CVE-2026-93239 in Linux
Summary
by MITRE • 09/24/2026
In the Linux kernel, the following vulnerability has been resolved:
arm64: mm: Fix the lockless page-table walk in show_pte()
show_pte() walks page tables locklessly and can run with interrupts enabled. A concurrent teardown can free a table page while it is being walked. It can also clear a parent entry after show_pte() checked it; the regular pXd_offset() helpers then reread the cleared entry and can derive a bogus lower-level pointer and fault again.
Use the lockless offset helpers with the saved parent entries, as gup_fast() does, and pass the saved PMD to pte_offset_map().
For task page tables, arm64 selects MMU_GATHER_RCU_TABLE_FREE. Disable local interrupts around the walk to hold off RCU-deferred table frees and block the tlb_remove_table_sync_one() IPI until the walk is finished.
Place the IRQ guard after the header print. This does not make the output a consistent snapshot, but prevents the task page-table walk from dereferencing a released table page or deriving a pointer from a different parent value.
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Analysis
by VulDB Data Team • 09/24/2026
The Linux kernel on ARM64 architectures contains a concurrency vulnerability within the show_pte function, which is responsible for displaying page table entries for debugging and diagnostic purposes. This function performs lockless walks of the page tables while interrupts remain enabled during execution. The fundamental flaw lies in the lack of synchronization between the read-only traversal of memory management structures and concurrent modifications initiated by other kernel threads or interrupt handlers. Specifically, a concurrent teardown operation can free a page table page that is currently being accessed by show_pte. Furthermore, a parent entry may be cleared after show_pte has already checked it but before subsequent operations utilize that information to derive lower-level pointers. This race condition allows the regular pXd_offset helpers to reread an entry that has been cleared or modified concurrently, leading to the derivation of bogus lower-level pointers and resulting in page faults when these invalid addresses are dereferenced.
This vulnerability falls under CWE-362, which describes concurrent execution using shared resources with improper synchronization. The specific mechanism involves a race condition where one thread reads state that is being simultaneously invalidated by another thread without adequate locking mechanisms to ensure atomicity or consistency of the read operation. In the context of ARM64 memory management, page tables are hierarchical structures where each level depends on pointers from the previous level. If a parent pointer changes between the time it is read and the time it is used to index into the next level, the resulting address calculation becomes invalid. This can lead to kernel panics or unexpected behavior when the system attempts to access memory at these corrupted addresses. The issue highlights the complexity of maintaining consistency in lockless data structures within a highly concurrent operating system environment.
The operational impact of this vulnerability includes potential denial of service through kernel crashes, as well as possible information disclosure if the invalid pointer dereferences lead to reading unintended memory regions before faulting occurs. Although show_pte is primarily used for debugging and not typically invoked in high-frequency production paths, its presence in critical diagnostic routines means that any trigger condition can destabilize the system. The vulnerability affects systems relying on ARM64 architecture where MMU_GATHER_RCU_TABLE_FREE is selected, indicating environments using Read-Copy-Update mechanisms for deferred table freeing. Such configurations are common in modern Linux distributions optimizing memory management performance by delaying actual deallocation until all readers have completed their access. Without proper safeguards, the RCU grace period may not sufficiently protect against rapid teardowns that occur during active diagnostic walks.
To mitigate this vulnerability, the kernel developers implemented a multi-layered fix strategy focusing on synchronization and safe pointer handling. First, the implementation adopts lockless offset helpers similar to those used in gup_fast, ensuring that parent entries are saved before proceeding with lower-level lookups. This prevents the scenario where a cleared entry is reread incorrectly by using consistent snapshots of parent pointers passed directly to pte_offset_map. Second, for task page tables, local interrupts are disabled around the duration of the walk. This action holds off RCU-deferred table frees and blocks tlb_remove_table_sync_one IPIs until the traversal completes. By disabling interrupts, the system ensures that no concurrent teardown can free a table page while it is being walked. The IRQ guard is strategically placed after header printing to avoid affecting output consistency but effectively prevents dereferencing released pages or deriving pointers from inconsistent parent values.
This remediation aligns with best practices for handling lockless data structures in kernel space, emphasizing the need for explicit synchronization when accessing shared memory management resources that are subject to concurrent modification. The approach mirrors techniques used elsewhere in the kernel such as gup_fast, demonstrating a standardized pattern for safely traversing page tables without acquiring heavy locks that could impact performance significantly. Security professionals should verify that systems running affected ARM64 kernels have applied this patch or equivalent updates from their distribution vendors. Regular auditing of kernel logs for unexpected faults related to memory management can help detect residual issues in environments where the fix has not yet been deployed. Maintaining up-to-date kernel versions is critical as these low-level concurrency bugs often require precise architectural knowledge to identify and resolve correctly through code patches rather than configuration changes alone.