CVE-2021-31440: Linux Kernel eBPF Privilege Escalation

CVE-2021-31440 Overview

CVE-2021-31440 is a local privilege escalation vulnerability in the Linux Kernel 5.11.15 eBPF subsystem. The flaw stems from improper validation of user-supplied extended Berkeley Packet Filter (eBPF) programs prior to execution. A local attacker with the ability to run low-privileged code can leverage this weakness to execute arbitrary code in the context of the kernel. The issue was reported through Trend Micro's Zero Day Initiative as ZDI-CAN-13661 and is tracked under CWE-682: Incorrect Calculation. NetApp shipped multiple firmware and software products containing the affected kernel code, expanding the impact beyond mainline Linux distributions.

Critical Impact

Successful exploitation grants kernel-level code execution, allowing full system compromise from an unprivileged local user account.

Affected Products

• Linux Kernel 5.11.15
• NetApp SolidFire Baseboard Management Controller firmware
• NetApp H-Series (H300S, H410S, H500S, H700S, H300E, H500E, H700E) firmware and Cloud Backup

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

The vulnerability resides in the eBPF verifier component of the Linux kernel. eBPF allows user-space programs to load bytecode that executes inside the kernel for purposes such as packet filtering, tracing, and observability. Before execution, the verifier must prove that submitted programs are memory-safe and cannot harm the kernel.

In kernel 5.11.15, the verifier performs incorrect calculations when tracking bounds of 32-bit values. The mismatch between the verifier's tracked register state and the actual runtime state allows crafted programs to bypass safety checks. An attacker can use this gap to read and write arbitrary kernel memory, achieving kernel-mode arbitrary code execution from an unprivileged context.

Root Cause

The root cause is an incorrect calculation [CWE-682] in the eBPF verifier's handling of register bounds. The verifier fails to properly propagate 32-bit signed and unsigned bounds during program analysis. As a result, the in-kernel JIT compiler emits code based on a register state that does not reflect runtime reality, enabling out-of-bounds memory access.

Attack Vector

Exploitation requires local code execution with low privileges and the ability to invoke the bpf() syscall. The attacker submits a malicious eBPF program designed to defeat the verifier's bounds tracking. Once loaded, the program manipulates kernel pointers to read or overwrite arbitrary kernel memory, typically targeting credential structures to elevate the calling process to root. On systems where unprivileged eBPF is disabled via kernel.unprivileged_bpf_disabled, the attack surface is significantly reduced. See the Zero Day Initiative Advisory ZDI-21-503 for additional technical context.

Detection Methods for CVE-2021-31440

Indicators of Compromise

• Unexpected bpf() syscall invocations from non-administrative user accounts or service contexts that do not normally use eBPF.
• Loaded eBPF programs of type BPF_PROG_TYPE_SOCKET_FILTER or similar originating from unusual processes.
• Unprivileged processes suddenly executing with uid=0 or capability sets inconsistent with their parent.

Detection Strategies

• Audit bpf() syscall usage with auditd rules to capture program loads, including the calling UID and program type.
• Monitor kernel logs for verifier rejections and unusual JIT activity that may indicate exploitation attempts.
• Correlate process credential changes with prior eBPF program loads to identify privilege escalation chains.

Monitoring Recommendations

• Centralize host telemetry, including syscall and process lineage data, into a SIEM for cross-host correlation.
• Alert on any process performing a setuid-equivalent transition without an authorized parent such as sudo or su.
• Track the value of /proc/sys/kernel/unprivileged_bpf_disabled across the fleet to detect configuration drift.

How to Mitigate CVE-2021-31440

Immediate Actions Required

• Apply the upstream Linux kernel fix from commit 10bf4e83167c or update to a kernel version that includes it.
• For NetApp deployments, review and apply guidance in NetApp Security Advisory NTAP-20210706-0003.
• Inventory all Linux hosts running kernel 5.11.x and prioritize patching systems with shell access for non-administrative users.

Patch Information

The upstream fix is committed to the Linux kernel tree at commit 10bf4e83167cc68595b85fd73bb91e8f2c086e36. The patch corrects the eBPF verifier's handling of 32-bit bounds tracking so that register state during verification matches runtime behavior. Distribution maintainers backported the fix into supported stable kernel branches. NetApp issued advisory NTAP-20210706-0003 covering its affected SolidFire BMC, Cloud Backup, and H-Series firmware products.

Workarounds

• Disable unprivileged eBPF by setting kernel.unprivileged_bpf_disabled=1 to prevent non-root users from loading eBPF programs.
• Restrict access to the bpf() syscall using seccomp profiles in container runtimes and systemd units.
• Limit CAP_BPF and CAP_SYS_ADMIN capabilities to a minimal set of trusted administrative processes.

# Configuration example
# Disable unprivileged eBPF program loading
sudo sysctl -w kernel.unprivileged_bpf_disabled=1
echo 'kernel.unprivileged_bpf_disabled=1' | sudo tee -a /etc/sysctl.d/90-disable-unpriv-bpf.conf

# Verify the setting
sysctl kernel.unprivileged_bpf_disabled