{
  "schemaVersion": 1,
  "attribution": [
    {
      "source": "CISA Known Exploited Vulnerabilities",
      "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog",
      "licence": "US government work"
    },
    {
      "source": "GitHub Advisory Database",
      "url": "https://github.com/advisories",
      "licence": "CC BY 4.0"
    },
    {
      "source": "Have I Been Pwned",
      "url": "https://haveibeenpwned.com/",
      "licence": "CC BY 4.0"
    },
    {
      "source": "CVE Program (CVE records)",
      "url": "https://www.cve.org/Legal/TermsOfUse",
      "licence": "CVE Terms of Use",
      "notice": "Copyright © 1999-2026, The MITRE Corporation. CVE is a trademark and the CVE logo is a registered trademark of The MITRE Corporation.",
      "terms": "CVE Usage: MITRE hereby grants you a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable copyright license to reproduce, prepare derivative works of, publicly display, publicly perform, sublicense, and distribute Common Vulnerabilities and Exposures (CVE™). Any copy you make for such purposes is authorized provided that you reproduce MITRE's copyright designation and this license in any such copy."
    },
    {
      "source": "FIRST EPSS",
      "url": "https://www.first.org/epss/",
      "licence": "free to use; FIRST requests attribution"
    }
  ],
  "record": {
    "id": "CVE-2024-46787",
    "sourceURL": "https://www.cve.org/CVERecord?id=CVE-2024-46787",
    "idYear": 2024,
    "status": "PUBLISHED",
    "publishedAt": "2024-09-18T07:12:43.264Z",
    "updatedAt": "2026-08-05T11:38:33.153Z",
    "importedAt": "2026-10-24T01:57:16.009Z",
    "snapshot": {
      "id": "cve_2026-10-24_midnight_baseline",
      "at": "2026-10-24T00:00:00.000Z",
      "source": "https://github.com/CVEProject/cvelistV5/releases/tag/cve_2026-10-24_0100Z"
    },
    "facts": {
      "title": "userfaultfd: fix checks for huge PMDs",
      "summary": "In the Linux kernel, the following vulnerability has been resolved:\n\nuserfaultfd: fix checks for huge PMDs\n\nPatch series \"userfaultfd: fix races around pmd_trans_huge() check\", v2.\n\nThe pmd_trans_huge() code in mfill_atomic() is wrong in three different\nways depending on kernel version:\n\n1. The pmd_trans_huge() check is racy and can lead to a BUG_ON() (if you hit\n   the right two race windows) - I've tested this in a kernel build with\n   some extra mdelay() calls. See the commit message for a description\n   of the race scenario.\n   On older kernels (before 6.5), I think the same bug can even\n   theoretically lead to accessing transhuge page contents as a page table\n   if you hit the right 5 narrow race windows (I haven't tested this case).\n2. As pointed out by Qi Zheng, pmd_trans_huge() is not sufficient for\n   detecting PMDs that don't point to page tables.\n   On older kernels (before 6.5), you'd just have to win a single fairly\n   wide race to hit this.\n   I've tested this on 6.1 stable by racing migration (with a mdelay()\n   patched into try_to_migrate()) against UFFDIO_ZEROPAGE - on my x86\n   VM, that causes a kernel oops in ptlock_ptr().\n3. On newer kernels (>=6.5), for shmem mappings, khugepaged is allowed\n   to yank page tables out from under us (though I haven't tested that),\n   so I think the BUG_ON() checks in mfill_atomic() are just wrong.\n\nI decided to write two separate fixes for these (one fix for bugs 1+2, one\nfix for bug 3), so that the first fix can be backported to kernels\naffected by bugs 1+2.\n\n\nThis patch (of 2):\n\nThis fixes two issues.\n\nI discovered that the following race can occur:\n\n  mfill_atomic                other thread\n  ============                ============\n                              <zap PMD>\n  pmdp_get_lockless() [reads none pmd]\n  <bail if trans_huge>\n  <if none:>\n                              <pagefault creates transhuge zeropage>\n    __pte_alloc [no-op]\n                              <zap PMD>\n  <bail if pmd_trans_huge(*dst_pmd)>\n  BUG_ON(pmd_none(*dst_pmd))\n\nI have experimentally verified this in a kernel with extra mdelay() calls;\nthe BUG_ON(pmd_none(*dst_pmd)) triggers.\n\nOn kernels newer than commit 0d940a9b270b (\"mm/pgtable: allow\npte_offset_map[_lock]() to fail\"), this can't lead to anything worse than\na BUG_ON(), since the page table access helpers are actually designed to\ndeal with page tables concurrently disappearing; but on older kernels\n(<=6.4), I think we could probably theoretically race past the two\nBUG_ON() checks and end up treating a hugepage as a page table.\n\nThe second issue is that, as Qi Zheng pointed out, there are other types\nof huge PMDs that pmd_trans_huge() can't catch: devmap PMDs and swap PMDs\n(in particular, migration PMDs).\n\nOn <=6.4, this is worse than the first issue: If mfill_atomic() runs on a\nPMD that contains a migration entry (which just requires winning a single,\nfairly wide race), it will pass the PMD to pte_offset_map_lock(), which\nassumes that the PMD points to a page table.\n\nBreakage follows: First, the kernel tries to take the PTE lock (which will\ncrash or maybe worse if there is no \"struct page\" for the address bits in\nthe migration entry PMD - I think at least on X86 there usually is no\ncorresponding \"struct page\" thanks to the PTE inversion mitigation, amd64\nlooks different).\n\nIf that didn't crash, the kernel would next try to write a PTE into what\nit wrongly thinks is a page table.\n\nAs part of fixing these issues, get rid of the check for pmd_trans_huge()\nbefore __pte_alloc() - that's redundant, we're going to have to check for\nthat after the __pte_alloc() anyway.\n\nBackport note: pmdp_get_lockless() is pmd_read_atomic() in older kernels.",
      "affected": [
        {
          "vendor": "Linux",
          "product": "Linux",
          "defaultStatus": "unaffected",
          "versions": [
            {
              "version": "c1a4de99fada21e2e9251e52cbb51eff5aadc757",
              "status": "affected",
              "versionType": "git",
              "lessThan": "3c6b4bcf37845c9359aed926324bed66bdd2448d"
            },
            {
              "version": "c1a4de99fada21e2e9251e52cbb51eff5aadc757",
              "status": "affected",
              "versionType": "git",
              "lessThan": "98cc18b1b71e23fe81a5194ed432b20c2d81a01a"
            },
            {
              "version": "c1a4de99fada21e2e9251e52cbb51eff5aadc757",
              "status": "affected",
              "versionType": "git",
              "lessThan": "71c186efc1b2cf1aeabfeff3b9bd5ac4c5ac14d8"
            }
          ]
        },
        {
          "vendor": "Linux",
          "product": "Linux",
          "defaultStatus": "affected",
          "versions": [
            {
              "version": "4.3",
              "status": "affected"
            },
            {
              "version": "0",
              "status": "unaffected",
              "versionType": "semver",
              "lessThan": "4.3"
            },
            {
              "version": "6.6.51",
              "status": "unaffected",
              "versionType": "semver",
              "lessThanOrEqual": "6.6.*"
            },
            {
              "version": "6.10.10",
              "status": "unaffected",
              "versionType": "semver",
              "lessThanOrEqual": "6.10.*"
            },
            {
              "version": "6.11",
              "status": "unaffected",
              "versionType": "original_commit_for_fix",
              "lessThanOrEqual": "*"
            }
          ]
        }
      ]
    }
  }
}