CVE-2021-20194 in Linux
Summary
by MITRE • 02/24/2021
There is a vulnerability in the linux kernel versions higher than 5.2 (if kernel compiled with config params CONFIG_BPF_SYSCALL=y , CONFIG_BPF=y , CONFIG_CGROUPS=y , CONFIG_CGROUP_BPF=y , CONFIG_HARDENED_USERCOPY not set, and BPF hook to getsockopt is registered). As result of BPF execution, the local user can trigger bug in __cgroup_bpf_run_filter_getsockopt() function that can lead to heap overflow (because of non-hardened usercopy). The impact of attack could be deny of service or possibly privileges escalation.
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Analysis
by VulDB Data Team • 08/09/2025
The vulnerability identified as CVE-2021-20194 represents a critical security flaw within the Linux kernel that emerges specifically in versions exceeding 5.2. This weakness manifests when the kernel is compiled with particular configuration parameters including CONFIG_BPF_SYSCALL=y, CONFIG_BPF=y, CONFIG_CGROUPS=y, CONFIG_CGROUP_BPF=y, and when CONFIG_HARDENED_USERCOPY is not enabled. The vulnerability operates through the BPF (Berkeley Packet Filter) subsystem which provides a powerful mechanism for filtering network packets and other kernel events. The flaw resides in the __cgroup_bpf_run_filter_getsockopt() function where a heap overflow condition can be triggered by a local user through malicious BPF hook registration to the getsockopt system call. This vulnerability directly maps to CWE-121, heap-based buffer overflow, and aligns with ATT&CK technique T1068 which involves local privilege escalation through kernel exploits. The attack vector requires a local user with the ability to register BPF hooks, typically achievable through existing user privileges or by leveraging other initial compromise vectors that grant access to the system.
The technical exploitation of this vulnerability occurs when a malicious BPF program is registered to intercept getsockopt calls within a cgroup context. When the kernel processes these intercepted calls, the __cgroup_bpf_run_filter_getsockopt() function fails to properly validate or bound-check data copied from user space to kernel space, creating a scenario where user-supplied data can overwrite adjacent heap memory regions. The absence of CONFIG_HARDENED_USERCOPY in the kernel configuration removes crucial protections against such usercopy operations, allowing for potentially arbitrary memory corruption. This heap overflow condition can result in kernel memory corruption that manifests as either a denial of service through system crashes or more critically, privilege escalation that could allow a local attacker to gain root privileges. The vulnerability is particularly concerning because it leverages the legitimate BPF subsystem functionality while exploiting a specific interaction between BPF hooks, cgroup management, and memory copy operations within the kernel's networking stack.
The operational impact of CVE-2021-20194 extends beyond simple system instability to potentially enable full system compromise. A local attacker who can register BPF hooks can craft malicious programs that trigger the heap overflow, leading to unpredictable kernel behavior. In a denial of service scenario, the system may crash or become unresponsive, causing service interruption that affects availability. However, the more severe implications involve privilege escalation where successful exploitation could allow attackers to execute code with kernel-level privileges, potentially enabling full system compromise. The vulnerability's exploitation requires specific kernel configuration parameters to be present, making it less likely to affect systems running with hardened configurations, but it remains a significant threat to systems that have not implemented proper kernel hardening measures. Organizations should consider the broader implications of this vulnerability in their security posture, particularly those running kernel versions affected by this issue.
Mitigation strategies for CVE-2021-20194 focus on both immediate remediation and long-term hardening measures. The primary solution involves updating to kernel versions that contain the appropriate patches, typically those released after the vulnerability disclosure. System administrators should also consider disabling or restricting BPF functionality where it is not strictly required for operations, particularly by ensuring CONFIG_BPF_SYSCALL is not enabled in kernel configurations. The implementation of CONFIG_HARDENED_USERCOPY becomes critical as it provides essential protections against usercopy-based buffer overflows that are fundamental to this vulnerability. Additional defensive measures include implementing strict BPF program validation and monitoring for unauthorized BPF hook registration activities, as well as conducting regular kernel configuration reviews to ensure hardened security settings are maintained. Organizations should also consider implementing runtime protections and monitoring systems that can detect anomalous BPF activity patterns that might indicate exploitation attempts. The vulnerability underscores the importance of maintaining current kernel versions and implementing comprehensive kernel hardening practices as recommended by security frameworks and industry best practices.