CVE-2026-31769: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31769 Overview

CVE-2026-31769 is a use-after-free vulnerability in the Linux kernel gpib subsystem. The flaw resides in the IBRD, IBWRT, IBCMD, and IBWAIT ioctl handlers, which dereference a gpib_descriptor pointer after board->big_gpib_mutex has been released. A concurrent IBCLOSEDEV ioctl can free the descriptor through close_dev_ioctl() during this unprotected window. Local attackers with access to the GPIB device can trigger memory corruption that leads to privilege escalation or denial of service. The kernel maintainers have committed fixes upstream that introduce a descriptor_busy reference count to prevent the race.

Critical Impact
Local attackers can exploit a race condition between IO ioctl handlers and IBCLOSEDEV to trigger a use-after-free, enabling kernel memory corruption and privilege escalation.

Affected Products

Linux kernel versions containing the gpib subsystem prior to the fix commits
Distributions shipping the unpatched gpib driver module
Systems exposing GPIB device nodes to unprivileged users

Discovery Timeline

2026-05-01 - CVE-2026-31769 published to NVD
2026-05-03 - Last updated in NVD database

Technical Details for CVE-2026-31769

Vulnerability Analysis

The vulnerability is a kernel use-after-free [CWE-416] caused by improper synchronization between IO ioctl handlers and the device close path in the gpib subsystem. The IO handlers read_ioctl, write_ioctl, and command_ioctl explicitly release big_gpib_mutex before invoking their underlying handlers. The wait_ioctl() path is entered with the lock held, but ibwait() releases it internally when wait_mask is non-zero.

In each of these four handlers, the gpib_descriptor pointer obtained from handle_to_descriptor() becomes unprotected after the mutex is dropped. A concurrent IBCLOSEDEV ioctl invoking close_dev_ioctl() can free the descriptor while another thread is still using it, producing a classic time-of-check to time-of-use condition that corrupts kernel memory.

Root Cause

The root cause is the absence of a stable reference count protecting the gpib_descriptor lifetime across lock boundaries. Relying on io_in_progress is insufficient because that flag can be cleared from unprivileged userspace through IBWAIT via general_ibstatus() with set_mask containing CMPL. An attacker can manipulate this state to bypass any check that depends solely on io_in_progress.

Attack Vector

A local attacker with permission to open the GPIB character device opens two file descriptors against the same handle. From one thread the attacker issues an IBRD, IBWRT, IBCMD, or IBWAIT ioctl while a second thread races an IBCLOSEDEV against the same descriptor. After the IO handler releases big_gpib_mutex, the close path frees the descriptor, and the IO handler dereferences freed memory. Successful exploitation enables kernel heap corruption that can be shaped into local privilege escalation.

The upstream fix introduces a kernel-only descriptor_busy reference counter in struct gpib_descriptor. Each handler atomically increments the counter under file_priv->descriptors_mutex before releasing big_gpib_mutex, and decrements it on completion. close_dev_ioctl() checks descriptor_busy under the same lock and returns -EBUSY when the count is non-zero. The lock ordering remains big_gpib_mutex → descriptors_mutex, avoiding deadlock.

See Kernel Git Commit 28c75dd, Kernel Git Commit cae26ef, and Kernel Git Commit d1857f8 for the patch details.

Detection Methods for CVE-2026-31769

Indicators of Compromise

Kernel oops or panic messages referencing gpib, handle_to_descriptor, close_dev_ioctl, or KASAN use-after-free reports in dmesg.
Unexpected processes opening /dev/gpib* device nodes, particularly with multiple concurrent file descriptors against the same handle.
Crashes correlated with rapid sequences of IBRD, IBWRT, IBCMD, IBWAIT, and IBCLOSEDEV ioctls from the same UID.

Detection Strategies

Enable Kernel Address Sanitizer (KASAN) on test systems to surface use-after-free conditions during fuzzing of GPIB ioctls.
Audit auditd or eBPF telemetry for ioctl numbers associated with IBCLOSEDEV issued in tight loops alongside IO ioctls on /dev/gpib*.
Hunt for unprivileged users invoking GPIB ioctls on systems where the GPIB stack is not part of normal workflows.

Monitoring Recommendations

Centralize kernel logs and alert on BUG:, KASAN:, or general protection fault entries that mention the gpib module.
Track installed kernel versions across the fleet and flag hosts running gpib builds that predate the fix commits.
Monitor process behavior for non-instrument workloads opening /dev/gpib*, which may indicate exploitation attempts.

How to Mitigate CVE-2026-31769

Immediate Actions Required

Apply the upstream kernel patches referenced by commits 28c75dd, cae26ef, and d1857f8 once they ship in your distribution's kernel update.
Restrict permissions on /dev/gpib* device nodes so that only trusted users and service accounts can issue ioctls against the GPIB subsystem.
Where the gpib driver is not required, blacklist the module to remove the attack surface entirely.

Patch Information

The Linux kernel maintainers resolved the issue by adding a descriptor_busy reference count to struct gpib_descriptor. IO ioctl handlers now hold a reference across the unlocked window, and close_dev_ioctl() returns -EBUSY while the count is non-zero. Apply the fix from Kernel Git Commit 28c75dd, Kernel Git Commit cae26ef, or Kernel Git Commit d1857f8 depending on the stable branch in use.

Workarounds

Unload the driver with modprobe -r gpib_common and add it to /etc/modprobe.d/blacklist.conf on hosts that do not need GPIB instrument support.
Tighten device node permissions, for example chmod 0660 /dev/gpib* and assign ownership to a dedicated gpib group containing only trusted users.
Restrict local shell access on systems where the driver must remain loaded, since exploitation requires a local user able to open the GPIB device.

bash

# Configuration example
# Block the vulnerable module on systems that do not require GPIB
echo 'blacklist gpib_common' | sudo tee /etc/modprobe.d/disable-gpib.conf
sudo modprobe -r gpib_common

# Restrict device node access where the driver must stay loaded
sudo groupadd -f gpib
sudo chown root:gpib /dev/gpib*
sudo chmod 0660 /dev/gpib*