CVE-2026-46215: Linux Kernel Race Condition Vulnerability

CVE-2026-46215 Overview

CVE-2026-46215 is a race condition vulnerability in the Linux kernel's Direct Rendering Manager (DRM) subsystem. The flaw resides in the change_handle ioctl path, where a single object could briefly hold two IDR (ID Radix tree) entries during a prime swap operation. A concurrent gem_close call could delete the object and remove one handle while leaving the other dangling. The dangling handle could then be dereferenced, producing a use-after-free condition in kernel memory. The Linux kernel maintainers resolved the issue by setting the old handle to NULL via idr_replace before performing the prime swap, mirroring the existing safe pattern used in gem_close.

Critical Impact

Local attackers with access to the DRM ioctl interface can trigger a use-after-free in kernel memory, potentially leading to privilege escalation or kernel crashes.

Affected Products

• Linux kernel — DRM (Direct Rendering Manager) subsystem
• Affected versions are addressed by upstream commits 5e28b7b94408, 61bd96d3e547, and 672464dd5323
• Linux distributions shipping vulnerable DRM code prior to these stable backports

Discovery Timeline

• 2026-05-28 - CVE-2026-46215 published to NVD
• 2026-05-28 - Last updated in NVD database

Technical Details for CVE-2026-46215

Vulnerability Analysis

The vulnerability is a race condition [CWE-362] in the DRM subsystem's change_handle ioctl. During the handle swap operation, the kernel briefly maintained a state in which a single GEM (Graphics Execution Manager) object was referenced by two IDR entries. This transient duplication created a window during which another thread executing gem_close could delete the object and remove one handle while the second handle remained valid but pointed to freed memory. A subsequent dereference of the dangling handle results in a use-after-free [CWE-416] in kernel context.

Root Cause

The root cause is the lack of atomic handle transitions in change_handle. The original implementation did not invalidate the old IDR entry before introducing the new one, leaving an exploitable window. The upstream fix applies the same pattern previously used in commits f6cd7daecff5 and bd46cece51a3: the old handle is replaced with NULL via idr_replace first, and the object is only closed after the prime operations succeed. The change_handle path holds the prime lock for the full duration, which limits the races to those involving gem_close.

Attack Vector

Exploitation requires local access to the DRM device node, typically /dev/dri/card* or /dev/dri/renderD*. An unprivileged user with access to these device files can issue concurrent change_handle and gem_close ioctls from separate threads. Winning the race produces a dangling handle that, when dereferenced through subsequent DRM operations, accesses freed kernel memory. Successful exploitation can yield kernel information disclosure, denial of service through panic, or privilege escalation depending on heap layout control.

The vulnerable mechanism is described in detail in the upstream patches. See the Kernel Git Commit Note for the full source-level fix.

Detection Methods for CVE-2026-46215

Indicators of Compromise

• Unexpected kernel oops or panic messages referencing drm_gem_*, change_handle, or idr_* functions in dmesg or journalctl -k output
• Kernel Address Sanitizer (KASAN) reports flagging use-after-free in DRM code paths
• Processes performing repeated, rapid DRM_IOCTL_* calls combined with GEM_CLOSE from multiple threads on the same file descriptor

Detection Strategies

• Enable CONFIG_KASAN on test and staging kernels to surface use-after-free conditions in DRM ioctl paths during fuzzing or workload replay
• Audit kernel ring buffer logs for crashes in the DRM subsystem and correlate with the user identity that opened the DRM device node
• Monitor for unprivileged processes that open /dev/dri/* and perform high-frequency ioctl operations against GEM handles

Monitoring Recommendations

• Forward kernel.crit and kernel.err syslog facilities to centralized logging for review of DRM-related faults
• Track installed kernel versions across the fleet and alert on hosts running kernels predating the fix commits listed in the NVD references
• Instrument auditd to log ioctl syscalls against DRM device nodes by non-graphics service accounts

How to Mitigate CVE-2026-46215

Immediate Actions Required

• Apply the upstream Linux kernel patches referenced in the NVD entry, or upgrade to a distribution kernel that includes commits 5e28b7b94408, 61bd96d3e547, or 672464dd5323
• Restrict access to /dev/dri/* device nodes to trusted users and graphics service accounts via group membership and udev rules
• Identify hosts running unpatched kernels and prioritize multi-user systems, container hosts, and shared workstations for remediation

Patch Information

The fix is committed to the upstream Linux stable tree. Review the Kernel Git Commit Note, the Kernel Git Commit Note, and the Kernel Git Commit Note for the source-level changes. Apply distribution-provided kernel updates from Red Hat, SUSE, Debian, Ubuntu, or other vendors as they become available.

Workarounds

• Where patching is not immediately possible, remove access to DRM device nodes from untrusted users by tightening permissions on /dev/dri/card* and /dev/dri/renderD*
• In container environments, avoid passing DRM devices into untrusted containers and exclude the video and render groups from low-trust workloads
• Disable GPU passthrough or DRM access for guest workloads that do not require hardware-accelerated graphics

# Configuration example: restrict DRM device access to the 'video' group
sudo chgrp video /dev/dri/card* /dev/dri/renderD*
sudo chmod 0660 /dev/dri/card* /dev/dri/renderD*

# Verify the running kernel version against patched releases
uname -r

# Check distribution advisories for the relevant fixed kernel package
# Example for Debian/Ubuntu:
apt list --upgradable | grep linux-image