CVE-2026-68369: Linux Kernel USB Gadget Printer DoS Flaw

CVE-2026-68369 Overview

CVE-2026-68369 is a denial-of-service vulnerability in the Linux kernel USB gadget printer driver. The flaw resides in the printer_read() function, which reuses a single variable to track both the requested copy size and the number of bytes actually copied to user space. When copy_to_user() fails to write any bytes to the target buffer, the driver enters an infinite loop because the read state variables remain unchanged across iterations. The issue has been resolved upstream by tracking the copied length as a separate variable and returning -EFAULT when no progress is made.

Critical Impact
A local user with access to the USB gadget printer device can trigger an infinite kernel loop by supplying an unwritable user buffer while receive (RX) data is pending, resulting in CPU exhaustion and denial of service.

Affected Products

Linux kernel USB gadget printer driver (drivers/usb/gadget/function/f_printer.c)
Linux kernel stable branches referenced in the upstream fix commits
Systems configured with USB gadget printer functionality enabled

Discovery Timeline

2026-08-10 - CVE-2026-68369 published to NVD
2026-08-10 - Last updated in NVD database

Technical Details for CVE-2026-68369

Vulnerability Analysis

The vulnerability is an infinite loop condition [CWE-835] in the Linux kernel USB gadget printer driver. The printer_read() function is responsible for delivering received print data from the USB gadget to user space. The function relies on a single variable to represent both the number of bytes the caller requested and the number of bytes successfully transferred by copy_to_user(). This dual use of one variable creates a state inconsistency when the copy operation fails.

The kernel copy_to_user() helper returns the number of bytes that were not copied. When it fails to copy any bytes, the calculated copied length becomes zero. As a result, the local variables len, buf, current_rx_bytes, and current_rx_buf remain unchanged. The surrounding loop then re-examines the same RX buffer and repeats the failed copy without ever making forward progress.

Root Cause

The root cause is improper progress tracking inside printer_read(). Because the same variable represents both the request size and the actual copied amount, a zero-byte successful copy is indistinguishable from a completely failed copy. The loop lacks a distinct error path for the case where an iteration transfers no data, allowing execution to remain trapped inside the read handler.

Attack Vector

Exploitation requires local access to the USB gadget printer character device. An unprivileged process reading from the device using a user-space buffer that cannot be written to, such as a buffer backed by unmapped or read-only memory, triggers repeated copy_to_user() failures. When RX data is present, the read loop repeats indefinitely, consuming CPU cycles inside kernel context and degrading overall system responsiveness.

No verified public exploit code is available. The vulnerability is a logic flaw in kernel control flow rather than a memory corruption primitive, so it is not suitable for privilege escalation or code execution. Technical details are documented across the upstream stable-tree fix commits, including the primary fix commit and its companion backports.

Detection Methods for CVE-2026-68369

Indicators of Compromise

Sustained high CPU utilization attributed to a kernel thread servicing a process blocked in a read on a USB gadget printer device node.
Soft lockup or RCU stall warnings in dmesg referencing printer_read or the f_printer module.
Processes stuck in uninterruptible sleep (D state) while holding file descriptors on /dev/g_printer* device nodes.

Detection Strategies

Audit systems for the presence of the usb_f_printer kernel module and confirm whether USB gadget printer functionality is required.
Monitor kernel logs for repeated copy_to_user failures or watchdog warnings tied to the USB gadget subsystem.
Compare running kernel versions against the fixed commit hashes referenced in the upstream advisory to identify unpatched hosts.

Monitoring Recommendations

Collect and centralize dmesg output and /proc/pressure/cpu metrics to spot sustained kernel-side CPU saturation.
Alert on processes accumulating extended CPU time while blocked on USB gadget device file descriptors.
Track kernel package versions across the fleet to ensure timely rollout of stable-tree updates that include the printer driver fix.

How to Mitigate CVE-2026-68369

Immediate Actions Required

Apply the upstream Linux kernel stable-tree updates that include the printer_read() fix on all affected hosts.
If patching is not immediately possible, unload the usb_f_printer module and disable the USB gadget printer configuration on systems that do not require it.
Restrict access to USB gadget device nodes so that only trusted service accounts can open the printer character device.

Patch Information

The fix tracks the copied length in a separate variable and returns -EFAULT, or the number of bytes already copied, when an iteration makes no progress. The change is distributed across multiple stable branches. Refer to the upstream commits: e41bbbbb1740, 994afccfdcce, c2e819be6a5c, e03597ad9494, and 4cde0b38cc0c.

Workarounds

Blacklist the usb_f_printer module in /etc/modprobe.d/ on hosts that do not use USB printer gadget functionality.
Tighten permissions on /dev/g_printer* device nodes so only administrative users can open them.
Deploy a soft-lockup watchdog with automatic reporting so unpatched hosts triggering the loop are detected quickly.

bash

# Prevent the vulnerable module from loading until patches are applied
echo 'blacklist usb_f_printer' | sudo tee /etc/modprobe.d/disable-usb-printer-gadget.conf
sudo rmmod usb_f_printer 2>/dev/null || true

# Restrict device node access to root only
sudo chmod 600 /dev/g_printer* 2>/dev/null || true