In recent Linux kernel security updates, a critical vulnerability was discovered with the identifier CVE-2021-3600. This vulnerability affects the eBPF (extended Berkeley Packet Filter) implementation in the Linux kernel, and a successful exploit could result in the execution of arbitrary code. In this post, we will discuss what this vulnerability entails, how it occurs, and the potential impact on affected systems.

To understand the context of this vulnerability, it's important to first understand what eBPF is and how it works. eBPF is a Linux kernel feature that allows for the safe execution of custom user-provided programs in the kernel space. These programs can be used for various purposes like tracing, monitoring, filtering, and networking.

CVE-2021-3600 Description

The vulnerability (CVE-2021-3600) is related to the eBPF implementation in the Linux kernel, specifically how it handles bounds tracking for 32-bit registers during "div" (division) and "mod" (modulo) operations.

When performing these operations, the kernel fails to properly track the bounds, which means that a local attacker could craft and execute a malicious eBPF program, manipulating 32-bit register bounds and potentially gaining the ability to execute arbitrary code.

Here's an example code snippet to illustrate the problem

// eBPF code example
#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>

int example(struct xdp_md *ctx) {
    int64_t a = x100000000;
    int64_t b = x100000000;
    int64_t res;
    // Unsafe division operation with 32-bit bounds
    res = a / b;
    // Do something with the result
    return XDP_PASS;

char _license[] SEC("license") = "GPL";

In this example, the res variable could be incorrectly calculated due to the improper bounds tracking, leading to unexpected behavior.

Original References

- Official CVE entry:
- Linux kernel patch details:
- Upstream fix:

Exploit Details

At this time, there are no known public exploits for CVE-2021-3600. However, as the details of the vulnerability are publicly available, a motivated attacker could potentially develop a working exploit based on this information. It's crucial for Linux kernel developers and systems administrators to apply the necessary patches to mitigate the risk associated with this vulnerability.

Mitigation and Patching

The Linux kernel developers have introduced a patch that resolves the issue by correctly tracking 32-bit register bounds when performing the "div" and "mod" operations. The upstream fix can be found at the following GitHub commit: 493196ffce5c.

To mitigate the vulnerability, it's advised to keep the Linux kernel updated to the latest version that includes this patch. For system administrators who cannot update the kernel immediately, it's recommended to monitor and restrict access to eBPF functionality on affected systems as a precautionary measure.


CVE-2021-3600 is a critical vulnerability that could potentially allow an attacker to execute arbitrary code. It highlights the importance of proper bounds tracking in eBPF programs and also serves as a reminder to keep operating systems updated with the latest security patches. Although there aren't any known public exploits available, it's essential to apply the necessary patches and take preventive measures to mitigate the risk posed by this vulnerability.


Published on: 01/08/2024 19:15:08 UTC
Last modified on: 01/08/2024 19:30:10 UTC