CVE-2026-72174 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
fs/proc/task_mmu: fix hugetlb self-deadlock in pagemap_scan_pte_hole()
A PAGEMAP_SCAN ioctl requesting PM_SCAN_WP_MATCHING on a hugetlb VMA hangs the calling thread, unkillably, as soon as the scan reaches an unpopulated part of the range:
do_pagemap_scan() walk_page_range() walk_hugetlb_range() hugetlb_vma_lock_read() # take the vma lock for read ... pagemap_scan_pte_hole() # ... ->pte_hole() for a hole uffd_wp_range() change_protection() hugetlb_change_protection() hugetlb_vma_lock_write() # ... and block taking it for write
walk_hugetlb_range() holds the hugetlb vma lock for read across the whole walk. A present entry goes to ->hugetlb_entry(); an unpopulated one goes to ->pte_hole(), i.e. pagemap_scan_pte_hole(). To write-protect the hole that handler calls uffd_wp_range(), which on a hugetlb VMA reaches hugetlb_change_protection() and takes the same vma lock for write. The thread then blocks in down_write() waiting for the read lock it is itself holding.
The populated path avoids this: pagemap_scan_hugetlb_entry() write-protects the entry inline under the page-table lock and never enters hugetlb_change_protection().
Do the same for holes. Fault in the page table and install the uffd-wp marker directly with make_uffd_wp_huge_pte() under the page-table lock, rather than routing through uffd_wp_range(). That is the same sequence hugetlb_change_protection() runs for an unpopulated entry, minus the vma write lock -- which is safe to skip because PMD sharing is disabled on uffd-wp VMAs (hugetlb_unshare_all_pmds() runs at registration), leaving nothing for that lock to serialise against.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described represents a critical self-deadlock condition within the Linux kernel's memory management subsystem, specifically affecting hugetlb (huge page) virtual memory areas during pagemap scanning operations. This issue manifests when a process attempts to perform a PAGEMAP_SCAN ioctl operation with PM_SCAN_WP_MATCHING flag on a hugetlb VMA that contains unpopulated memory regions. The flaw arises from improper locking mechanisms that create a circular dependency where the same thread attempts to acquire both read and write locks on the same virtual memory area lock, resulting in an unkillable hang that can only be resolved through system reboot.
The technical root cause stems from the walk_hugetlb_range() function holding a read lock on the hugetlb vma lock throughout the entire memory range traversal process. When encountering unpopulated memory areas, the code path directs execution to pagemap_scan_pte_hole(), which subsequently calls uffd_wp_range(). This function eventually reaches hugetlb_change_protection() that attempts to acquire the same vma lock but in write mode, creating an immediate deadlock situation. The kernel's page table walk logic properly handles populated entries through pagemap_scan_hugetlb_entry() by write-protecting entries inline under the page-table lock without requiring additional vma locking, but this optimization is absent for unpopulated regions.
This vulnerability directly maps to CWE-367, which describes Time-of-Check Time-of-Use (TOCTOU) flaws and resource management issues. The deadlock scenario represents a classic resource contention problem where a single thread becomes blocked while holding a lock that it cannot release due to its own subsequent locking attempts. From an ATT&CK perspective, this vulnerability could be exploited by malicious processes attempting to cause denial of service conditions through careful crafting of memory access patterns that trigger the specific code path leading to deadlock.
The operational impact of this vulnerability is severe as it renders the calling thread completely unresponsive and unkillable, effectively creating a system-wide resource exhaustion scenario. This type of deadlock can be particularly problematic in production environments where kernel-level operations are critical for system stability and performance. The fix addresses the issue by implementing a direct page table modification approach using make_uffd_wp_huge_pte() function that operates under the page-table lock without invoking the problematic hugetlb_change_protection() path, thereby eliminating the need for additional vma locking operations.
The solution implements a pattern similar to how populated entries are handled, where write-protection is applied directly within the page table context rather than routing through the complex vma locking hierarchy. This approach leverages the fact that uffd-wp VMAs have PMD sharing disabled through hugetlb_unshare_all_pmds() during registration, making it safe to skip the vma write lock operation. The fix aligns with kernel memory management best practices by maintaining atomicity within page table operations and avoiding cross-layer locking dependencies that could lead to deadlocks.
Security implications extend beyond simple denial of service as this vulnerability could potentially be exploited in privilege escalation scenarios or as part of broader attack chains targeting system stability. The issue affects all Linux kernel versions supporting hugetlb functionality where PAGEMAP_SCAN operations with PM_SCAN_WP_MATCHING are possible, making it a widespread concern for systems utilizing huge page memory management features. System administrators should prioritize applying patches addressing this vulnerability to prevent potential exploitation and maintain overall system reliability.
The resolution demonstrates proper kernel design principles by ensuring that locking operations remain within appropriate scope and avoid creating circular dependencies. The fix maintains the semantic correctness of uffd-wp functionality while eliminating the race condition that led to the deadlock, preserving both performance characteristics and memory management integrity. This approach follows established kernel development patterns where complex locking scenarios are simplified through direct manipulation of underlying data structures rather than relying on higher-level abstractions that may introduce unintended dependencies.