CVE-2026-31412: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-31412 Overview

A potential integer overflow vulnerability has been discovered in the Linux kernel's USB gadget mass storage driver (f_mass_storage). The vulnerability exists in the check_command_size_in_blocks() function, which calculates data size in bytes by left shifting common->data_size_from_cmnd by the block size (common->curlun->blkbits). Without proper validation, this shift operation can cause an integer overflow, potentially leading to memory corruption or out-of-bounds memory access.

Critical Impact

A malicious USB host can send crafted SCSI READ or WRITE commands requesting large data sizes, causing the left shift operation to wrap around and bypass boundary checks, potentially leading to memory corruption or out-of-bounds accesses.

Affected Products

• Linux kernel (USB gadget f_mass_storage driver)
• Systems using the USB mass storage gadget functionality
• Embedded devices implementing USB gadget mode

Discovery Timeline

• 2026-04-10 - CVE CVE-2026-31412 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-31412

Vulnerability Analysis

The integer overflow vulnerability resides in the USB gadget mass storage driver's check_command_size_in_blocks() function within the Linux kernel. The core issue stems from the unsafe calculation of data size when processing SCSI commands from a USB host.

When the function calculates the data size in bytes, it performs a left shift operation on common->data_size_from_cmnd using the block size value stored in common->curlun->blkbits. The block size is initially configured in fsg_lun_open(), while common->data_size_from_cmnd is established in do_scsi_command(). Neither initialization includes validation to prevent integer overflow when these two values interact through the shift operation.

An attacker with physical access to a system acting as a USB device can exploit this by connecting the vulnerable system to a malicious USB host that sends specially crafted SCSI READ or WRITE commands with excessively large data size values. When the left shift operation overflows, the resulting truncated data size can bypass subsequent boundary validation checks, potentially causing memory corruption or unauthorized memory access.

Root Cause

The root cause is the absence of overflow checking when performing the left shift operation in check_command_size_in_blocks(). The function blindly shifts common->data_size_from_cmnd left by common->curlun->blkbits bits without verifying that the result will fit within the expected integer bounds. This is a classic integer overflow condition where the product of the calculation exceeds the maximum value representable by the data type.

Attack Vector

The attack requires physical access to exploit. An attacker must connect a vulnerable Linux system operating in USB gadget mode to a malicious USB host controller. The malicious host then issues SCSI commands (such as READ or WRITE operations) with carefully crafted parameters designed to trigger the integer overflow. When the overflow occurs, the truncated size value causes the driver to operate on incorrect memory boundaries, potentially allowing memory corruption or information disclosure.

The vulnerability is exploited through crafted SCSI command sequences sent via USB. The malicious host sends commands with large data_size_from_cmnd values that, when left-shifted by the block size bits, cause integer wraparound. The fix implemented in the kernel patches uses the check_shl_overflow() macro to safely perform the shift operation and detect any overflow conditions before proceeding with the calculation.

Detection Methods for CVE-2026-31412

Indicators of Compromise

• Unexpected system crashes or kernel panics when USB gadget mode is active
• Memory corruption errors in kernel logs related to f_mass_storage driver
• Unusual SCSI command sequences in USB traffic when system is connected to external hosts

Detection Strategies

• Monitor kernel logs for errors or warnings originating from the USB gadget subsystem
• Implement USB traffic analysis to detect anomalous SCSI command patterns with unusually large data size parameters
• Deploy kernel-level monitoring to detect out-of-bounds memory access attempts in the mass storage driver context

Monitoring Recommendations

• Enable verbose logging for the USB gadget subsystem during debugging or high-security scenarios
• Monitor system stability metrics when USB gadget functionality is in use
• Implement endpoint detection solutions capable of identifying kernel-level memory corruption attempts

How to Mitigate CVE-2026-31412

Immediate Actions Required

• Update to a patched Linux kernel version that includes the overflow check fix
• If patching is not immediately possible, consider disabling USB gadget mass storage functionality if not required
• Restrict physical access to systems operating in USB gadget mode

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability by implementing the check_shl_overflow() macro to safely perform the shift operation and prevent integer overflow conditions. Multiple patch commits are available for different kernel branches:

• Kernel Git Commit 228b379
• Kernel Git Commit 3428dc5
• Kernel Git Commit 387ebb0
• Kernel Git Commit 8479891
• Kernel Git Commit 91817ad
• Kernel Git Commit ce0caae

Workarounds

• Disable the USB gadget mass storage driver module (g_mass_storage or usb_f_mass_storage) if not required for operations
• Implement physical security controls to prevent unauthorized USB host connections to vulnerable systems
• Use application-level controls to restrict USB gadget functionality to trusted environments only

# Disable USB mass storage gadget module if not needed
modprobe -r usb_f_mass_storage
# Blacklist the module to prevent automatic loading
echo "blacklist usb_f_mass_storage" >> /etc/modprobe.d/blacklist-usb-gadget.conf