CVE-2020-15393: Linux Kernel Memory Leak Vulnerability

CVE-2020-15393 Overview

CVE-2020-15393 is a memory leak vulnerability in the Linux kernel affecting versions 4.4 through 5.7.6. The flaw exists in the usbtest_disconnect function within drivers/usb/misc/usbtest.c, where improper resource deallocation leads to memory not being freed when a USB test device is disconnected. This vulnerability can be exploited by a local attacker to cause denial of service through memory exhaustion.

Critical Impact

Local attackers with low privileges can trigger repeated memory leaks by connecting and disconnecting USB test devices, eventually exhausting system memory and causing system instability or denial of service.

Affected Products

• Linux Kernel versions 4.4 through 5.7.6
• Debian Linux 9.0
• openSUSE Leap 15.1 and 15.2
• Ubuntu Linux 14.04 ESM, 16.04 ESM, 18.04 LTS, and 20.04 LTS

Discovery Timeline

• June 29, 2020 - CVE-2020-15393 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-15393

Vulnerability Analysis

This vulnerability represents a classic memory leak scenario (CWE-401: Missing Release of Memory after Effective Lifetime) in kernel-level USB driver code. The usbtest_disconnect function fails to properly release all allocated memory when a USB test device is disconnected from the system. Each disconnect event leaves orphaned memory allocations that cannot be reclaimed by the kernel's memory management subsystem.

The impact is primarily availability-focused, as repeated exploitation can lead to gradual memory exhaustion. While the vulnerability requires local access and low-level privileges, it poses a significant risk to system stability, particularly in environments where USB devices are frequently connected and disconnected or where the usbtest driver is actively used for development and testing purposes.

Root Cause

The root cause lies in the incomplete cleanup logic within the usbtest_disconnect function. When a USB test device is disconnected, the driver fails to free all dynamically allocated memory structures associated with the device context. The specific commit identifier CID-28ebeb8db770 tracks this issue in the Linux kernel source tree. The fix ensures that all allocated resources are properly released during the device disconnection handler, preventing memory from being leaked.

Attack Vector

The attack vector requires local access to the system. An attacker with low privileges can exploit this vulnerability by repeatedly triggering USB device connect/disconnect cycles involving the usbtest driver. This can be accomplished through:

The vulnerability is triggered when the usbtest_disconnect function executes during USB device removal. The function's incomplete resource cleanup allows memory allocations to persist beyond their intended lifetime. An attacker can automate this process to rapidly consume available system memory, leading to degraded performance or complete system unavailability.

Since no user interaction is required beyond the initial local access, and the attack complexity is low, this vulnerability can be reliably exploited on affected systems where the usbtest driver is loaded.

Detection Methods for CVE-2020-15393

Indicators of Compromise

• Gradual increase in kernel memory usage without corresponding application activity
• Growing count of unreleased memory allocations tracked in /proc/meminfo or via kernel memory debugging tools
• System log entries indicating memory pressure or out-of-memory conditions on systems with USB test activity
• Unusual USB device connect/disconnect patterns in kernel logs (dmesg output)

Detection Strategies

• Monitor kernel memory statistics for anomalous growth patterns using tools like vmstat, free, or custom monitoring scripts
• Enable kernel memory leak detection tools such as kmemleak during development or testing to identify specific leak sources
• Audit loaded kernel modules to determine if the vulnerable usbtest driver is active on production systems
• Implement USB device monitoring to detect rapid or unusual connect/disconnect cycles

Monitoring Recommendations

• Deploy SentinelOne agents configured to monitor kernel-level resource consumption and system stability metrics
• Configure alerts for memory utilization thresholds that may indicate active exploitation or memory leak conditions
• Review system logs regularly for USB-related errors or warnings that may correlate with memory issues
• Establish baseline memory consumption metrics to facilitate detection of gradual memory leaks

How to Mitigate CVE-2020-15393

Immediate Actions Required

• Update to a patched Linux kernel version that includes the fix (commit 28ebeb8db77035e058a510ce9bd17c2b9a009dba)
• Apply distribution-specific security updates from Debian, Ubuntu, or openSUSE as available
• If patching is not immediately possible, consider unloading the usbtest kernel module on production systems where it is not required
• Restrict physical access to USB ports on critical systems to limit attack surface

Patch Information

The vulnerability has been addressed in the upstream Linux kernel. The fix is tracked by commit ID 28ebeb8db77035e058a510ce9bd17c2b9a009dba in the Torvalds Linux tree and 831eebad70a25f55b5745453ac252d4afe997187 in the tip tree. Administrators should apply patches through their distribution's package management system:

• Debian: See Debian LTS Security Announcement
• Ubuntu: Apply updates from Ubuntu Security Notice #4463-1, USN-4465-1, USN-4483-1, or USN-4485-1
• openSUSE: Refer to openSUSE Security Announcement

Workarounds

• Unload the usbtest kernel module if not required for operations: sudo modprobe -r usbtest
• Add the usbtest module to the kernel module blacklist to prevent automatic loading
• Implement USB device whitelisting policies to restrict which devices can connect to the system
• Monitor memory usage and implement automated system restarts if memory pressure becomes critical

# Configuration example - Disable usbtest module
# Check if usbtest module is loaded
lsmod | grep usbtest

# Unload the usbtest module if present
sudo modprobe -r usbtest

# Blacklist the module to prevent automatic loading
echo "blacklist usbtest" | sudo tee /etc/modprobe.d/blacklist-usbtest.conf

# Update initramfs to apply changes
sudo update-initramfs -u