CVE-2026-72170 (GCVE-0-2026-72170)
Vulnerability from cvelistv5
Published
2026-08-15 05:53
Modified
2026-08-23 12:46
Severity ?
7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: 9p: skip nlink update in cacheless mode to fix WARN_ON v9fs_dec_count() unconditionally calls drop_nlink() on regular files, even when the inode's nlink is already zero. In cacheless mode the client refetches inode metadata from the server (the source of truth) on every operation, so by the time v9fs_remove() returns, the locally cached nlink may already reflect the post-unlink value: 1. Client initiates unlink, server processes it and sets nlink to 0 2. Client refetches inode metadata (nlink=0) before unlink returns 3. Client's v9fs_remove() completes successfully 4. Client calls v9fs_dec_count() which calls drop_nlink() on nlink=0 This race is easily triggered under heavy unlink workloads, such as stress-ng's unlink stressor, producing the following warning: WARNING: fs/inode.c:417 at drop_nlink+0x4c/0xc8 Call trace: drop_nlink+0x4c/0xc8 v9fs_remove+0x1e0/0x250 [9p] v9fs_vfs_unlink+0x20/0x38 [9p] vfs_unlink+0x13c/0x258 ... In cacheless mode the server is authoritative and the inode is on its way out, so locally adjusting nlink buys nothing. Skip v9fs_dec_count() entirely when neither CACHE_META nor CACHE_LOOSE is set, which both avoids the warning and removes a class of nlink races (two concurrent unlinkers observing nlink > 0 and both calling drop_nlink()) that an nlink == 0 guard alone would only narrow rather than close.
References
Impacted products
Vendor Product Version
Linux Linux Version: ac89b2ef9b55924bcf922251f043ba73a32d05bb
Version: ac89b2ef9b55924bcf922251f043ba73a32d05bb
Version: ac89b2ef9b55924bcf922251f043ba73a32d05bb
Version: ac89b2ef9b55924bcf922251f043ba73a32d05bb
Version: ac89b2ef9b55924bcf922251f043ba73a32d05bb
Version: ac89b2ef9b55924bcf922251f043ba73a32d05bb
Version: ac89b2ef9b55924bcf922251f043ba73a32d05bb
Version: ac89b2ef9b55924bcf922251f043ba73a32d05bb
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/9p/vfs_inode.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
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            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
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          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "4.17"
            },
            {
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              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "5.10.*",
              "status": "unaffected",
              "version": "5.10.266",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "5.15.*",
              "status": "unaffected",
              "version": "5.15.217",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.1.*",
              "status": "unaffected",
              "version": "6.1.184",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.6.*",
              "status": "unaffected",
              "version": "6.6.145",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.12.*",
              "status": "unaffected",
              "version": "6.12.97",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.40",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.1.*",
              "status": "unaffected",
              "version": "7.1.5",
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            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.2",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
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                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "5.10.266",
                  "versionStartIncluding": "4.17",
                  "vulnerable": true
                },
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                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
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                  "vulnerable": true
                },
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                  "vulnerable": true
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                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.6.145",
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                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
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                {
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                  "versionEndExcluding": "7.1.5",
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                {
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                  "versionEndExcluding": "7.2",
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                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\n9p: skip nlink update in cacheless mode to fix WARN_ON\n\nv9fs_dec_count() unconditionally calls drop_nlink() on regular files,\neven when the inode\u0027s nlink is already zero. In cacheless mode the\nclient refetches inode metadata from the server (the source of truth)\non every operation, so by the time v9fs_remove() returns, the locally\ncached nlink may already reflect the post-unlink value:\n\n  1. Client initiates unlink, server processes it and sets nlink to 0\n  2. Client refetches inode metadata (nlink=0) before unlink returns\n  3. Client\u0027s v9fs_remove() completes successfully\n  4. Client calls v9fs_dec_count() which calls drop_nlink() on nlink=0\n\nThis race is easily triggered under heavy unlink workloads, such as\nstress-ng\u0027s unlink stressor, producing the following warning:\n\n  WARNING: fs/inode.c:417 at drop_nlink+0x4c/0xc8\n  Call trace:\n   drop_nlink+0x4c/0xc8\n   v9fs_remove+0x1e0/0x250 [9p]\n   v9fs_vfs_unlink+0x20/0x38 [9p]\n   vfs_unlink+0x13c/0x258\n   ...\n\nIn cacheless mode the server is authoritative and the inode is on its\nway out, so locally adjusting nlink buys nothing. Skip v9fs_dec_count()\nentirely when neither CACHE_META nor CACHE_LOOSE is set, which both\navoids the warning and removes a class of nlink races (two concurrent\nunlinkers observing nlink \u003e 0 and both calling drop_nlink()) that an\nnlink == 0 guard alone would only narrow rather than close."
