CVE-2026-68086 (GCVE-0-2026-68086)

Vulnerability from cvelistv5 – Published: 2026-08-10 11:51 – Updated: 2026-08-10 11:51
VLAI
Title
mm/khugepaged: write all dirty file folios when collapsing
Summary
In the Linux kernel, the following vulnerability has been resolved: mm/khugepaged: write all dirty file folios when collapsing [There is no upstream commit, as this code was removed by upstream commit 044925f9b565 ("mm: fs: remove filemap_nr_thps*() functions and their users")] As-is, khugepaged and writable-file opening exclude each other. A file cannot be open writeable and have THPs (because the filesystem is not aware of them). khugepaged will never collapse file pages for files that are opened writeable. On an open(O_RDWR/O_WRONLY), the page cache for that particular file is dropped. This is fine because nothing could've been dirtied. However, there is an edge-case: collapse_file() might not be able to coexist with concurrent writers, but it can coexist with dirty folios (from previous writers). Therefore, the following can happen: open(file, O_RDWR) write(file) close(file) madvise(file_mapping, MADV_COLLAPSE, some non-dirty range) open(file, O_RDWR) nr_thps > 0 truncate_inode_pages() /* THPs are cleared out, but so are the dirty folios */ When this edge-case happens, there is data loss, as the dirty folios are fully discarded. Fix it by fully writing back the page cache (and waiting) when collapsing file THPs. Doing so provides the guarantee that no dirty folio will be observed while there are active THPs. To fully ensure this is safe, the invalidate_lock needs to be held while doing the writeout, so that do_dentry_open()'s page cache truncation excludes this write-and-wait. As a side effect, move the nr_thps counter bumping outside the i_pages lock. This is correct since the counter itself is an atomic_t and the producer <-> consumer correctness is provided by a full memory barrier: smp_mb() in collapse_file()/memory barrier implied by full ordering in get_write_access() -> atomic_inc_unless_negative().
Severity
No CVSS data available.
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 99cb0dbd47a15d395bf3faa78dc122bc5efe3fc0 , < 2dfe9f5c91d0963058f8a5e46e1c2a908382cc46 (git)
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Linux Linux Affected: 5.4
Unaffected: 0 , < 5.4 (semver)
Unaffected: 7.1.4 , ≤ 7.1.* (semver)
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{
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm/khugepaged: write all dirty file folios when collapsing\n\n[There is no upstream commit, as this code was removed by upstream\n commit 044925f9b565 (\"mm: fs: remove filemap_nr_thps*() functions and their users\")]\n\nAs-is, khugepaged and writable-file opening exclude each other. A file\ncannot be open writeable and have THPs (because the filesystem is not aware\nof them). khugepaged will never collapse file pages for files that are\nopened writeable. On an open(O_RDWR/O_WRONLY), the page cache for that\nparticular file is dropped. This is fine because nothing could\u0027ve been\ndirtied.\n\nHowever, there is an edge-case: collapse_file() might not be able to\ncoexist with concurrent writers, but it can coexist with dirty folios\n(from previous writers). Therefore, the following can happen:\n\nopen(file, O_RDWR)\nwrite(file)\nclose(file)\nmadvise(file_mapping, MADV_COLLAPSE, some non-dirty range)\nopen(file, O_RDWR)\n nr_thps \u003e 0\n  truncate_inode_pages()\n    /* THPs are cleared out, but so are the dirty folios */\n\nWhen this edge-case happens, there is data loss, as the dirty folios are\nfully discarded.\n\nFix it by fully writing back the page cache (and waiting) when collapsing\nfile THPs. Doing so provides the guarantee that no dirty folio will be\nobserved while there are active THPs. To fully ensure this is safe, the\ninvalidate_lock needs to be held while doing the writeout, so that\ndo_dentry_open()\u0027s page cache truncation excludes this write-and-wait.\n\nAs a side effect, move the nr_thps counter bumping outside the i_pages\nlock. This is correct since the counter itself is an atomic_t and the\nproducer \u003c-\u003e consumer correctness is provided by a full memory barrier:\nsmp_mb() in collapse_file()/memory barrier implied by full ordering in\nget_write_access() -\u003e atomic_inc_unless_negative()."
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-10T11:51:40.304Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
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          "url": "https://git.kernel.org/stable/c/2dfe9f5c91d0963058f8a5e46e1c2a908382cc46"
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      "title": "mm/khugepaged: write all dirty file folios when collapsing",
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    "cveId": "CVE-2026-68086",
    "datePublished": "2026-08-10T11:51:40.304Z",
    "dateReserved": "2026-07-30T09:28:09.367Z",
    "dateUpdated": "2026-08-10T11:51:40.304Z",
    "state": "PUBLISHED"
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        "cveTags": [],
        "descriptions": [
          {
            "lang": "en",
            "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm/khugepaged: write all dirty file folios when collapsing\n\n[There is no upstream commit, as this code was removed by upstream\n commit 044925f9b565 (\"mm: fs: remove filemap_nr_thps*() functions and their users\")]\n\nAs-is, khugepaged and writable-file opening exclude each other. A file\ncannot be open writeable and have THPs (because the filesystem is not aware\nof them). khugepaged will never collapse file pages for files that are\nopened writeable. On an open(O_RDWR/O_WRONLY), the page cache for that\nparticular file is dropped. This is fine because nothing could\u0027ve been\ndirtied.\n\nHowever, there is an edge-case: collapse_file() might not be able to\ncoexist with concurrent writers, but it can coexist with dirty folios\n(from previous writers). Therefore, the following can happen:\n\nopen(file, O_RDWR)\nwrite(file)\nclose(file)\nmadvise(file_mapping, MADV_COLLAPSE, some non-dirty range)\nopen(file, O_RDWR)\n nr_thps \u003e 0\n  truncate_inode_pages()\n    /* THPs are cleared out, but so are the dirty folios */\n\nWhen this edge-case happens, there is data loss, as the dirty folios are\nfully discarded.\n\nFix it by fully writing back the page cache (and waiting) when collapsing\nfile THPs. Doing so provides the guarantee that no dirty folio will be\nobserved while there are active THPs. To fully ensure this is safe, the\ninvalidate_lock needs to be held while doing the writeout, so that\ndo_dentry_open()\u0027s page cache truncation excludes this write-and-wait.\n\nAs a side effect, move the nr_thps counter bumping outside the i_pages\nlock. This is correct since the counter itself is an atomic_t and the\nproducer \u003c-\u003e consumer correctness is provided by a full memory barrier:\nsmp_mb() in collapse_file()/memory barrier implied by full ordering in\nget_write_access() -\u003e atomic_inc_unless_negative()."
          }
        ],
        "id": "CVE-2026-68086",
        "lastModified": "2026-08-10T12:17:21.207",
        "metrics": {},
        "published": "2026-08-10T12:17:21.207",
        "references": [
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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      "product_status:fixed": "612",
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      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: mm/khugepaged: write all dirty file folios when collapsing",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-68086.json",
      "version": "3"
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      "cve": "CVE-2026-68086",
      "id": "CVE-2026-68086",
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      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-68086",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-68086.json",
      "version": "9"
    }
  }
}



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