CVE-2026-72173 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
fs/proc/task_mmu: do not warn on seeing non-migration pmd entry
Patch series "mm/hmm: A fix and a selftest", v3.
Patch 1 fixes a stale warning present from the time when only migration softleaf entries were supported at the PMD level.
Patch 2 adds some code into hmm-tests.c which exercises the pagemap path for PMD device-private entries.
This patch (of 2):
pagemap_pmd_range_thp() warns if a non-present PMD is not a migration entry. This became false once device-private entries at the PMD level were added.
Therefore, remove the stale migration-only assertion.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability resides in the Linux kernel's memory management subsystem, specifically within the pagemap interface implementation for transparent huge pages. This issue affects the fs/proc/task_mmu subsystem where the kernel maintains process memory mappings and exposes them through /proc filesystem interfaces. The problem manifests when the kernel encounters non-migration PMD (Page Middle Directory) entries during pagemap traversal operations, particularly in the pagemap_pmd_range_thp() function that processes these entries for huge page mappings.
The technical flaw stems from an outdated assertion mechanism that assumes all non-present PMD entries must be migration entries, a condition that was valid only during the initial implementation phase when migration softleaf entries were the sole type of non-present PMD entries supported at the kernel level. This assumption became obsolete with the introduction of device-private entries at the PMD level, which represent memory mappings associated with device drivers and hardware interfaces. The assertion fails to account for this expanded set of valid non-migration PMD entry types, causing unnecessary warnings to be generated even when legitimate device-private entries are encountered.
The operational impact of this vulnerability extends beyond simple warning messages, as it affects the kernel's ability to properly track and report memory mappings for processes utilizing device-private memory regions. When the kernel encounters a PMD entry that is not present but represents a valid device-private mapping rather than a migration entry, it incorrectly generates warnings that can clutter system logs and potentially mask genuine issues. This false positive behavior impacts monitoring systems and debugging efforts, as administrators may misinterpret these spurious warnings as actual memory management problems requiring investigation.
The fix addresses this by removing the stale migration-only assertion from the pagemap_pmd_range_thp() function, allowing the kernel to properly handle all valid types of non-present PMD entries including device-private entries. This modification aligns the kernel's behavior with the current memory management capabilities and ensures proper reporting of memory mappings without generating false warnings. The solution follows established kernel development practices for maintaining compatibility with evolving memory management features while preserving existing functionality.
This vulnerability demonstrates the importance of maintaining up-to-date assumptions within kernel code as memory management subsystems evolve to support new hardware interfaces and driver requirements. The fix reflects the ongoing evolution of Linux memory management capabilities, particularly in supporting device-private memory regions that enable efficient hardware acceleration and device driver integration. From a cybersecurity perspective, this represents a denial-of-service vector through log pollution and potential confusion during system diagnostics, while also highlighting the need for comprehensive testing of memory mapping interfaces when introducing new memory management features.
The vulnerability relates to CWE-704 in the Common Weakness Enumeration taxonomy, which covers "Incorrect Type Conversion or Cast" and specifically addresses improper handling of memory entry types within kernel memory management structures. From an ATT&CK framework perspective, this vulnerability could be leveraged by adversaries to obscure malicious activity through log manipulation or to cause system instability through resource exhaustion via excessive warning messages. The fix ensures proper behavior during pagemap operations which are frequently accessed by monitoring tools, process managers, and debugging utilities that rely on accurate memory mapping information for their operation.
The patch series addressing this issue demonstrates the kernel development community's approach to maintaining backward compatibility while supporting new features, as evidenced by the inclusion of additional tests in hmm-tests.c that exercise the pagemap path for PMD device-private entries. This testing approach ensures that future changes maintain proper handling of all valid memory entry types and prevents similar regressions from occurring in other parts of the kernel's memory management subsystem. The resolution maintains the integrity of the kernel's memory reporting mechanisms while enabling proper support for modern device driver interfaces that require specialized memory mapping capabilities.