CVE-2026-31401: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-31401 Overview

A buffer overflow vulnerability has been identified in the Linux kernel's HID (Human Interface Device) BPF subsystem. The vulnerability exists in the hid_hw_request function where the return value from dispatch_hid_bpf_raw_requests() is improperly validated. This function calls struct_ops and the returned value can be arbitrarily large, leading to potential buffer overflow conditions when processing HID-BPF requests.

Critical Impact

Attackers with local access could potentially exploit this buffer overflow to corrupt memory, cause denial of service, or potentially achieve code execution within kernel context.

Affected Products

• Linux Kernel (HID-BPF subsystem)
• Linux systems utilizing HID-BPF functionality

Discovery Timeline

• April 3, 2026 - CVE-2026-31401 published to NVD
• April 7, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31401

Vulnerability Analysis

The vulnerability resides in the HID-BPF interface of the Linux kernel. When processing raw HID requests through the BPF (Berkeley Packet Filter) extension mechanism, the hid_hw_request function fails to properly validate the return value from dispatch_hid_bpf_raw_requests(). This function invokes struct_ops callbacks, which are user-provided BPF programs that can return arbitrary values without any bounds checking.

The core issue is that the kernel code assumes the returned value is always valid and within expected bounds. However, a malicious or buggy BPF program could return an excessively large value, which when used for buffer operations, can lead to out-of-bounds memory access.

Root Cause

The root cause is improper input validation of the return value from BPF struct_ops callbacks. The dispatch_hid_bpf_raw_requests() function returns values that originate from user-provided BPF programs, but these values are used without verification that they fall within safe operational bounds. This violates the principle of never trusting user-controlled data, even when that data flows through kernel BPF infrastructure.

Attack Vector

An attacker with the ability to load BPF programs on the target system could craft a malicious BPF program that returns an oversized value when handling HID requests. When this value is subsequently used in buffer operations, it could cause:

1. Buffer overflow: Writing beyond allocated buffer boundaries
2. Memory corruption: Overwriting adjacent kernel memory structures
3. Denial of service: Causing kernel panics or system instability
4. Potential privilege escalation: If memory corruption can be controlled precisely

The vulnerability requires local access and BPF program loading capabilities, which may be restricted on hardened systems but available on development or misconfigured systems.

Detection Methods for CVE-2026-31401

Indicators of Compromise

• Unexpected kernel panics or oops messages referencing HID or BPF subsystems
• Suspicious BPF program loading activity targeting HID device handlers
• Memory corruption errors in kernel logs related to HID drivers

Detection Strategies

• Monitor for BPF program attachments to HID-related hooks using bpftool or similar utilities
• Enable kernel auditing for BPF syscalls and CAP_BPF capability usage
• Deploy kernel integrity monitoring to detect unexpected memory modifications
• Review system logs for HID subsystem anomalies or crashes

Monitoring Recommendations

• Enable enhanced kernel logging for the HID subsystem
• Implement runtime monitoring for BPF program loading and execution patterns
• Configure alerts for CAP_BPF capability requests from unexpected processes
• Monitor for repeated HID subsystem errors that may indicate exploitation attempts

How to Mitigate CVE-2026-31401

Immediate Actions Required

• Update to a patched Linux kernel version containing the fix commits
• Restrict BPF program loading capabilities using kernel.unprivileged_bpf_disabled sysctl
• Audit existing BPF programs attached to HID subsystem hooks
• Consider disabling HID-BPF functionality if not required

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fix adds proper validation of return values from dispatch_hid_bpf_raw_requests() to prevent buffer overflow conditions. Multiple stable kernel branches have received patches:

• Linux Kernel Commit 2b658c1
• Linux Kernel Commit 73c5b5a
• Linux Kernel Commit d6efaa5
• Linux Kernel Commit eb57dae

Administrators should update their kernel packages from distribution repositories or compile a patched kernel from source.

Workarounds

• Disable unprivileged BPF by setting kernel.unprivileged_bpf_disabled=2 in sysctl configuration
• Restrict CAP_BPF capability to trusted administrators only
• Blacklist HID-BPF modules if the functionality is not required for system operation
• Implement mandatory access control policies (SELinux/AppArmor) to restrict BPF program loading

# Configuration example
# Disable unprivileged BPF access system-wide
echo "kernel.unprivileged_bpf_disabled=2" >> /etc/sysctl.d/99-disable-bpf.conf
sysctl -p /etc/sysctl.d/99-disable-bpf.conf

# Verify the setting is applied
sysctl kernel.unprivileged_bpf_disabled