CVE-2026-63950 (GCVE-0-2026-63950)

Vulnerability from cvelistv5 – Published: 2026-07-19 14:55 – Updated: 2026-07-20 13:41
VLAI
Title
mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one
Summary
In the Linux kernel, the following vulnerability has been resolved: mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one Initialize nr_pages to 1 at the start of each loop iteration, like folio_referenced_one() does. Without this, nr_pages computed by a previous folio_unmap_pte_batch() call can be reused on a later iteration that does not run folio_unmap_pte_batch() again. mmap a 64K large folio with MAP_ANONYMOUS | MAP_DROPPABLE, then call madvise(MADV_FREE), then make the last page device-exclusive via HMM_DMIRROR_EXCLUSIVE. Trigger node reclaim through sysfs. Now, in try_to_unmap_one(), we will first clear the first 15 out of 16 entries mapping the lazyfree folio. This will set nr_pages to 15. In the next pvmw walk, this nr_pages gets reused on a device-exclusive pte, thus potentially corrupting folio refcount/mapcount. At the moment, I have a userspace program which can make the kernel spit out a trace, but the blow up is in folio_referenced_one(), because there are existing bugs in the interaction between device-private and rmap (which too I am investigating). I did a one liner kernel change to avoid going into folio_referenced_one(), and the kernel blows up at folio_remove_rmap_ptes in try_to_unmap_one which is what I wanted. Note that the bug is there not since file folio batching but lazyfree folio batching, since device-exclusive only works for anonymous folios. Userspace visible effect is simply kernel crashing somewhere due to refcount/mapcount corruption.
Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: 354dffd29575cdf13154e8fb787322354aa9efc4 , < 0fcc34d0d8fefca4fea349e45c10e3a3d90350eb (git)
Affected: 354dffd29575cdf13154e8fb787322354aa9efc4 , < f611db9b771b2b6775357555d2517af044fca4f0 (git)
Affected: 354dffd29575cdf13154e8fb787322354aa9efc4 , < 3f8968e9cbf95d5d87d32218906cab0b9b9eddbe (git)
Create a notification for this product.
Linux Linux Affected: 6.15
Unaffected: 0 , < 6.15 (semver)
Unaffected: 6.18.35 , ≤ 6.18.* (semver)
Unaffected: 7.0.12 , ≤ 7.0.* (semver)
Unaffected: 7.1 , ≤ * (original_commit_for_fix)
Create a notification for this product.
Show details on NVD website
To clipboard 

{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "mm/rmap.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "0fcc34d0d8fefca4fea349e45c10e3a3d90350eb",
              "status": "affected",
              "version": "354dffd29575cdf13154e8fb787322354aa9efc4",
              "versionType": "git"
            },
            {
              "lessThan": "f611db9b771b2b6775357555d2517af044fca4f0",
              "status": "affected",
              "version": "354dffd29575cdf13154e8fb787322354aa9efc4",
              "versionType": "git"
            },
            {
              "lessThan": "3f8968e9cbf95d5d87d32218906cab0b9b9eddbe",
              "status": "affected",
              "version": "354dffd29575cdf13154e8fb787322354aa9efc4",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "mm/rmap.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "6.15"
            },
            {
              "lessThan": "6.15",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.35",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.0.*",
              "status": "unaffected",
              "version": "7.0.12",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.1",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.18.35",
                  "versionStartIncluding": "6.15",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.0.12",
                  "versionStartIncluding": "6.15",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.1",
                  "versionStartIncluding": "6.15",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one\n\nInitialize nr_pages to 1 at the start of each loop iteration, like\nfolio_referenced_one() does.\n\nWithout this, nr_pages computed by a previous folio_unmap_pte_batch() call\ncan be reused on a later iteration that does not run\nfolio_unmap_pte_batch() again.\n\nmmap a 64K large folio with MAP_ANONYMOUS | MAP_DROPPABLE, then call\nmadvise(MADV_FREE), then make the last page device-exclusive via\nHMM_DMIRROR_EXCLUSIVE.\n\nTrigger node reclaim through sysfs.  Now, in try_to_unmap_one(), we will\nfirst clear the first 15 out of 16 entries mapping the lazyfree folio. \nThis will set nr_pages to 15.  In the next pvmw walk, this nr_pages gets\nreused on a device-exclusive pte, thus potentially corrupting folio\nrefcount/mapcount.\n\nAt the moment, I have a userspace program which can make the kernel spit\nout a trace, but the blow up is in folio_referenced_one(), because there\nare existing bugs in the interaction between device-private and rmap\n(which too I am investigating).  I did a one liner kernel change to avoid\ngoing into folio_referenced_one(), and the kernel blows up at\nfolio_remove_rmap_ptes in try_to_unmap_one which is what I wanted.\n\nNote that the bug is there not since file folio batching but lazyfree\nfolio batching, since device-exclusive only works for anonymous folios.\n\nUserspace visible effect is simply kernel crashing somewhere due to\nrefcount/mapcount corruption."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.8,
