CVE-2026-64289 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
iommufd: Set upper bounds on cache invalidation entry_num and entry_len
iommufd_hwpt_invalidate() takes a user-controlled entry_num and entry_len, each bounded only by U32_MAX. An entry_len beyond the kernel's struct size makes the copy helper verify the extra bytes are zero, scanning that excess in one uninterruptible pass; a multi-gigabyte value over zeroed user memory trips the soft-lockup watchdog.
A large entry_num is the other half, driving the backend invalidation loop with no reschedule. The VT-d nested handler, for one, copies each entry and flushes caches per iteration, pinning the CPU on a non-preemptible kernel.
Cap both in the ioctl. entry_len is held under PAGE_SIZE, above any request struct, and entry_num under 1 << 19, the order of a hardware invalidation queue and well beyond any real batch, bounding the per-call loop length.
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Analysis
by VulDB Data Team • 07/25/2026
The vulnerability resides in the Linux kernel's iommufd subsystem, specifically within the iommufd_hwpt_invalidate() function that handles cache invalidation operations for IOMMU (Input-Output Memory Management Unit) hardware page tables. This function processes user-controlled parameters entry_num and entry_len without adequate bounds checking, creating a potential denial-of-service condition that can severely impact system stability and performance.
The technical flaw manifests through insufficient input validation where both entry_num and entry_len parameters accept values up to U32_MAX, effectively unlimited within the 32-bit integer range. When entry_len exceeds the kernel's internal struct size, the copy helper function performs verification by scanning the excess bytes in a single uninterruptible pass, which can trigger soft-lockup watchdog mechanisms when dealing with multi-gigabyte values. This behavior directly violates the principle of bounded resource consumption and can lead to system hang conditions. The entry_num parameter creates additional risk through an unbounded loop that drives backend invalidation operations without rescheduling opportunities, preventing kernel preemption and CPU starvation.
The operational impact of this vulnerability extends beyond simple resource exhaustion to include complete system instability. The VT-d nested handler demonstrates how each invalidation iteration can copy individual entries and flush caches, creating a scenario where the CPU becomes pinned on non-preemptible kernel code execution. This condition violates the ATT&CK framework's concept of privilege escalation through resource exhaustion attacks and aligns with CWE-704 which addresses improper input validation leading to system instability. The vulnerability affects systems using IOMMU hardware page table invalidation mechanisms, particularly those implementing virtualization technologies that rely on VT-d support.
The mitigation strategy involves implementing strict upper bounds on both parameters within the ioctl interface. The entry_len parameter is constrained to remain under PAGE_SIZE, ensuring it cannot exceed the size of any request structure while maintaining compatibility with legitimate use cases. The entry_num parameter is capped at 1 << 19, which corresponds to the order of a hardware invalidation queue and provides sufficient capacity for real-world batch operations while preventing excessive loop iterations. This approach aligns with security best practices for input validation and resource management, preventing both the soft-lockup conditions and CPU pinning scenarios that could compromise system availability. The solution ensures that kernel execution remains within predictable time bounds while maintaining functional integrity of the IOMMU subsystem.
The vulnerability represents a classic example of insufficient input validation leading to denial-of-service conditions in kernel space operations. It demonstrates how user-controlled inputs can be leveraged to create system-level stability issues through resource exhaustion and unbounded loop execution, highlighting the critical importance of proper bounds checking in kernel drivers and subsystems that handle external data inputs. The fix ensures compliance with security standards by preventing unbounded resource consumption and maintaining predictable system behavior under all operational conditions.