CVE-2025-38084 (GCVE-0-2025-38084)
Vulnerability from cvelistv5
Published
2025-06-28 07:44
Modified
2026-08-05 11:59
Severity ?
7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: mm/hugetlb: unshare page tables during VMA split, not before Currently, __split_vma() triggers hugetlb page table unsharing through vm_ops->may_split(). This happens before the VMA lock and rmap locks are taken - which is too early, it allows racing VMA-locked page faults in our process and racing rmap walks from other processes to cause page tables to be shared again before we actually perform the split. Fix it by explicitly calling into the hugetlb unshare logic from __split_vma() in the same place where THP splitting also happens. At that point, both the VMA and the rmap(s) are write-locked. An annoying detail is that we can now call into the helper hugetlb_unshare_pmds() from two different locking contexts: 1. from hugetlb_split(), holding: - mmap lock (exclusively) - VMA lock - file rmap lock (exclusively) 2. hugetlb_unshare_all_pmds(), which I think is designed to be able to call us with only the mmap lock held (in shared mode), but currently only runs while holding mmap lock (exclusively) and VMA lock Backporting note: This commit fixes a racy protection that was introduced in commit b30c14cd6102 ("hugetlb: unshare some PMDs when splitting VMAs"); that commit claimed to fix an issue introduced in 5.13, but it should actually also go all the way back. [jannh@google.com: v2]
References
Impacted products
Vendor Product Version
Linux Linux Version: 39dde65c9940c97fcd178a3d2b1c57ed8b7b68aa
Version: 39dde65c9940c97fcd178a3d2b1c57ed8b7b68aa
Version: 39dde65c9940c97fcd178a3d2b1c57ed8b7b68aa
Version: 39dde65c9940c97fcd178a3d2b1c57ed8b7b68aa
Version: 39dde65c9940c97fcd178a3d2b1c57ed8b7b68aa
Version: 39dde65c9940c97fcd178a3d2b1c57ed8b7b68aa
Version: 39dde65c9940c97fcd178a3d2b1c57ed8b7b68aa
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm/hugetlb: unshare page tables during VMA split, not before\n\nCurrently, __split_vma() triggers hugetlb page table unsharing through\nvm_ops-\u003emay_split().  This happens before the VMA lock and rmap locks are\ntaken - which is too early, it allows racing VMA-locked page faults in our\nprocess and racing rmap walks from other processes to cause page tables to\nbe shared again before we actually perform the split.\n\nFix it by explicitly calling into the hugetlb unshare logic from\n__split_vma() in the same place where THP splitting also happens.  At that\npoint, both the VMA and the rmap(s) are write-locked.\n\nAn annoying detail is that we can now call into the helper\nhugetlb_unshare_pmds() from two different locking contexts:\n\n1. from hugetlb_split(), holding:\n    - mmap lock (exclusively)\n    - VMA lock\n    - file rmap lock (exclusively)\n2. hugetlb_unshare_all_pmds(), which I think is designed to be able to\n   call us with only the mmap lock held (in shared mode), but currently\n   only runs while holding mmap lock (exclusively) and VMA lock\n\nBackporting note:\nThis commit fixes a racy protection that was introduced in commit\nb30c14cd6102 (\"hugetlb: unshare some PMDs when splitting VMAs\"); that\ncommit claimed to fix an issue introduced in 5.13, but it should actually\nalso go all the way back.\n\n[jannh@google.com: v2]"
        }
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            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
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          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - The race is driven entirely through local syscalls \u2014 `mmap(MAP_SHARED|MAP_HUGETLB)` plus `munmap`/`mprotect`/`madvise` to force a non-PUD-aligned `__split_vma()`, with concurrent threads faulting the mapping. No network or physical component is involved.\nAC:L - The attacker controls both sides of the race and the window is very wide \u2014 several sleeping GFP_KERNEL allocations sit between `may_split()` and `vma_start_write()` \u2014 and it can be retried indefinitely and widened with memory pressure. PMD sharing is compiled in by default on x86_64/arm64/riscv64, and the only environmental dependency (preallocated hugepages) is standard in the KVM/cloud, database and DPDK deployments this code targets.\nPR:L - An ordinary unprivileged user can create the required VM_MAYSHARE hugetlb mapping, because `mmap(MAP_ANONYMOUS|MAP_HUGETLB)` uses `HUGETLB_ANONHUGE_INODE` and therefore bypasses the `can_do_hugetlb_shm()`/`CAP_IPC_LOCK` check that guards SysV `SHM_HUGETLB`; `fork()` supplies the second sharing VMA. No capability is needed for the splitting syscalls either.\nUI:N - The whole sequence is performed programmatically by the attacker\u0027s own multi-threaded process; no victim action or interaction is required.\nS:U - The corrupted page tables belong to the attacking process\u0027s own mm within the host kernel\u0027s security authority, so this is standard local kernel compromise rather than a crossing of a VM/IOMMU boundary.\nC:H - A freed page-table page that is still referenced by a live PUD entry is the classic \"dirty pagetable\" primitive \u2014 once reallocated, the attacker can read arbitrary physical memory, disclosing kernel data and other processes\u0027 memory in full.\nI:H - The same page-table UAF yields an arbitrary physical-memory write primitive (writing PTEs into a reused page table), enabling kernel data modification, credential overwrite and full privilege escalation; it also leaves dangling KVM MMU mappings.\nA:H - Use-after-free of page tables reliably corrupts the memory management state, producing oopses, BUG_ON hits and kernel panics even in unsuccessful exploitation attempts, and it can be triggered repeatedly."
            }
          ]
        }
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      "providerMetadata": {
        "dateUpdated": "2026-08-05T11:59:22.391Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
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          "url": "https://git.kernel.org/stable/c/e8847d18cd9fff1edbb45e963d9141273c3b539c"
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        {
          "url": "https://git.kernel.org/stable/c/366298f2b04d2bf1f2f2b7078405bdf9df9bd5d0"
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          "url": "https://git.kernel.org/stable/c/2511ac64bc1617ca716d3ba8464e481a647c1902"
        },
        {
          "url": "https://git.kernel.org/stable/c/af6cfcd0efb7f051af221c418ec8b37a10211947"
        },
        {
          "url": "https://git.kernel.org/stable/c/9cf5b2a3b72c23fb7b84736d5d19ee6ea718762b"
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          "url": "https://git.kernel.org/stable/c/8a21d5584826f4880f45bbf8f72375f4e6c0ff2a"
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          "url": "https://git.kernel.org/stable/c/081056dc00a27bccb55ccc3c6f230a3d5fd3f7e0"
        },
        {
          "url": "https://project-zero.issues.chromium.org/issues/420715744"
        }
      ],
      "title": "mm/hugetlb: unshare page tables during VMA split, not before",
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    "cveId": "CVE-2025-38084",
    "datePublished": "2025-06-28T07:44:25.379Z",
    "dateReserved": "2025-04-16T04:51:23.981Z",
    "dateUpdated": "2026-08-05T11:59:22.391Z",
    "state": "PUBLISHED"
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