CVE-2021-3489 in Linux
Summary
by MITRE • 06/04/2021
The eBPF RINGBUF bpf_ringbuf_reserve() function in the Linux kernel did not check that the allocated size was smaller than the ringbuf size, allowing an attacker to perform out-of-bounds writes within the kernel and therefore, arbitrary code execution. This issue was fixed via commit 4b81ccebaeee ("bpf, ringbuf: Deny reserve of buffers larger than ringbuf") (v5.13-rc4) and backported to the stable kernels in v5.12.4, v5.11.21, and v5.10.37. It was introduced via 457f44363a88 ("bpf: Implement BPF ring buffer and verifier support for it") (v5.8-rc1).
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Analysis
by VulDB Data Team • 06/07/2021
The vulnerability identified as CVE-2021-3489 resides within the Linux kernel's eBPF (extended Berkeley Packet Filter) implementation, specifically within the RINGBUF functionality that enables efficient data transfer between user-space and kernel-space. This flaw manifests in the bpf_ringbuf_reserve() function which serves as a critical component for managing ring buffer allocations in the kernel's BPF subsystem. The vulnerability represents a classic buffer overflow condition that occurs when memory management checks are insufficiently enforced, creating a path for privilege escalation and arbitrary code execution within the kernel space. The issue was introduced in kernel version 5.8-rc1 with the initial implementation of BPF ring buffer support and remained unpatched until the release of kernel versions 5.10.37, 5.11.21, and 5.12.4, with the primary fix appearing in commit 4b81ccebaeef.
The technical flaw stems from a missing validation check within the ring buffer allocation mechanism that fails to verify whether the requested buffer size exceeds the maximum capacity of the ring buffer itself. When an attacker successfully exploits this vulnerability through a crafted eBPF program, the bpf_ringbuf_reserve() function allocates memory without proper bounds checking, allowing subsequent write operations to overflow the allocated buffer boundaries. This out-of-bounds write capability enables attackers to overwrite adjacent kernel memory regions, potentially corrupting critical data structures or executing arbitrary code with kernel privileges. The vulnerability directly maps to CWE-121, which describes heap-based buffer overflow conditions, and more specifically to CWE-787, representing out-of-bounds write vulnerabilities. The flaw operates at the kernel level and can be exploited through eBPF programs, making it particularly dangerous as it bypasses traditional user-space security controls and operates within the trusted kernel environment.
The operational impact of CVE-2021-3489 extends beyond simple privilege escalation, as it provides attackers with a pathway to achieve complete system compromise. Once an attacker gains kernel-level execution privileges through this vulnerability, they can manipulate system memory, escalate privileges further, access sensitive data, modify system configurations, or even install persistent backdoors. The vulnerability affects systems running Linux kernel versions prior to 5.10.37, 5.11.21, or 5.12.4, encompassing a broad range of enterprise and cloud environments that rely on BPF functionality for network monitoring, security enforcement, and system performance tracking. Organizations using eBPF-based security tools such as Falco, Cilium, or other network security solutions are particularly at risk, as these systems often leverage the ring buffer functionality for efficient data collection and analysis. The attack surface is further expanded due to the widespread adoption of BPF features in modern Linux distributions and containerized environments where kernel security is paramount.
Mitigation strategies for CVE-2021-3489 require immediate kernel updates to the patched versions mentioned in the advisory, specifically versions 5.10.37, 5.11.21, and 5.12.4, or the application of the relevant security patch from commit 4b81ccebaeef. Organizations should also implement additional security measures including eBPF program validation, monitoring for suspicious BPF activity, and restricting BPF program loading where possible. The fix addresses the root cause by introducing proper size validation within the bpf_ringbuf_reserve() function, ensuring that any allocation request exceeding the ring buffer's maximum capacity is properly rejected. System administrators should conduct thorough vulnerability assessments to identify systems running affected kernel versions and prioritize patching activities accordingly. Given the nature of the vulnerability and its potential for system compromise, organizations should also consider implementing runtime protection mechanisms and monitoring for unusual kernel memory access patterns that might indicate exploitation attempts. The vulnerability demonstrates the critical importance of proper input validation in kernel space and highlights the need for comprehensive security reviews of system-level memory management functions that are exposed to potentially untrusted user inputs through eBPF programs.