CVE-2026-98004 in Linux
Summary
by MITRE • 09/25/2026
In the Linux kernel, the following vulnerability has been resolved:
iommu/riscv: Serialize command queue publishing
Serialize command queue publishing so software producer state advances only after a command is written and the hardware tail is updated. Wait for hardware consumption outside the queue lock when the command queue is full so other CPUs are not blocked behind a long poll.
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Analysis
by VulDB Data Team • 09/25/2026
The vulnerability addressed in this update pertains to a race condition within the RISC-V IOMMU driver subsystem of the Linux kernel, specifically involving the serialization of command queue publishing operations. In systems utilizing an Input/Output Memory Management Unit (IOMMU), software components must communicate with hardware devices through a shared command queue mechanism. This interaction relies on precise synchronization between the producer state maintained by the operating system and the consumer tail pointer managed by the hardware. The core technical flaw lies in the atomicity of these updates, where prior to this fix, there was insufficient guarantee that the software producer index would be advanced only after both the command had been fully written to memory and the hardware tail register had been updated to reflect consumption readiness. This lack of strict ordering can lead to scenarios where subsequent commands are published before previous ones are properly acknowledged or processed by the IOMMU hardware, resulting in data corruption or undefined behavior within the DMA translation tables.
From a security perspective, this issue aligns with CWE-362, which describes concurrent execution using shared resources with improper synchronization. The absence of proper memory barriers and atomic operations allows for potential race conditions that could be exploited to manipulate IOMMU state. An attacker who has achieved code execution privileges on the host system or can influence kernel module loading might leverage this flaw to corrupt DMA mappings. Such corruption could lead to unauthorized access to physical memory regions, effectively bypassing isolation boundaries enforced by the IOMMU. This undermines the fundamental security model of virtualization and containerized environments that rely heavily on strict memory protection mechanisms provided by hardware-assisted virtualization features. The vulnerability represents a significant risk in multi-core systems where concurrent threads may attempt to submit commands simultaneously without adequate locking or barrier instructions, leading to unpredictable system states.
The operational impact of this flaw extends beyond mere data integrity issues. When the command queue is full and software waits for hardware consumption, previous implementations might have held locks during long polling loops. This design choice caused other CPUs attempting to access shared IOMMU resources to block indefinitely or for extended periods, leading to severe performance degradation and potential system hangs. By separating the wait logic from the critical section protected by the queue lock, the updated implementation ensures that high-priority tasks on other cores are not starved of CPU time while waiting for DMA operations to complete. This improvement enhances overall system stability and responsiveness, particularly in workloads with heavy I/O throughput where frequent command submissions occur. The fix prevents denial-of-service conditions caused by resource contention and lock hold times exceeding acceptable thresholds during high-load scenarios.
Mitigation strategies primarily involve applying the upstream kernel patch that corrects the synchronization logic within the RISC-V IOMMU driver. System administrators should ensure their systems are updated to a kernel version containing this specific commit, which enforces strict ordering between memory writes and hardware register updates using appropriate memory barriers such as dma_wmb or equivalent architecture-specific primitives. For environments where immediate patching is not feasible, limiting concurrent DMA submission rates through application-level throttling may reduce the likelihood of triggering the race condition, though this is a less robust defense. Continuous monitoring for IOMMU fault logs and system latency spikes can help detect potential exploitation attempts or instability caused by the flaw. Adherence to secure coding practices that emphasize atomicity in hardware-software interfaces remains critical for preventing similar vulnerabilities in future kernel developments.