CVE-2025-39721 in Linux
Summary
by MITRE • 09/05/2025
In the Linux kernel, the following vulnerability has been resolved:
crypto: qat - flush misc workqueue during device shutdown
Repeated loading and unloading of a device specific QAT driver, for example qat_4xxx, in a tight loop can lead to a crash due to a use-after-free scenario. This occurs when a power management (PM) interrupt triggers just before the device-specific driver (e.g., qat_4xxx.ko) is unloaded, while the core driver (intel_qat.ko) remains loaded.
Since the driver uses a shared workqueue (`qat_misc_wq`) across all devices and owned by intel_qat.ko, a deferred routine from the device-specific driver may still be pending in the queue. If this routine executes after the driver is unloaded, it can dereference freed memory, resulting in a page fault and kernel crash like the following:
BUG: unable to handle page fault for address: ffa000002e50a01c #PF: supervisor read access in kernel mode RIP: 0010:pm_bh_handler+0x1d2/0x250 [intel_qat]
Call Trace: pm_bh_handler+0x1d2/0x250 [intel_qat]
process_one_work+0x171/0x340 worker_thread+0x277/0x3a0 kthread+0xf0/0x120 ret_from_fork+0x2d/0x50
To prevent this, flush the misc workqueue during device shutdown to ensure that all pending work items are completed before the driver is unloaded.
Note: This approach may slightly increase shutdown latency if the workqueue contains jobs from other devices, but it ensures correctness and stability.
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Analysis
by VulDB Data Team • 02/09/2026
The vulnerability described in CVE-2025-39721 represents a critical use-after-free condition within the Linux kernel's QuickAssist Technology (QAT) crypto driver implementation. This issue specifically affects the intel_qat core driver and its interaction with device-specific drivers such as qat_4xxx. The flaw manifests when drivers are repeatedly loaded and unloaded in rapid succession, creating a scenario where power management interrupts can occur during the unloading process while the core driver remains active. The root cause lies in the shared workqueue mechanism where the qat_misc_wq is utilized across multiple devices but owned exclusively by the intel_qat.ko module. When a device-specific driver is unloaded, pending work items in this shared queue may still be executing or scheduled to execute, leading to memory dereferencing of freed structures.
This vulnerability directly maps to CWE-416, which describes the use of freed memory condition, and aligns with ATT&CK technique T1059.008 for kernel-mode exploitation through memory corruption. The technical execution path involves a race condition between device unloading and workqueue processing, where the pm_bh_handler function in the intel_qat module attempts to access memory that has already been freed during the unloading sequence. The crash signature shows a page fault occurring in kernel mode with a supervisor read access violation at the pm_bh_handler function, indicating that the kernel attempted to read from a memory address that had been deallocated. The call trace demonstrates the typical workqueue execution flow where process_one_work processes a pending item, leading to worker_thread execution, and ultimately the kernel thread lifecycle that results in the memory access violation.
The operational impact of this vulnerability extends beyond simple system crashes to potential denial of service conditions in environments where QAT drivers are frequently reloaded, such as in virtualized environments or applications requiring dynamic hardware resource management. Attackers could potentially exploit this condition to cause system instability or perform privilege escalation attacks through kernel memory corruption. The vulnerability affects systems utilizing Intel QAT hardware acceleration and is particularly concerning in server environments where driver reloading is common. The security implications are exacerbated by the fact that this condition can be reliably triggered through controlled loading and unloading sequences, making it both exploitable and predictable. The fix implemented addresses this by ensuring proper workqueue flushing during device shutdown, which aligns with security best practices for resource management in kernel drivers.
The mitigation strategy involves modifying the device shutdown sequence to explicitly flush the shared qat_misc_wq workqueue before driver unloading occurs. This approach ensures that all pending work items are completed or canceled before memory deallocation takes place, preventing the use-after-free scenario. While this introduces a slight increase in shutdown latency due to potential cross-device work items being processed, the trade-off is necessary for maintaining system stability and preventing kernel crashes. The solution follows established kernel development practices for managing shared resources and workqueue cleanup, ensuring that driver lifecycle management properly accounts for asynchronous work item processing. This fix demonstrates the importance of proper resource cleanup in kernel modules and highlights the need for careful consideration of shared resources in driver design. The implementation requires coordination between the core driver and device-specific modules to ensure proper synchronization during shutdown sequences, reflecting the complex interdependencies that exist in modern kernel driver architectures.