CVE-2021-43976: Linux Kernel mwifiex USB DoS Vulnerability

CVE-2021-43976 Overview

CVE-2021-43976 is a denial of service vulnerability in the Linux kernel through version 5.15.2. The vulnerability exists in the mwifiex_usb_recv function within drivers/net/wireless/marvell/mwifiex/usb.c. An attacker with physical access who can connect a specially crafted USB device can trigger an skb_over_panic condition, causing a kernel panic and system crash.

This vulnerability affects the Marvell WiFi-Ex (mwifiex) USB wireless driver, which is commonly used in embedded systems, laptops, and networking appliances that support Marvell wireless chipsets via USB interface.

Critical Impact

Physical access exploitation allows an attacker to crash affected Linux systems by connecting a malicious USB device, causing complete denial of service and potential data loss from unexpected system termination.

Affected Products

• Linux Kernel through version 5.15.2
• Fedora 34 and 35
• Debian Linux 9.0, 10.0, and 11.0
• NetApp Cloud Backup and H-Series Storage Firmware (H300S, H500S, H700S, H300E, H500E, H700E, H410S, H410C)
• Oracle Communications Cloud Native Core (Binding Support Function 22.1.3, Network Exposure Function 22.1.1, Policy 22.2.0)

Discovery Timeline

• 2021-11-17 - CVE-2021-43976 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-43976

Vulnerability Analysis

The vulnerability resides in the USB receive path of the Marvell mwifiex wireless driver. When the driver processes incoming data from a USB wireless device, it allocates socket buffers (skb) to store the received packets. The mwifiex_usb_recv function fails to properly validate the size of incoming data against the allocated buffer capacity.

When a crafted USB device sends data that exceeds the expected buffer boundaries, the kernel's skb_over_panic protection mechanism is triggered. This function is designed to halt the system when a buffer overflow in network code is detected, preventing potential memory corruption but resulting in a complete system crash.

The attack requires physical access to the target system and the ability to connect a malicious USB device. This makes it particularly relevant for scenarios involving:

• Shared computing environments
• Unattended kiosk systems
• Physical security breaches
• Supply chain attacks involving compromised USB devices

Root Cause

The root cause is insufficient input validation in the USB data receive path. The mwifiex_usb_recv function does not adequately verify that incoming packet sizes from the USB device match the expected boundaries before copying data into the socket buffer. This allows an attacker-controlled USB device to send oversized data that overflows the allocated buffer space.

The Marvell mwifiex driver trusts the USB device to provide correctly formatted data within expected size limits. A malicious device can exploit this trust by advertising itself as a legitimate Marvell wireless adapter while sending malformed packets designed to trigger the buffer overflow condition.

Attack Vector

The attack requires physical access to connect a crafted USB device to the target system. The malicious device impersonates a Marvell wireless USB adapter and sends data packets designed to exceed buffer boundaries in the receive path.

When the victim system loads the mwifiex_usb driver and attempts to communicate with the malicious device, the oversized data triggers skb_over_panic, causing an immediate kernel panic. The attack does not require user interaction beyond the initial USB device connection, as the driver automatically processes incoming data from recognized devices.

The attack flow involves:

1. Attacker prepares a USB device with modified firmware to mimic a Marvell wireless adapter
2. Attacker gains physical access to target system
3. Malicious USB device is connected
4. Linux kernel loads mwifiex_usb driver
5. Device sends crafted oversized packets
6. mwifiex_usb_recv copies data without proper size validation
7. skb_over_panic is triggered, causing kernel panic

Detection Methods for CVE-2021-43976

Indicators of Compromise

• Unexpected system crashes or kernel panics with skb_over_panic in stack traces
• Kernel log entries showing buffer overflow conditions in mwifiex_usb driver
• Unusual USB device connection events for wireless adapters in /var/log/kern.log or dmesg
• System reboots correlated with USB device insertion events

Detection Strategies

• Monitor kernel logs for skb_over_panic messages referencing the mwifiex driver module
• Implement USB device whitelisting using udev rules to block unauthorized wireless adapters
• Deploy endpoint detection solutions that monitor USB device enumeration events
• Configure audit logging for USB device connections using the Linux audit framework

Monitoring Recommendations

• Enable enhanced kernel logging for USB subsystem events
• Monitor for repeated crash patterns that correlate with USB device insertion
• Implement SentinelOne Singularity for real-time detection of kernel-level anomalies and crash events
• Review crash dump analysis for mwifiex-related stack traces in kdump or crash reports

How to Mitigate CVE-2021-43976

Immediate Actions Required

• Update the Linux kernel to a patched version that addresses the buffer validation issue
• Disable the mwifiex_usb driver module if Marvell USB wireless adapters are not required
• Implement physical access controls to prevent unauthorized USB device connections
• Consider using USB port blockers or disabling USB ports on sensitive systems

Patch Information

The Linux kernel community has released a fix for this vulnerability. The patch adds proper size validation in the mwifiex_usb_recv function to prevent buffer overflows from malicious USB devices.

Apply the appropriate kernel update for your distribution:

• Debian: Apply updates from DSA-5092 or DSA-5096
• Fedora: Apply the kernel package update from Fedora Package Announcement
• Upstream Linux: Review the kernel commit for patch details
• Oracle Products: Refer to the Oracle Security Alert July 2022
• NetApp Products: Consult the NetApp Security Advisory

Workarounds

• Blacklist the mwifiex_usb module by adding blacklist mwifiex_usb to /etc/modprobe.d/blacklist.conf
• Physically disable or lock unused USB ports on critical systems
• Implement USBGuard to create strict device whitelisting policies
• Use USB data blockers (USB condoms) that only allow charging, not data transfer

# Disable mwifiex_usb module loading
echo "blacklist mwifiex_usb" | sudo tee /etc/modprobe.d/blacklist-mwifiex.conf
echo "install mwifiex_usb /bin/false" | sudo tee -a /etc/modprobe.d/blacklist-mwifiex.conf

# Remove module if currently loaded
sudo modprobe -r mwifiex_usb 2>/dev/null

# Update initramfs to apply blacklist on boot
sudo update-initramfs -u