{"id":"CVE-2026-72170","state":"PUBLISHED","title":"9p: skip nlink update in cacheless mode to fix WARN_ON","description":"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's 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's 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's 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 > 0 and both calling drop_nlink()) that an\nnlink == 0 guard alone would only narrow rather than close.","publishedAt":"2026-08-15T05:53:36.332Z","updatedAt":"2026-10-16T18:45:00.097Z","severity":{"score":7.8,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"6086469f7d469549bfd070348b717a6e43736200","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"ab257019cb72f467b55c95384d99c94e3908b928","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"4ec4ebe40c82cb4c60756732f6593055d010c59c","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"a5a682b016ef5b5384e28f6d652d47a8f8e73d37","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"de79c3f3643841b8659a71958df7cf2a66bfd409","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"8d610017c992de705b304d3d727a6e3a86af6149","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"8faccac11e1369adddf5d80f4a45af93f13b2e1a","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"574aa0b4799470ac814479f1138d19efe6262255","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["fs/9p/vfs_inode.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"4.17","status":"affected"},{"version":"0","status":"unaffected","lessThan":"4.17","versionType":"semver"},{"version":"5.10.266","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.217","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.184","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.145","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.97","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.40","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.1.5","status":"unaffected","lessThanOrEqual":"7.1.*","versionType":"semver"},{"version":"7.2","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["fs/9p/vfs_inode.c"]}],"references":["https://git.kernel.org/stable/c/6086469f7d469549bfd070348b717a6e43736200","https://git.kernel.org/stable/c/ab257019cb72f467b55c95384d99c94e3908b928","https://git.kernel.org/stable/c/4ec4ebe40c82cb4c60756732f6593055d010c59c","https://git.kernel.org/stable/c/a5a682b016ef5b5384e28f6d652d47a8f8e73d37","https://git.kernel.org/stable/c/de79c3f3643841b8659a71958df7cf2a66bfd409","https://git.kernel.org/stable/c/8d610017c992de705b304d3d727a6e3a86af6149","https://git.kernel.org/stable/c/8faccac11e1369adddf5d80f4a45af93f13b2e1a","https://git.kernel.org/stable/c/574aa0b4799470ac814479f1138d19efe6262255"],"weaknesses":[],"ingestedAt":"2026-10-29T20:12:58.252Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/72xxx/CVE-2026-72170.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-72170","state":"PUBLISHED","datePublished":"2026-08-15T05:53:36.332Z","dateUpdated":"2026-10-16T18:45:00.097Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-08-09T03:40:39.910Z"},"containers":{"cna":{"title":"9p: skip nlink update in cacheless mode to fix WARN_ON","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's 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's 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's 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 > 0 and both calling drop_nlink()) that an\nnlink == 0 guard alone would only narrow rather than close."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"6086469f7d469549bfd070348b717a6e43736200","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"ab257019cb72f467b55c95384d99c94e3908b928","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"4ec4ebe40c82cb4c60756732f6593055d010c59c","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"a5a682b016ef5b5384e28f6d652d47a8f8e73d37","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"de79c3f3643841b8659a71958df7cf2a66bfd409","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"8d610017c992de705b304d3d727a6e3a86af6149","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"8faccac11e1369adddf5d80f4a45af93f13b2e1a","versionType":"git"},{"version":"ac89b2ef9b55924bcf922251f043ba73a32d05bb","status":"affected","lessThan":"574aa0b4799470ac814479f1138d19efe6262255","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["fs/9p/vfs_inode.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"4.17","status":"affected"},{"version":"0","status":"unaffected","lessThan":"4.17","versionType":"semver"},{"version":"5.10.266","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.217","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.184","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.145","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.97","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.40","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.1.5","status":"unaffected","lessThanOrEqual":"7.1.*","versionType":"semver"},{"version":"7.2","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["fs/9p/vfs_inode.c"]}],"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"}],"metrics":[{"cvssV3_1":{"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"},"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":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-08-23T12:46:59.271Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.17","versionEndExcluding":"5.10.266"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.17","versionEndExcluding":"5.15.217"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.17","versionEndExcluding":"6.1.184"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.17","versionEndExcluding":"6.6.145"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.17","versionEndExcluding":"6.12.97"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.17","versionEndExcluding":"6.18.40"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.17","versionEndExcluding":"7.1.5"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.17","versionEndExcluding":"7.2"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"id":"CVE-2026-72170","role":"CISA Coordinator","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"total"}],"version":"2.0.3","timestamp":"2026-10-16T18:43:16.365463Z"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-16T18:45:00.097Z"}}]}}}