CVE-2021-38204: Linux Kernel MAX-3421 DoS Vulnerability

CVE-2021-38204 Overview

CVE-2021-38204 is a use-after-free vulnerability in the Linux kernel's MAX-3421 USB host controller driver (drivers/usb/host/max3421-hcd.c). The vulnerability exists in Linux kernel versions prior to 5.13.6 and allows physically proximate attackers to cause a denial of service condition, resulting in system panic, by removing a MAX-3421 USB device in certain situations.

Critical Impact

Physical attackers can trigger a kernel panic and system crash by exploiting memory corruption during USB device removal, potentially leading to data loss and system unavailability.

Affected Products

• Linux Kernel versions before 5.13.6
• Debian Linux 9.0
• Systems using the MAX-3421 USB host controller driver

Discovery Timeline

• 2021-08-08 - CVE-2021-38204 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-38204

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue that occurs when the MAX-3421 USB host controller driver accesses memory that has already been freed. The flaw resides in the max3421-hcd.c driver, which manages the MAX-3421 USB peripheral/host controller IC.

When a MAX-3421 USB device is physically removed during active operations, the driver fails to properly handle the deallocation sequence, leading to a condition where freed memory structures are still referenced. This results in memory corruption that triggers a kernel panic, causing complete system denial of service.

Root Cause

The root cause lies in improper memory management within the MAX-3421 host controller driver. Specifically, the driver maintains references to device structures (loaded_dev) and endpoint numbers (loaded_epnum) that can become stale after the associated memory is freed during device removal. The vulnerable code tracked which device was "loaded into the chip" but failed to properly invalidate these references when the device was disconnected.

Attack Vector

The attack requires physical access to the target system. An attacker must:

1. Have physical proximity to a system with a MAX-3421 USB controller
2. Insert a MAX-3421 USB device to initialize the driver structures
3. Remove the device at a specific timing window during active operations
4. The abrupt removal triggers the use-after-free condition, causing kernel memory corruption and system panic

The physical access requirement limits the attack surface, but the vulnerability can be exploited reliably in environments where physical security is not strictly enforced.

// Security patch removing vulnerable device tracking variables
// Source: https://github.com/torvalds/linux/commit/b5fdf5c6e6bee35837e160c00ac89327bdad031b

 	*/
	struct urb *curr_urb;
	enum scheduling_pass sched_pass;
-	struct usb_device *loaded_dev;	/* dev that's loaded into the chip */
-	int loaded_epnum;		/* epnum whose toggles are loaded */
	int urb_done;			/* > 0 -> no errors, < 0: errno */
	size_t curr_len;
	u8 hien;

The fix removes the loaded_dev and loaded_epnum tracking variables that were susceptible to corruption after device removal, preventing the use-after-free condition.

Detection Methods for CVE-2021-38204

Indicators of Compromise

• Unexpected kernel panic messages referencing max3421-hcd or USB subsystem
• System crashes occurring after USB device removal events
• Kernel oops messages indicating use-after-free in USB host controller code
• Repeated system instability when using MAX-3421 USB controllers

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for panic messages related to max3421-hcd.c
• Implement kernel crash dump analysis to identify use-after-free patterns in USB driver code
• Deploy endpoint detection solutions that alert on unusual kernel crash patterns
• Use static analysis tools to identify systems running vulnerable kernel versions

Monitoring Recommendations

• Enable kernel crash reporting and analyze dumps for USB subsystem memory corruption
• Monitor for repeated system reboots that may indicate exploitation attempts
• Implement physical security monitoring for USB device insertion/removal events
• Track kernel version across infrastructure to identify unpatched systems

How to Mitigate CVE-2021-38204

Immediate Actions Required

• Update Linux kernel to version 5.13.6 or later to apply the security fix
• Review and apply vendor-specific kernel updates from distributions like Debian
• Restrict physical access to systems using MAX-3421 USB controllers
• Consider disabling the max3421-hcd driver module if not required for operations

Patch Information

The vulnerability is addressed in Linux kernel version 5.13.6. The fix is documented in the Linux Kernel ChangeLog 5.13.6 and the specific commit is available at GitHub Linux Commit b5fdf5c. Debian users should refer to the Debian LTS Security Announcement for distribution-specific updates.

Workarounds

• Blacklist the max3421_hcd kernel module if the hardware is not in use
• Implement strict physical access controls to prevent unauthorized USB device manipulation
• Use USB port blockers or disable unused USB ports at the BIOS/UEFI level
• Deploy endpoint security solutions to monitor for unusual kernel activity

# Configuration example - Blacklist the vulnerable driver if not needed
echo "blacklist max3421_hcd" | sudo tee /etc/modprobe.d/blacklist-max3421.conf
sudo update-initramfs -u