mm: userfaultfd: fix race of userfaultfd_move and swap cache

{
  "id": "5b11821ce11a557457069673385b0e09de9a068bf1c16817fcf8691960462b17",
  "url": "https://osv.dev/vulnerability/CVE-2025-38242",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "mm: userfaultfd: fix race of userfaultfd_move and swap cache",
    "summary": "In the Linux kernel, the following vulnerability has been resolved: mm: userfaultfd: fix race of userfaultfd_move and swap cache This commit fixes two kinds of races, they may have different results: Barry reported a BUG_ON in commit c50f8e6053b0, we may see the same BUG_ON if the filemap lookup returned NULL and folio is added to swap cache after that. If another kind of race is triggered (folio changed after lookup) we may see RSS counter is corrupted: [ 406.893936] BUG: Bad rss-counter state mm:ffff0000c5a9ddc0 type:MM_ANONPAGES val:-1 [ 406.894071] BUG: Bad rss-counter state mm:ffff0000c5a9ddc0 type:MM_SHMEMPAGES val:1 Because the folio is being accounted to the wrong VMA. I'm not sure if there will be any data corruption though, seems no. The issues above are critical already. On seeing a swap entry PTE, userfaultfd_move does a lockless swap cache lookup, and tries to move the found folio to the faulting vma. Currently, it relies on checking the PTE value to ensure that the moved folio still belongs to the src swap entry and that no new folio has been added to the swap cache, which turns out to be unreliable. While working and reviewing the swap table series with Barry, following existing races are observed and reproduced [1]: In the example below, move_pages_pte is moving src_pte to dst_pte, where src_pte is a swap entry PTE holding swap entry S1, and S1 is not in the swap cache: CPU1 CPU2 userfaultfd_move move_pages_pte() entry = pte_to_swp_entry(orig_src_pte); // Here",
    "evidence": "osv:CVE-2025-38242",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
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            "value": "6.12.37",
            "evidence": "/affected/1/ranges"
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    "severity": "high",
    "structured": {
      "id": "CVE-2025-38242",
      "aliases": [
        "CVE-2025-38242"
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      "metrics": [
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          "type": "CVSS_V3",
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      "modified": "2026-10-28T02:30:25.478159297Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/0ea148a799198518d8ebab63ddd0bb6114a103bc",
        "https://git.kernel.org/stable/c/4c443046d8c9ed8724a4f4c3c2457d3ac8814b2f",
        "https://git.kernel.org/stable/c/db2ca8074955ca64187a4fb596dd290b9c446cd3",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/38xxx/CVE-2025-38242.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2025-38242",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
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    "severity_evidence": "/severity/0/score"
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  "source_id": "core-osv",
  "document_id": 491768,
  "published_at": "2025-07-09T10:42:25.396+00:00"
}