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 resides in the usbtest_disconnect function within drivers/usb/misc/usbtest.c. When a USB test device is disconnected, the driver fails to release allocated memory, causing kernel memory to leak over time. The issue is tracked as commit identifier CID-28ebeb8db770 in the upstream Linux repository. Repeated triggering of the disconnect path can exhaust kernel memory and lead to denial-of-service conditions on affected hosts. The vulnerability is classified under [CWE-401] Missing Release of Memory After Effective Lifetime.

Critical Impact
A local attacker with access to the affected system can repeatedly trigger the leak to exhaust kernel memory and cause a denial of service on Linux hosts running vulnerable kernel versions.

Affected Products

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

Discovery Timeline

2020-06-29 - CVE-2020-15393 published to NVD
2020-07 - openSUSE publishes security advisories addressing the issue
2020-08 - Debian LTS and Ubuntu (USN-4463-1, USN-4465-1) issue patched kernels
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-15393

Vulnerability Analysis

The vulnerability is a kernel-mode memory leak in the USB test driver (drivers/usb/misc/usbtest.c). The usbtest_disconnect callback is invoked when a USB device handled by the usbtest driver is removed from the system. In vulnerable versions, this function does not free all memory allocated during device probe and use. Each connect-disconnect cycle leaks kernel heap memory.

The leak occurs in kernel space and is therefore not reclaimed when user-space processes terminate. Over time, repeated triggering reduces available kernel memory, degrading system responsiveness and potentially preventing new allocations. Because the impact is limited to availability, the issue carries no confidentiality or integrity consequences.

The upstream fix is recorded in commits 28ebeb8db770 (Torvalds tree) and 831eebad70a2 (tip tree), which add the missing deallocation logic to the disconnect path.

Root Cause

The root cause is missing cleanup in the driver disconnect routine. Resources allocated when the USB test device is attached are not released when the device detaches. This is a classic [CWE-401] pattern where the lifetime management of kernel objects is incomplete across the device lifecycle.

Attack Vector

Exploitation requires local access to the system and the ability to bind a USB device to the usbtest driver. An attacker with physical access can plug and unplug a compatible USB device, or use a programmable USB emulator, to repeatedly invoke the disconnect path. On systems supporting USB/IP or virtualized USB pass-through, a low-privileged local user with access to those interfaces may also trigger the leak. The result is gradual kernel memory exhaustion, leading to a denial of service.

No verified public exploit code is associated with CVE-2020-15393. The vulnerability mechanism is described in the Linux kernel mailing list discussion and the upstream commit referenced by the advisories.

Detection Methods for CVE-2020-15393

Indicators of Compromise

Steady, unexplained growth in kernel slab memory consumption visible in /proc/meminfo and /proc/slabinfo.
Repeated USB connect-disconnect events in dmesg or journal logs referencing the usbtest driver.
Out-of-memory (OOM) killer activity or allocation failures on hosts with prolonged uptime and USB device churn.

Detection Strategies

Inventory running kernel versions across the fleet and flag any host running Linux 4.4 through 5.7.6 without the upstream fix 28ebeb8db770.
Monitor kernel log messages for usbtest driver bind and unbind events, especially on servers and workstations where USB testing devices are not expected.
Track slab allocator growth trends over time to identify hosts exhibiting progressive leakage tied to USB activity.

Monitoring Recommendations

Centralize dmesg, auditd, and /proc/slabinfo telemetry to a SIEM or data lake for longitudinal analysis.
Alert on USB device attachments to production servers, where such events are typically anomalous.
Correlate kernel memory pressure metrics with USB subsystem events to surface exploitation patterns.

How to Mitigate CVE-2020-15393

Immediate Actions Required

Apply the kernel update from your distribution: Ubuntu USN-4463-1, USN-4465-1, USN-4483-1, USN-4485-1; Debian LTS announcements from August and October 2020; openSUSE advisories from July and August 2020.
Reboot affected hosts after package installation to load the patched kernel.
Restrict physical and logical access to USB ports on servers and sensitive endpoints.

Patch Information

The upstream fix is included in Linux kernel 5.7.7 and later. The corrective commit is 28ebeb8db77035e058a510ce9bd17c2b9a009dba in the Torvalds tree and 831eebad70a25f55b5745453ac252d4afe997187 in the tip tree. Distribution-backported kernels are available via Ubuntu USN-4463-1, Debian LTS announcements, and openSUSE security announcements.

Workarounds

Blacklist the usbtest module on systems that do not require USB testing functionality by adding blacklist usbtest to /etc/modprobe.d/.
Disable USB autosuspend or unused USB controllers where feasible to limit exposure.
Apply USB device authorization policies through udev to prevent unauthorized devices from binding to the usbtest driver.

bash

# Configuration example: blacklist the vulnerable module
echo "blacklist usbtest" | sudo tee /etc/modprobe.d/blacklist-usbtest.conf
sudo rmmod usbtest 2>/dev/null
sudo update-initramfs -u