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

Vulnerability from cvelistv5 – Published: 2026-07-19 14:55 – Updated: 2026-08-05 12:37
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 CPE status
Linux Linux Affected: 354dffd29575cdf13154e8fb787322354aa9efc4 , < 0fcc34d0d8fefca4fea349e45c10e3a3d90350eb (git)
Affected: 354dffd29575cdf13154e8fb787322354aa9efc4 , < f611db9b771b2b6775357555d2517af044fca4f0 (git)
Affected: 354dffd29575cdf13154e8fb787322354aa9efc4 , < 3f8968e9cbf95d5d87d32218906cab0b9b9eddbe (git)
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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)
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          "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."
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            "baseSeverity": "HIGH",
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            "version": "3.1"
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              "value": "AV:L - Exploitation requires local syscalls (mmap with MAP_DROPPABLE, madvise MADV_FREE/MADV_PAGEOUT, and a device-exclusive ioctl such as HMM_DMIRROR_EXCLUSIVE or GPU SVM) that drive kernel memory reclaim into try_to_unmap_one(); there is no network-facing entry point.\nAC:L - The reporter provides a deterministic userspace reproducer and the attacker fully controls the memory layout (64K lazyfree large folio with one device-exclusive PTE) and reclaim triggering; no race or layout condition outside attacker control is required beyond having DEVICE_PRIVATE/THP enabled.\nPR:L - A normal unprivileged local process can perform the required mmap/madvise sequence and trigger reclaim via MADV_PAGEOUT or memory pressure; marking a page device-exclusive only needs access to a GPU render node or hmm_dmirror test device, which is routinely granted to unprivileged users on GPU-equipped systems.\nUI:N - No victim interaction is required; the attacker runs the exploit sequence in their own process to corrupt folio refcount/mapcount during reclaim.\nS:U - Impact is confined to kernel memory management within the same security authority; this is standard kernel memory corruption/privilege-escalation territory, not a VM escape or cross-authority boundary bypass.\nC:H - Stale nr_pages causes folio_remove_rmap_ptes() and folio_put_refs() to operate on the wrong page count, corrupting mapcount and refcount; the author confirms refcount/mapcount corruption and this class of bug can yield use-after-free with arbitrary read primitives.\nI:H - Incorrect multi-page rmap removal and reference dropping on a single device-exclusive PTE is exploitable memory corruption that can lead to use-after-free and arbitrary kernel write/control-flow hijack, not merely a bounded modification.\nA:H - The fix commit and reporter explicitly state the userspace-visible effect is kernel crashes (oops/panic) from refcount/mapcount corruption, observed at folio_remove_rmap_ptes in try_to_unmap_one."
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        "dateUpdated": "2026-08-05T12:37:35.244Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
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        {
          "url": "https://git.kernel.org/stable/c/3f8968e9cbf95d5d87d32218906cab0b9b9eddbe"
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    "cveId": "CVE-2026-63950",
    "datePublished": "2026-07-19T14:55:43.404Z",
    "dateReserved": "2026-07-19T07:54:57.023Z",
    "dateUpdated": "2026-08-05T12:37:35.244Z",
    "state": "PUBLISHED"
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    "redhat_vex": {
      "aggregate_severity": "Important",
      "current_release_date": "2026-07-21T22:21:45+00:00",
      "cve": "CVE-2026-63950",
      "id": "CVE-2026-63950",
      "initial_release_date": "2026-07-19T00:00:00+00:00",
      "product_status:known_affected": "76",
      "product_status:known_not_affected": "184",
      "product_status:under_investigation": "14",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-63950.json",
      "version": "3"
    }
  }
}



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