            "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"
          }
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-07-20T13:41:49.078Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/0fcc34d0d8fefca4fea349e45c10e3a3d90350eb"
        },
        {
          "url": "https://git.kernel.org/stable/c/f611db9b771b2b6775357555d2517af044fca4f0"
        },
        {
          "url": "https://git.kernel.org/stable/c/3f8968e9cbf95d5d87d32218906cab0b9b9eddbe"
        }
      ],
      "title": "mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63950",
    "datePublished": "2026-07-19T14:55:43.404Z",
    "dateReserved": "2026-07-19T07:54:57.023Z",
    "dateUpdated": "2026-07-20T13:41:49.078Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-63950",
      "date": "2026-07-21",
      "epss": "0.00161",
      "percentile": "0.0575"
    },
    "nvd": "{\"cve\":{\"id\":\"CVE-2026-63950\",\"sourceIdentifier\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"published\":\"2026-07-19T16:17:13.730\",\"lastModified\":\"2026-07-20T15:16:58.600\",\"vulnStatus\":\"Received\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"In the Linux kernel, the following vulnerability has been resolved:\\n\\nmm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one\\n\\nInitialize nr_pages to 1 at the start of each loop iteration, like\\nfolio_referenced_one() does.\\n\\nWithout this, nr_pages computed by a previous folio_unmap_pte_batch() call\\ncan be reused on a later iteration that does not run\\nfolio_unmap_pte_batch() again.\\n\\nmmap a 64K large folio with MAP_ANONYMOUS | MAP_DROPPABLE, then call\\nmadvise(MADV_FREE), then make the last page device-exclusive via\\nHMM_DMIRROR_EXCLUSIVE.\\n\\nTrigger node reclaim through sysfs.  Now, in try_to_unmap_one(), we will\\nfirst clear the first 15 out of 16 entries mapping the lazyfree folio. \\nThis will set nr_pages to 15.  In the next pvmw walk, this nr_pages gets\\nreused on a device-exclusive pte, thus potentially corrupting folio\\nrefcount/mapcount.\\n\\nAt the moment, I have a userspace program which can make the kernel spit\\nout a trace, but the blow up is in folio_referenced_one(), because there\\nare existing bugs in the interaction between device-private and rmap\\n(which too I am investigating).  I did a one liner kernel change to avoid\\ngoing into folio_referenced_one(), and the kernel blows up at\\nfolio_remove_rmap_ptes in try_to_unmap_one which is what I wanted.\\n\\nNote that the bug is there not since file folio batching but lazyfree\\nfolio batching, since device-exclusive only works for anonymous folios.\\n\\nUserspace visible effect is simply kernel crashing somewhere due to\\nrefcount/mapcount corruption.\"}],\"affected\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"affectedData\":[{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"unaffected\",\"programFiles\":[\"mm/rmap.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"354dffd29575cdf13154e8fb787322354aa9efc4\",\"lessThan\":\"0fcc34d0d8fefca4fea349e45c10e3a3d90350eb\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"354dffd29575cdf13154e8fb787322354aa9efc4\",\"lessThan\":\"f611db9b771b2b6775357555d2517af044fca4f0\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"354dffd29575cdf13154e8fb787322354aa9efc4\",\"lessThan\":\"3f8968e9cbf95d5d87d32218906cab0b9b9eddbe\",\"versionType\":\"git\",\"status\":\"affected\"}]},{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"affected\",\"programFiles\":[\"mm/rmap.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"6.15\",\"status\":\"affected\"},{\"version\":\"0\",\"lessThan\":\"6.15\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.18.35\",\"lessThanOrEqual\":\"6.18.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.0.12\",\"lessThanOrEqual\":\"7.0.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.1\",\"lessThanOrEqual\":\"*\",\"versionType\":\"original_commit_for_fix\",\"status\":\"unaffected\"}]}]}],\"metrics\":{\"cvssMetricV31\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"type\":\"Secondary\",\"cvssData\":{\"version\":\"3.1\",\"vectorString\":\"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H\",\"baseScore\":7.8,\"baseSeverity\":\"HIGH\",\"attackVector\":\"LOCAL\",\"attackComplexity\":\"LOW\",\"privilegesRequired\":\"LOW\",\"userInteraction\":\"NONE\",\"scope\":\"UNCHANGED\",\"confidentialityImpact\":\"HIGH\",\"integrityImpact\":\"HIGH\",\"availabilityImpact\":\"HIGH\"},\"exploitabilityScore\":1.8,\"impactScore\":5.9}]},\"references\":[{\"url\":\"https://git.kernel.org/stable/c/0fcc34d0d8fefca4fea349e45c10e3a3d90350eb\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/3f8968e9cbf95d5d87d32218906cab0b9b9eddbe\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/f611db9b771b2b6775357555d2517af044fca4f0\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"}]}}"
  }
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.