when the scheduler encounters certain conditions involving leaf cfs_rq structures, creating a scenario where the update_blocked_averages function becomes trapped in an infinite loop, effectively rendering the system unresponsive.
The technical nature of this vulnerability stems from improper handling of the relationship between leaf cfs_rq structures and their parent entities within the scheduler's hierarchical tree structure. When the system experiences high load conditions, particularly those involving complex scheduling scenarios with multiple processes competing for CPU resources, the scheduler fails to properly maintain the integrity of the cfs_rq linked lists. This breakdown in state management causes the update_blocked_averages function to repeatedly traverse the same data structures without proper termination conditions, leading to the infinite loop behavior that constitutes the primary denial of service vector.
The operational impact of this vulnerability extends beyond simple system unresponsiveness to potentially encompass broader system stability issues that could affect critical infrastructure operations. Under high load conditions, an attacker could exploit this vulnerability to cause sustained denial of service attacks against systems running affected kernel versions, potentially leading to complete system lockup or requiring manual intervention to restore normal operation. The unspecified other impacts mentioned in the CVE description suggest that the infinite loop condition might also potentially lead to memory corruption or other unpredictable behaviors that could be leveraged for more sophisticated attacks.
This vulnerability aligns with CWE-835, which specifically addresses infinite loops in software systems, and demonstrates how improper state management in kernel-level code can create persistent denial of service conditions. The attack vector requires an attacker to induce high load conditions that trigger the specific scheduling scenario where the cfs_rq handling fails, making this a challenging vulnerability to exploit in controlled environments but potentially devastating when successfully triggered. The ATT&CK framework categorizes this under privilege escalation and denial of service techniques, as the vulnerability can be exploited to gain control over system resources and potentially disrupt service availability.
The recommended mitigation strategy involves upgrading to Linux kernel version 4.20.2 or later, where the scheduler's handling of leaf cfs_rq structures has been corrected to prevent the infinite loop condition. System administrators should prioritize this update, particularly for production systems running high load workloads where the vulnerability could be exploited. Additionally, monitoring systems for unusual CPU load patterns or scheduler-related performance degradation could help identify potential exploitation attempts, though the vulnerability's nature makes detection challenging once the infinite loop has been initiated. Organizations should also consider implementing process isolation and resource limiting mechanisms as additional defensive measures to reduce the impact of such vulnerabilities on overall system availability.