FKIE_CVE-2026-68086

Vulnerability from fkie_nvd - Published: 2026-08-10 12:17 - Updated: 2026-08-10 12:17
Severity
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().
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/2dfe9f5c91d0963058f8a5e46e1c2a908382cc46
Impacted products
Vendor Product Version
To clipboard 

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "mm/khugepaged.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "2dfe9f5c91d0963058f8a5e46e1c2a908382cc46",
              "status": "affected",
              "version": "99cb0dbd47a15d395bf3faa78dc122bc5efe3fc0",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "mm/khugepaged.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "5.4"
            },
            {
              "lessThan": "5.4",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.1.*",
              "status": "unaffected",
              "version": "7.1.4",
              "versionType": "semver"
            }
          ]
        }
      ],
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "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",
      "url": "https://git.kernel.org/stable/c/2dfe9f5c91d0963058f8a5e46e1c2a908382cc46"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Received"
}



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