CVE-2026-98521 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
drm/i915: fix incorrect RCU teardown order
i915_gem_busy_ioctl uses dma_resv_for_each_fence_unlocked() to iterate over the fences in an GEM object without holding a reference but only the RCU read side lock.
What can happen here is that the GEM object is destroyed concurrently while i915_gem_busy_ioctl is still running. This won't free the GEM objects memory, but still drops all the dma_fence references.
Now when dma_resv_for_each_fence_unlocked() sees a destroyed dma_fence it assumes that a new fence list was installed and re-starts the loop.
But in the case of a destroyed GEM object a new fence list is never installed, only the old one freed and therefore the iteration never finishes resulting in an endless loop.
The solution is to drop the fence references only after the RCU grace period.
The fixes tag is not necessary the patch introducing the problem, but the one making it so worse that we need to address it.
This problem was pointed out by Sashiko-bot.
(cherry picked from commit 5113479556025093bf8133bb2dcaa33be2d50921)
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Analysis
by VulDB Data Team • 10/18/2026
The vulnerability identified in the Linux kernel's Intel graphics driver, specifically within the drm/i915 subsystem, stems from a race condition involving Resource-Conditional Execution (RCU) synchronization and object lifecycle management. The core issue resides in the i915_gem_busy_ioctl function, which utilizes dma_resv_for_each_fence_unlocked to iterate over fences associated with a Graphics Execution Manager GEM object. This iteration is performed without holding a reference count on the underlying objects, relying solely on the RCU read-side lock for temporary protection against concurrent memory reclamation. While this approach is generally efficient for high-frequency operations, it introduces a critical timing window where the structural integrity of the data structures being traversed can be compromised by concurrent destruction events that are not fully synchronized with the iteration logic.
The operational flaw manifests when a GEM object is destroyed concurrently while i915_gem_busy_ioctl is actively iterating over its associated fences. Although the kernel's memory management subsystem ensures that the actual memory backing the GEM object is not immediately freed, it does proceed to drop all dma_fence references and potentially modify or free internal data structures like fence lists during the teardown process. The function dma_resv_for_each_fence_unlocked contains logic designed to handle dynamic updates to reservation objects; specifically, if it encounters a destroyed or invalidated dma_fence, it assumes that a new fence list has been installed by another thread and attempts to restart the iteration from the beginning of this presumed new list. This mechanism is intended to support live updates where fences are swapped out atomically without stopping execution for long periods.
However, in the specific context of GEM object destruction, no new fence list is ever installed. Instead, the existing fence list is simply freed and invalidated as part of the cleanup routine. Consequently, when the iteration logic detects a destroyed fence and attempts to restart based on the assumption of a replacement structure, it encounters an invalid or null state because the expected data has been deallocated rather than replaced. This mismatch between the iterator's expectation of structural continuity and the reality of total resource teardown results in an infinite loop. The thread executing i915_gem_busy_ioctl becomes permanently stuck, consuming CPU resources indefinitely and potentially causing a denial of service for other processes or system stability issues depending on how the kernel handles hung tasks.
This vulnerability is categorized under CWE-835, which describes loops with unreachable exit conditions, specifically arising from incorrect synchronization primitives leading to livelock scenarios. From an ATT&CK perspective, this aligns with techniques related to resource exhaustion and denial of service, as a local user could potentially trigger this condition through crafted ioctl calls that force rapid creation and destruction of GEM objects while simultaneously querying their busy status. The lack of proper reference counting during the iteration phase allows the concurrent teardown path to race ahead and dismantle the data structures before the iterator has safely completed its traversal, highlighting a classic use-after-free variant where logical consistency is broken even if physical memory access remains within bounds due to RCU delays.
The resolution involves adjusting the lifecycle management of fence references to ensure they are not dropped until after the RCU grace period has elapsed. By deferring the release of these references, the kernel guarantees that any ongoing RCU read-side critical sections will complete their execution before the underlying data structures become invalid or inaccessible in a way that breaks iteration logic. This change ensures that dma_resv_for_each_fence_unlocked either completes its traversal successfully or safely handles termination without entering an infinite loop state. The fix prioritizes correctness of synchronization over minimal latency, acknowledging that the performance benefit of not holding references is outweighed by the stability risk introduced by the race condition.
Mitigation strategies for this vulnerability primarily involve applying the kernel patch associated with commit 5113479556025093bf8133bb2dcaa33be2d50921 to affected systems. For environments where immediate patching is not feasible, administrators should monitor for high CPU usage in i915-related processes and consider limiting the rate of GEM object creation and destruction if possible through application-level controls. Furthermore, ensuring that kernel updates are applied promptly remains the most effective defense against such low-level synchronization bugs. The issue was identified via automated testing tools indicated by the Sashiko-bot reference, underscoring the importance of continuous fuzzing and static analysis in detecting subtle concurrency flaws in complex subsystems like graphics drivers.