CVE-2026-31751: Linux Kernel Race Condition Vulnerability

CVE-2026-31751 Overview

CVE-2026-31751 affects the Linux kernel's comedi subsystem, specifically the dt2815 driver. The driver crashes when attached to I/O ports without actual hardware present. A local user with permissions to issue the COMEDI_DEVCONFIG ioctl can attach the driver to arbitrary I/O addresses. When no hardware exists at the specified port, outb() write operations trigger page faults due to undefined behavior, particularly under race conditions [CWE-362]. This results in a kernel panic and denial of service on affected systems.

Critical Impact

Local users can trigger a kernel page fault and system crash by attaching the dt2815 comedi driver to I/O ports lacking real hardware, causing denial of service.

Affected Products

• Linux Kernel (multiple stable branches)
• Linux Kernel 7.0-rc1 through 7.0-rc6
• Distributions shipping the comedidt2815 driver module

Discovery Timeline

• 2026-05-01 - CVE-2026-31751 published to NVD
• 2026-05-07 - Last updated in NVD database

Technical Details for CVE-2026-31751

Vulnerability Analysis

The dt2815 driver in the Linux comedi subsystem accepts arbitrary I/O port base addresses through the COMEDI_DEVCONFIG ioctl. The dt2815_attach function previously performed write operations via outb() against the configured I/O range without first verifying that genuine hardware was present. When no device responds at the address, write accesses to the floating bus generate undefined behavior. Under race conditions, the kernel dereferences an invalid pointer and produces a page fault as shown in the upstream report:

BUG: unable to handle page fault for address: 000000007fffff90
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
RIP: 0010:dt2815_attach+0x6e0/0x1110

The crash was reproduced by syzkaller fuzzing against arbitrary I/O addresses.

Root Cause

The driver's attach routine assumed valid hardware at the user-supplied I/O base. It issued outb() writes before any presence check. The race condition between port probing and write operations corrupts kernel state when no responder exists on the bus, classified as a concurrent execution flaw [CWE-362].

Attack Vector

Exploitation requires local access and the ability to call the COMEDI_DEVCONFIG ioctl on a comedi device node. The attacker supplies a crafted I/O port address pointing to unpopulated bus space. The fix adds a hardware detection step that reads the status register before any write. If inb() returns 0xff, the driver assumes no hardware is present and fails the attach with -ENODEV, preventing the crash path.

Detection Methods for CVE-2026-31751

Indicators of Compromise

• Kernel oops or panic logs containing dt2815_attach in the call trace
• BUG: unable to handle page fault for address messages referencing supervisor write access in kernel mode
• Unexpected loading of the dt2815 kernel module on systems without legitimate Data Translation DT2815 hardware

Detection Strategies

• Audit kernel logs (dmesg, journalctl -k) for page fault traces involving comedi driver attach functions
• Monitor invocations of the COMEDI_DEVCONFIG ioctl via auditd or eBPF tracing on systems where comedi is not operationally required
• Inventory loaded kernel modules and flag presence of dt2815 on non-laboratory or production systems

Monitoring Recommendations

• Forward kernel ring buffer events to a centralized log platform and alert on repeated kernel page faults
• Track unprivileged users opening /dev/comedi* device nodes
• Correlate kernel crashes with preceding ioctl activity to identify attempted exploitation patterns

How to Mitigate CVE-2026-31751

Immediate Actions Required

• Apply the upstream stable kernel patches that introduce hardware detection in dt2815_attach
• Blacklist the dt2815 module on systems that do not use Data Translation DT2815 analog output hardware
• Restrict access to /dev/comedi* device nodes to trusted users and groups only

Patch Information

The fix adds a status register read before any write operation in the dt2815 attach path. If inb() returns 0xff, the driver fails attach with -ENODEV. The patch is available across multiple stable branches via the following commits: 0dcf3399, 34b13250, 34c8b3a9, 65c528fb, 8d631618, 93853512, d2a786ef, and d5d9df8b.

Workarounds

• Add dt2815 to module blacklists by creating /etc/modprobe.d/blacklist-comedi.conf with blacklist dt2815
• Remove or restrict the comedi subsystem entirely if data acquisition hardware is not in use
• Apply Linux capability and Mandatory Access Control (MAC) policies to prevent unprivileged invocation of the COMEDI_DEVCONFIG ioctl

# Configuration example: blacklist the vulnerable driver
echo "blacklist dt2815" | sudo tee /etc/modprobe.d/blacklist-dt2815.conf
sudo update-initramfs -u
sudo modprobe -r dt2815 2>/dev/null || true