CVE-2020-8835 in Linux
Summary
by MITRE
In the Linux kernel 5.5.0 and newer, the bpf verifier (kernel/bpf/verifier.c) did not properly restrict the register bounds for 32-bit operations, leading to out-of-bounds reads and writes in kernel memory. The vulnerability also affects the Linux 5.4 stable series, starting with v5.4.7, as the introducing commit was backported to that branch. This vulnerability was fixed in 5.6.1, 5.5.14, and 5.4.29. (issue is aka ZDI-CAN-10780)
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Analysis
by VulDB Data Team • 08/17/2025
The vulnerability identified as CVE-2020-8835 represents a critical flaw in the Linux kernel's eBPF (extended Berkeley Packet Filter) verifier component that affects versions 5.5.0 and newer, including the 5.4 stable series from v5.4.7 onwards. This issue stems from improper register bounds checking during 32-bit operations processing within the kernel's BPF subsystem, creating a pathway for privilege escalation and arbitrary code execution. The bpf verifier serves as a crucial security mechanism that validates BPF programs before execution to prevent malicious code from compromising system integrity, making this flaw particularly dangerous as it undermines the fundamental security assumptions of the kernel's sandboxing capabilities.
The technical implementation flaw occurs in kernel/bpf/verifier.c where the verifier fails to properly constrain register values during 32-bit arithmetic operations, allowing for register overflow conditions that can result in out-of-bounds memory access patterns. This vulnerability specifically targets the register state management during BPF program validation, where 32-bit operations do not adequately account for potential overflow conditions that could extend beyond the intended memory boundaries. The flaw enables attackers to craft malicious BPF programs that manipulate register states in ways that bypass normal memory access controls, potentially allowing for read and write operations to kernel memory regions that should remain protected from user-space access.
The operational impact of this vulnerability is severe and multifaceted, as it provides attackers with the capability to achieve kernel-level privilege escalation without requiring local access or specific user privileges. An attacker could leverage this vulnerability to execute arbitrary code with kernel-level privileges, potentially leading to complete system compromise, data exfiltration, or persistent backdoor installation. The vulnerability affects systems running affected kernel versions where BPF functionality is enabled, which is common across modern Linux distributions and cloud environments where eBPF is used for network filtering, system monitoring, and security enforcement. The backport of the introducing commit to the 5.4 stable series means that organizations maintaining older kernel versions are also at risk, extending the attack surface significantly.
Mitigation strategies for CVE-2020-8835 primarily involve applying the appropriate kernel security patches released in versions 5.6.1, 5.5.14, and 5.4.29, which contain the necessary fixes to properly enforce register bounds checking during 32-bit operations. System administrators should prioritize patching affected systems, particularly in production environments where kernel-level vulnerabilities can be exploited by remote attackers. Additional protective measures include disabling BPF functionality if not required, implementing strict kernel module loading policies, and monitoring for unusual BPF program execution patterns that might indicate exploitation attempts. This vulnerability aligns with CWE-129, which addresses improper validation of array indices, and maps to ATT&CK technique T1059.007 for kernel module loading and T1068 for privilege escalation through kernel vulnerabilities. Organizations should also consider implementing kernel hardening measures such as KASLR (Kernel Address Space Layout Randomization) and SMEP (Supervisor Mode Execution Prevention) to further reduce the attack surface and limit the impact of potential exploitation attempts.