CVE-2025-68757: Linux Kernel Race Condition Vulnerability

CVE-2025-68757 Overview

A potential deadlock vulnerability has been identified in the Linux kernel's DRM (Direct Rendering Manager) subsystem, specifically within the vgem-fence module. The issue arises when a timer that expires a vgem fence automatically in 10 seconds is released using timer_delete_sync() from fence->ops.release() called on the last dma_fence_put(). In certain scenarios, this operation can run in IRQ context, which is not safe unless TIMER_IRQSAFE is used, leading to an inconsistent lock state and potential deadlock conditions.

Critical Impact

Systems running affected Linux kernel versions with the vgem (Virtual GEM provider) module loaded may experience kernel deadlocks, resulting in system hangs or denial of service conditions, particularly in graphics-intensive or DRM-dependent workloads.

Affected Products

• Linux Kernel (versions with vgem-fence module)
• Systems using Intel DRM drivers (as demonstrated in Intel DRM CI)
• Platforms utilizing DMA fence chain operations

Discovery Timeline

• 2026-01-05 - CVE CVE-2025-68757 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-68757

Vulnerability Analysis

This vulnerability is classified as a Race Condition/Deadlock issue within the Linux kernel's memory management and synchronization mechanisms. The root problem occurs in the vgem (Virtual GEM provider) fence implementation, which is part of the DRM subsystem used for graphics buffer management and synchronization.

The vgem fence mechanism employs a timer that automatically expires fences after 10 seconds. When a fence is released via dma_fence_put() reaching zero references, the fence->ops.release() callback invokes timer_delete_sync() to clean up the associated timer. However, the timer deletion operation may occur in IRQ (Interrupt Request) context when triggered through interrupt handling paths such as dma_fence_chain_irq_work().

The lock inconsistency arises because the timer lock is acquired in different contexts - sometimes with hardware interrupts enabled (HARDIRQ-ON-W state) during normal timer execution, and sometimes within a hardware interrupt handler (IN-HARDIRQ-W) during fence release. This creates a classic deadlock scenario where CPU0 holds the timer lock and an interrupt fires attempting to acquire the same lock.

Root Cause

The underlying cause is the absence of the TIMER_IRQSAFE flag when setting up the vgem fence timer. Without this flag, timer_delete_sync() is not safe to call from IRQ context. The Intel DRM CI testing revealed this during stress testing of syncobj_timeline operations, which exercise the DMA fence chain functionality heavily.

The lock state inconsistency is evidenced by the kernel warning showing the timer lock (&fence->timer) being acquired in incompatible contexts - first registered with {HARDIRQ-ON-W} state and later accessed with {IN-HARDIRQ-W} state, creating an ABBA deadlock potential.

Attack Vector

The attack vector for this vulnerability is local, requiring execution context on the affected system. The deadlock condition can be triggered through:

1. Heavy utilization of DRM fence operations through graphics workloads
2. Specific timing conditions where fence release coincides with timer IRQ handling
3. DMA fence chain operations that invoke dma_fence_chain_irq_work() during fence cleanup

The vulnerability was demonstrated on Intel Alder Lake platforms during CI testing, indicating that systems with Intel graphics drivers are particularly susceptible. However, any system loading the vgem module and performing fence-intensive operations could be affected.

The kernel stack trace shows the deadlock path: dma_fence_chain_irq_work() → dma_fence_release() → vgem_fence_release() → timer_delete_sync() → deadlock when attempting to acquire an already-held timer lock.

Detection Methods for CVE-2025-68757

Indicators of Compromise

• Kernel warning messages containing "inconsistent lock state" related to (&fence->timer)
• System hangs or freezes during graphics-intensive operations
• Kernel log entries showing HARDIRQ-ON-W to IN-HARDIRQ-W lock state transitions
• Stack traces containing vgem_fence_release, dma_fence_chain_release, or timer_delete_sync

Detection Strategies

• Monitor kernel logs for lockdep warnings using dmesg | grep -i "inconsistent lock state"
• Check for vgem module presence with lsmod | grep vgem
• Review kernel version against patched releases in vendor security advisories
• Enable kernel lock debugging (CONFIG_PROVE_LOCKING) to detect potential deadlock scenarios

Monitoring Recommendations

• Implement continuous kernel log monitoring for DRM subsystem warnings
• Set up alerts for system unresponsiveness that may indicate kernel deadlock conditions
• Monitor CPU utilization patterns that show stuck processes in D (uninterruptible sleep) state
• Track graphics driver crash reports and recovery events

How to Mitigate CVE-2025-68757

Immediate Actions Required

• Apply the latest kernel security patches addressing CVE-2025-68757
• Evaluate the necessity of the vgem module and unload it if not required using rmmod vgem
• Monitor affected systems for signs of deadlock conditions until patches can be applied
• Consider restricting access to DRM-related system calls for untrusted users

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fix involves ensuring proper timer handling in IRQ-safe contexts within the vgem fence implementation. Multiple commits have been published to the stable kernel tree:

• Git Kernel Commit 1f0ca9d3
• Git Kernel Commit 338e388c
• Git Kernel Commit 4f335cb8
• Git Kernel Commit 78b4d646

Organizations should update to kernel versions containing these fixes through their distribution's package management system.

Workarounds

• Unload the vgem kernel module if not actively required: sudo modprobe -r vgem
• Add vgem to the module blacklist to prevent automatic loading: echo "blacklist vgem" | sudo tee /etc/modprobe.d/blacklist-vgem.conf
• Reduce concurrent DMA fence operations in graphics workloads where possible
• Implement kernel watchdog mechanisms to detect and recover from potential deadlock conditions

# Configuration example - Blacklist vgem module
echo "blacklist vgem" >> /etc/modprobe.d/blacklist-vgem.conf
echo "install vgem /bin/false" >> /etc/modprobe.d/blacklist-vgem.conf
update-initramfs -u