CVE-2019-19922 in Linux
Summary
by MITRE
kernel/sched/fair.c in the Linux kernel before 5.3.9, when cpu.cfs_quota_us is used (e.g., with Kubernetes), allows attackers to cause a denial of service against non-cpu-bound applications by generating a workload that triggers unwanted slice expiration, aka CID-de53fd7aedb1. (In other words, although this slice expiration would typically be seen with benign workloads, it is possible that an attacker could calculate how many stray requests are required to force an entire Kubernetes cluster into a low-performance state caused by slice expiration, and ensure that a DDoS attack sent that number of stray requests. An attack does not affect the stability of the kernel; it only causes mismanagement of application execution.)
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Analysis
by VulDB Data Team • 10/15/2024
The vulnerability described in CVE-2019-19922 represents a critical issue within the Linux kernel's Completely Fair Scheduler (CFS) implementation that specifically affects systems utilizing CPU bandwidth control mechanisms. This flaw exists in kernel/sched/fair.c and manifests when the cpu.cfs_quota_us parameter is configured, which is commonly used in container orchestration platforms like Kubernetes to enforce CPU resource limits. The vulnerability allows attackers to manipulate the scheduler's time slice expiration behavior in a manner that can severely degrade system performance without causing kernel instability or crashes.
The technical mechanism behind this vulnerability stems from how the CFS scheduler handles time slice expiration when CPU bandwidth quotas are enforced. When cpu.cfs_quota_us is set, the scheduler allocates time slices to processes based on their assigned CPU shares, but the implementation contains a flaw in how it calculates when slice expiration occurs. Attackers can exploit this by creating specific workload patterns that cause the scheduler to prematurely expire time slices, particularly affecting non-cpu-bound applications that rely on predictable execution patterns. This occurs because the scheduler's accounting mechanism fails to properly handle certain edge cases in workload distribution, leading to cascading effects where legitimate processes are starved of CPU time.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially compromise entire containerized environments, particularly those running Kubernetes clusters. An attacker who can craft the appropriate workload patterns can force the entire cluster into a state of significantly reduced performance where legitimate applications experience severe delays and reduced throughput. The vulnerability is particularly dangerous in cloud environments where multiple tenants share the same infrastructure, as a single malicious workload can impact the performance of all applications running on the affected nodes. This type of attack does not crash the kernel or cause system instability, but rather creates a subtle but effective denial of service condition that can be difficult to detect and isolate.
The attack vector specifically targets the interaction between container orchestration systems and the Linux kernel's CPU scheduling mechanisms, making it particularly relevant to cloud-native environments. According to CWE classification, this vulnerability maps to CWE-20: Improper Input Validation, as the scheduler fails to properly validate or account for certain workload patterns that could cause unintended behavior. From an ATT&CK framework perspective, this represents a privilege escalation and denial of service technique that could be classified under T1499.004: Endpoint Denial of Service, where an attacker leverages system-level resources to degrade performance. The vulnerability demonstrates how seemingly benign scheduling algorithms can be exploited when integrated with containerization technologies, highlighting the importance of proper resource accounting in virtualized environments.
Mitigation strategies for this vulnerability require immediate kernel updates to versions 5.3.9 or later, where the scheduler's slice expiration logic has been corrected. Organizations should also implement proper resource limits and monitoring for CPU usage patterns to detect anomalous behavior that might indicate exploitation attempts. Additionally, Kubernetes administrators should review their CPU quota configurations and consider implementing more granular resource controls to limit the impact of potentially malicious workloads. The fix addresses the root cause by improving the scheduler's handling of time slice expiration when bandwidth quotas are active, ensuring that legitimate applications receive appropriate CPU allocation without being affected by artificial workload patterns designed to trigger the vulnerability.