CVE-2021-31440: Linux Kernel eBPF Privilege Escalation

CVE-2021-31440 Overview

CVE-2021-31440 is a privilege escalation vulnerability in the Linux Kernel's extended Berkeley Packet Filter (eBPF) subsystem. This vulnerability allows local attackers to escalate privileges on affected installations of Linux Kernel 5.11.15. An attacker must first obtain the ability to execute low-privileged code on the target system in order to exploit this vulnerability. The specific flaw exists within the handling of eBPF programs, where the issue results from the lack of proper validation of user-supplied eBPF programs prior to executing them. Successful exploitation enables an attacker to escalate privileges and execute arbitrary code in the context of the kernel.

Critical Impact

Local attackers with low-privileged access can leverage this eBPF validation flaw to achieve kernel-level code execution, resulting in complete system compromise.

Affected Products

• Linux Kernel (versions prior to and including 5.11.15)
• NetApp SolidFire Baseboard Management Controller Firmware
• NetApp Cloud Backup
• NetApp H500S, H700S, H300E, H500E, H700E, H410S, H300S (and associated firmware)

Discovery Timeline

• 2021-05-21 - CVE-2021-31440 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-31440

Vulnerability Analysis

This vulnerability is classified under CWE-682 (Incorrect Calculation), indicating a computational error in the eBPF verifier logic. The eBPF subsystem in the Linux kernel is designed to allow safe execution of sandboxed programs within the kernel context. However, the verifier component responsible for validating eBPF programs before execution contains a flaw that allows improperly crafted programs to bypass security checks.

The vulnerability requires local access and low privileges to exploit, but successful exploitation results in complete compromise of confidentiality, integrity, and availability at the kernel level. The complexity of exploitation is considered high due to the technical requirements of crafting a malicious eBPF program that can bypass the verifier.

Root Cause

The root cause of CVE-2021-31440 lies in an incorrect calculation within the eBPF verifier. The verifier is responsible for ensuring that eBPF programs cannot perform unsafe operations such as out-of-bounds memory access or privilege escalation. Due to a computational error in the verification logic, certain malicious eBPF programs can pass the safety checks despite containing operations that should be rejected. This allows an attacker to load and execute a crafted eBPF program that performs unauthorized operations with kernel privileges.

Attack Vector

The attack vector for CVE-2021-31440 is local, requiring the attacker to have already obtained low-privileged access to the target system. The exploitation process involves:

1. The attacker crafts a malicious eBPF program designed to exploit the verification bypass
2. The crafted program is submitted to the kernel's eBPF subsystem for loading
3. Due to the incorrect calculation in the verifier, the malicious program passes safety checks
4. Once loaded, the eBPF program executes within the kernel context
5. The attacker leverages this execution to escalate privileges to root/kernel level

The attack does not require user interaction and can be executed by any user capable of loading eBPF programs, which by default may include unprivileged users on some system configurations.

Detection Methods for CVE-2021-31440

Indicators of Compromise

• Unusual eBPF program loading activity, particularly from non-administrative users
• Unexpected privilege escalation events or new root-level processes
• Kernel log entries indicating eBPF verifier anomalies or warnings
• Evidence of exploitation tools targeting eBPF vulnerabilities on the system

Detection Strategies

• Monitor bpf() system calls for anomalous patterns, especially from low-privileged users
• Implement kernel auditing to track eBPF program loading events via auditd rules
• Deploy endpoint detection solutions capable of monitoring kernel-level activities
• Review system logs for signs of privilege escalation following eBPF-related activity

Monitoring Recommendations

• Configure Linux audit rules to log all bpf() syscall invocations: -a always,exit -F arch=b64 -S bpf -k ebpf_activity
• Monitor /proc/sys/kernel/unprivileged_bpf_disabled for unauthorized modifications
• Implement file integrity monitoring on critical kernel modules and system binaries
• Utilize SentinelOne's behavioral AI to detect privilege escalation patterns associated with kernel exploits

How to Mitigate CVE-2021-31440

Immediate Actions Required

• Update the Linux kernel to a patched version that includes commit 10bf4e83167cc68595b85fd73bb91e8f2c086e36
• Disable unprivileged eBPF by setting kernel.unprivileged_bpf_disabled=1
• Restrict access to systems where immediate patching is not possible
• Review system access to identify and limit users with capabilities to load eBPF programs

Patch Information

The vulnerability has been addressed in the upstream Linux kernel. The fix is available via the Linux Kernel Commit. Organizations using NetApp products should consult the NetApp Security Advisory for vendor-specific guidance. Additional technical details are available in the Zero Day Initiative Advisory.

Workarounds

• Disable unprivileged eBPF access by setting the kernel parameter kernel.unprivileged_bpf_disabled=1 at runtime or persistently via sysctl configuration
• Implement strict access controls to limit which users can interact with the eBPF subsystem
• Use SELinux or AppArmor policies to restrict eBPF program loading capabilities
• Consider removing or blacklisting unnecessary kernel modules related to eBPF if not required for operations

# Configuration example
# Disable unprivileged BPF access immediately
echo 1 > /proc/sys/kernel/unprivileged_bpf_disabled

# Make the change persistent across reboots
echo "kernel.unprivileged_bpf_disabled = 1" >> /etc/sysctl.d/99-disable-unprivileged-bpf.conf
sysctl --system

# Verify the setting is applied
sysctl kernel.unprivileged_bpf_disabled