        }
      ],
      "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"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - Triggered via unlink/rmdir/rename syscalls on a mounted 9p client filesystem (default cacheless mode). Common in QEMU/KVM virtio-9p guest shares and TCP-mounted exports; requires code execution on the client host, not direct remote packet delivery to this kernel path.\nAC:L - The fix commit states the race is easily triggered under heavy unlink workloads (e.g., stress-ng). An attacker can drive concurrent unlinks or getattr/unlink on the same inode to refresh server nlink=0 before drop_nlink(), controlling both sides of the race.\nPR:L - Exploitation requires only normal filesystem write permission on the 9p export (may_delete_dentry/inode_permission on the parent directory). Guest users on virtio-9p shares or tenants with write access do not need root/CAP_SYS_ADMIN beyond using an already-mounted filesystem.\nUI:N - No victim interaction is required beyond the attacker (or compromised local process) issuing unlink/rmdir/rename on paths they can already write; mounting 9p is an admin setup step, not per-exploit user action.\nS:U - The bug corrupts inode metadata in the 9p client kernel on the same host as the attacker process. It does not cross VM/host, container/host, or IOMMU security boundaries; virtio-9p guest triggers affect the guest kernel only.\nC:H - drop_nlink() on i_nlink==0 emits WARN_ON then underflows the unsigned link counter to UINT_MAX, corrupting kernel inode metadata. Per kernel CVSS guidance, memory/metadata corruption that could be leveraged for further exploitation warrants High confidentiality.\nI:H - Unsigned i_nlink underflow corrupts inode lifecycle accounting, preventing correct removal tracking (s_remove_count) and desynchronizing dentry unlink from inode state. This class of VFS inode corruption can enable further integrity violations and is scored High when uncertain.\nA:H - Each hit triggers kernel WARN_ON (easily spammed for denial-of-service via log flooding, and fatal where panic_on_warn is enabled) and leaves inodes with bogus nlink values that can provoke subsequent kernel faults during VFS operations on affected files."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-23T12:46:59.271Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/6086469f7d469549bfd070348b717a6e43736200"
        },
        {
          "url": "https://git.kernel.org/stable/c/ab257019cb72f467b55c95384d99c94e3908b928"
        },
        {
          "url": "https://git.kernel.org/stable/c/4ec4ebe40c82cb4c60756732f6593055d010c59c"
        },
        {
          "url": "https://git.kernel.org/stable/c/a5a682b016ef5b5384e28f6d652d47a8f8e73d37"
        },
        {
          "url": "https://git.kernel.org/stable/c/de79c3f3643841b8659a71958df7cf2a66bfd409"
        },
        {
          "url": "https://git.kernel.org/stable/c/8d610017c992de705b304d3d727a6e3a86af6149"
        },
        {
          "url": "https://git.kernel.org/stable/c/8faccac11e1369adddf5d80f4a45af93f13b2e1a"
        },
        {
          "url": "https://git.kernel.org/stable/c/574aa0b4799470ac814479f1138d19efe6262255"
        }
      ],
      "title": "9p: skip nlink update in cacheless mode to fix WARN_ON",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-72170",
    "datePublished": "2026-08-15T05:53:36.332Z",
    "dateReserved": "2026-08-09T03:40:39.910Z",
    "dateUpdated": "2026-08-23T12:46:59.271Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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