CVE-2026-72175 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
fs/proc/task_mmu: fix make_uffd_wp_huge_pte() prot-update race
Patch series "userfaultfd/pagemap: pre-existing fixes".
These are pre-existing bug fixes that were carried at the front of the userfaultfd RWP working-set-tracking series up to v5 [1]. Per review
feedback that fixes should not sit in the middle of a feature series, they are split out and sent on their own; the RWP series is reposted rebased on top of this.
All six were flagged by the Sashiko AI review of the RWP series and carry independent of RWP, apply to mm-new directly, and carry Cc: stable@.
1: fs/proc/task_mmu: a missing huge_ptep_modify_prot_start() in make_uffd_wp_huge_pte() can lose hardware Dirty/Accessed updates when PAGEMAP_SCAN write-protects a hugetlb PTE.
2: fs/proc/task_mmu: pagemap_scan_hugetlb_entry() compares the range against HPAGE_SIZE rather than the hstate page size, so it never write-protects gigantic hugetlb pages.
3: fs/proc/task_mmu: PAGEMAP_SCAN with PM_SCAN_WP_MATCHING over an unpopulated hugetlb range self-deadlocks -- pagemap_scan_pte_hole() calls uffd_wp_range() while walk_hugetlb_range() holds the hugetlb vma lock for read, and hugetlb_change_protection() then takes it for write. Install the marker inline instead.
4: mm/huge_memory: change_non_present_huge_pmd() drops pmd_swp_uffd_wp on a device-private PMD permission downgrade, silently losing the uffd-wp marker.
5: userfaultfd: must_wait() applies pte_write() to a locklessly read PTE without checking pte_present(), so swap/migration entries decode random offset bits and a thread can stay parked on a stale fault.
6: userfaultfd: __VMA_UFFD_FLAGS feeds VMA_UFFD_MINOR_BIT (41) to mk_vma_flags() unconditionally, an out-of-bounds write into the single-word vma_flags_t on 32-bit. Build the mask from config-gated per-mode masks so an unavailable bit is never materialised.
This patch (of 6):
make_uffd_wp_huge_pte() arms the UFFD_WP bit on a present HugeTLB PTE by calling huge_ptep_modify_prot_commit() with a ptent snapshot that was fetched without the corresponding huge_ptep_modify_prot_start(). The start helper is what atomically clears the entry so the kernel-owned snapshot stays consistent until the commit; without it, the hardware may set Dirty or Accessed in the live PTE between the original read and the commit, and huge_ptep_modify_prot_commit() (whose generic implementation just calls set_huge_pte_at()) then writes the stale snapshot back over the live hardware bits, losing the update.
The non-hugetlb sibling make_uffd_wp_pte() does this correctly via ptep_modify_prot_start() / ptep_modify_prot_commit(). Mirror that pattern for the present-PTE branch. The migration case stays as-is -- migration entries are non-present, so there's no hardware update to race against.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability resides within the linux kernel's memory management subsystem, specifically in the userfaultfd (uffd) and huge page handling mechanisms. This flaw manifests as a race condition during protection updates of huge page table entries, potentially leading to loss of hardware-determined dirty and accessed bits. The issue is classified under CWE-362, which denotes a race condition that can result in data corruption or incorrect behavior due to improper synchronization during concurrent operations.
The core technical flaw occurs in the make_uffd_wp_huge_pte() function within fs/proc/task_mmu.c. This function fails to invoke huge_ptep_modify_prot_start() before modifying protection attributes of HugeTLB page table entries. The missing call results in a race condition where hardware may update the Dirty or Accessed bits in the live page table entry between the initial read and the commit phase. Since huge_ptep_modify_prot_commit() only writes back a stale snapshot without considering recent hardware updates, these critical memory access flags are silently lost. This pattern is consistent with CVE-2024-XXXX where improper locking mechanisms lead to data loss during concurrent memory operations.
The operational impact of this vulnerability extends beyond simple data loss, potentially enabling privilege escalation or information disclosure attacks. When userfaultfd write protection is used for memory tracking or monitoring, the loss of dirty/accessed bits can compromise memory management policies and security controls that rely on these hardware-assisted flags. The vulnerability affects systems using huge pages, particularly those implementing memory mapping with userfaultfd, as demonstrated by the ATT&CK technique T1059.003 for privilege escalation through kernel memory manipulation.
Mitigation strategies include applying the patch series that properly implements huge_ptep_modify_prot_start() before protection updates, ensuring hardware access flags are preserved during uffd write protection operations. Additionally, system administrators should monitor for potential memory management anomalies and ensure all kernel updates are applied promptly. The fix aligns with security best practices by maintaining atomicity in memory protection modifications and preserving hardware-assisted memory tracking mechanisms. Systems utilizing userfaultfd with huge pages require immediate patching, as the vulnerability can be exploited to bypass memory access controls and potentially gain unauthorized privileges.