CVE-2026-90430 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
iommu/tegra241-cmdqv: Publish an LVCMDQ only after it is fully initialized
tegra241_vintf_init_lvcmdq() stores the freshly allocated vcmdq pointer to the vintf->lvcmdqs[] array, before tegra241_vcmdq_alloc_smmu_cmdq() builds
the vcmdq->cmdq. The error ISR dereferences that cmdq, so a latched LVCMDQ error (e.g. one inherited across a kexec) firing in this window would make tegra241_vintf0_handle_error() pass the still-zeroed arm_smmu_cmdq down to __arm_smmu_cmdq_skip_err(), dereferencing NULL queue register pointers.
Drop the store from tegra241_vintf_init_lvcmdq() and publish the vcmdq at the end of the allocation instead, with an smp_store_release() that pairs with an smp_load_acquire() in the ISR, which can see a fully built LVCMDQ or NULL.
The user-owned LVCMDQ allocation moves accordingly, publishing the vcmdq once tegra241_vcmdq_hw_init_user() succeeds, using a plain store since a user VINTF's lvcmdqs[] has no lockless reader -- the error ISR only walks
the VINTF0 array.
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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability identified in the Linux kernel involves a race condition within the NVIDIA Tegra241 IOMMU driver, specifically affecting the initialization sequence of the Low Virtual Command Queue (LVCMDQ). This flaw arises from an improper ordering of operations during the setup of virtual interface command queues. The core issue lies in how the system publishes references to these newly allocated structures before they are fully initialized and ready for hardware interaction. By exposing a partially constructed object to error handling routines, the kernel creates a window where accessing uninitialized memory can lead to critical failures such as null pointer dereferences or undefined behavior.
The technical flaw is rooted in the function tegra241_vintf_init_lvcmdq(), which previously stored the freshly allocated vcmdq pointer into the vintf->lvcmdqs[] array immediately after allocation but before calling tegra241_vcmdq_alloc_smmu_cmdq(). This latter function is responsible for building the actual cmdq structure. Consequently, there exists a temporal gap where the queue appears present in the global registry but lacks its essential hardware command queue components. If an error interrupt service routine (ISR) triggers during this specific window, it may attempt to process a latched LVCMDQ error that was inherited from a previous state, such as across a kexec transition. In such scenarios, the handler tegra241_vintf0_handle_error() receives a pointer to an arm_smmu_cmdq structure that is still zeroed out because its initialization has not yet completed.
This sequence leads directly to a null pointer dereference when the error handling logic passes this incomplete queue down to __arm_smmu_cmdq_skip_err(). The function attempts to access register pointers within the cmdq structure, which are currently NULL due to the premature publication of the object. This results in a kernel panic or system crash, effectively denying service and compromising the stability of the operating environment. Such vulnerabilities are particularly dangerous because they can be triggered by hardware events that occur during initialization phases, making them difficult to reproduce consistently but highly impactful when they do manifest.
From a classification perspective, this vulnerability aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, as the race condition stems from sharing a resource without ensuring its complete readiness before access by concurrent execution contexts like ISRs. It also relates to CWE-824: Access of Uninitialized Pointer, since the ISR accesses fields within an object that has not been fully initialized. In terms of the MITRE ATT&CK framework, this scenario reflects techniques associated with Initial Compromise or Defense Evasion where system instability is exploited, though primarily it represents a reliability issue rather than a direct exploitation vector for privilege escalation unless combined with other flaws.
The resolution involves restructuring the initialization logic to ensure that the vcmdq pointer is only published after all necessary hardware and software setup steps are complete. Specifically, the store operation in tegra241_vintf_init_lvcmdq() was removed from its previous location and moved to the end of the allocation process. This change ensures that the queue is fully built before it becomes visible to other parts of the system. Furthermore, synchronization primitives were introduced to manage access safely. An smp_store_release() call is used when publishing the vcmdq in the primary initialization path, which pairs with an smp_load_acquire() in the ISR. This memory barrier ensures that the ISR sees either a fully constructed LVCMDQ or NULL, preventing it from ever encountering a partially initialized state.
For user-owned LVCMDQ allocations, the logic was adjusted to publish the vcmdq only after tegra241_vcmdq_hw_init_user() succeeds. Since these specific queues are not accessed locklessly by error ISRs—which exclusively walk the VINTF0 array—a plain store operation suffices for synchronization in this context. This distinction optimizes performance while maintaining correctness, as it avoids unnecessary overhead where strict memory ordering guarantees are not required by concurrent readers.
To mitigate similar vulnerabilities and enhance system resilience, developers should adhere to strict initialization protocols that separate resource allocation from publication or exposure. All shared resources must be fully initialized before being made accessible to interrupt handlers or other asynchronous execution contexts. Implementing proper synchronization mechanisms such as atomic operations with appropriate memory barriers is crucial when dealing with concurrent access patterns. Additionally, robust error handling in ISR routines should include checks for null pointers and incomplete states to prevent crashes even if race conditions occur due to external factors like kexec transitions. Regular code reviews focusing on initialization order and concurrency control can help identify these subtle timing issues before they reach production environments.