CVE-2021-47506 (GCVE-0-2021-47506)
Vulnerability from cvelistv5 – Published: 2024-05-24 15:01 – Updated: 2026-08-05 08:48
VLAI
EPSS
VEX
Title
nfsd: fix use-after-free due to delegation race
Summary
In the Linux kernel, the following vulnerability has been resolved:
nfsd: fix use-after-free due to delegation race
A delegation break could arrive as soon as we've called vfs_setlease. A
delegation break runs a callback which immediately (in
nfsd4_cb_recall_prepare) adds the delegation to del_recall_lru. If we
then exit nfs4_set_delegation without hashing the delegation, it will be
freed as soon as the callback is done with it, without ever being
removed from del_recall_lru.
Symptoms show up later as use-after-free or list corruption warnings,
usually in the laundromat thread.
I suspect aba2072f4523 "nfsd: grant read delegations to clients holding
writes" made this bug easier to hit, but I looked as far back as v3.0
and it looks to me it already had the same problem. So I'm not sure
where the bug was introduced; it may have been there from the beginning.
Severity
9.8 (Critical)
SSVC
Exploitation: none
Automatable: no
Technical Impact: total
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2024-05-29 17:04 UTC
Assigner
References
8 references
Impacted products
2 products
| Vendor | Product | Version | CPE status | |
|---|---|---|---|---|
| Linux | Linux |
Affected:
dff1399f8addf7129c49bb2227469da79cc30b47 , < 04a8d07f3d58308b92630045560799a3faa3ebce
(git)
Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < 348714018139c39533c55661a0c7c990671396b4 (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < 33645d3e22720cac1e4548f8fef57bf0649536ee (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < 2becaa990b93cbd2928292c0b669d3abb6cf06d4 (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < e0759696de6851d7536efddfdd2dfed4c4df1f09 (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < eeb0711801f5e19ef654371b627682aed3b11373 (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < 148c816f10fd11df27ca6a9b3238cdd42fa72cd3 (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < 548ec0805c399c65ed66c6641be467f717833ab5 (git) |
guessed | |
| Linux | Linux |
Affected:
3.17
Unaffected: 0 , < 3.17 (semver) Unaffected: 4.4.296 , ≤ 4.4.* (semver) Unaffected: 4.9.294 , ≤ 4.9.* (semver) Unaffected: 4.14.259 , ≤ 4.14.* (semver) Unaffected: 4.19.222 , ≤ 4.19.* (semver) Unaffected: 5.4.168 , ≤ 5.4.* (semver) Unaffected: 5.10.85 , ≤ 5.10.* (semver) Unaffected: 5.15.8 , ≤ 5.15.* (semver) Unaffected: 5.16 , ≤ * (original_commit_for_fix) |
guessed |
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If we\\nthen exit nfs4_set_delegation without hashing the delegation, it will be\\nfreed as soon as the callback is done with it, without ever being\\nremoved from del_recall_lru.\\n\\nSymptoms show up later as use-after-free or list corruption warnings,\\nusually in the laundromat thread.\\n\\nI suspect aba2072f4523 \\\"nfsd: grant read delegations to clients holding\\nwrites\\\" made this bug easier to hit, but I looked as far back as v3.0\\nand it looks to me it already had the same problem. So I\u0027m not sure\\nwhere the bug was introduced; it may have been there from the beginning.\"},{\"lang\":\"es\",\"value\":\"En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: nfsd: corrige el use-after-free debido a la ejecuci\u00f3n de delegaci\u00f3n. Una interrupci\u00f3n de la delegaci\u00f3n podr\u00eda llegar tan pronto como hayamos llamado a vfs_setlease. Una interrupci\u00f3n de la delegaci\u00f3n ejecuta una devoluci\u00f3n de llamada que inmediatamente (en nfsd4_cb_recall_prepare) agrega la delegaci\u00f3n a del_recall_lru. Si luego salimos de nfs4_set_delegation sin aplicar hash a la delegaci\u00f3n, se liberar\u00e1 tan pronto como finalice la devoluci\u00f3n de llamada, sin eliminarse nunca de del_recall_lru. Los s\u00edntomas aparecen m\u00e1s tarde como advertencias de use-after-free o de corrupci\u00f3n en la lista, generalmente en el hilo de la lavander\u00eda. Sospecho que aba2072f4523 \\\"nfsd: otorgar delegaciones de lectura a clientes que tienen escrituras\\\" hizo que este error fuera m\u00e1s f\u00e1cil de corregir, pero mir\u00e9 hasta la versi\u00f3n 3.0 y me parece que ya ten\u00eda el mismo problema. 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"current_release_date": "2026-08-04T19:57:20+00:00",
"cve": "CVE-2021-47506",
"id": "CVE-2021-47506",
"initial_release_date": "2021-01-01T00:00:00+00:00",
"product_status:known_affected": "90",
"product_status:known_not_affected": "108",
"source": "Red Hat CSAF VEX",
"status": "final",
"title": "kernel: nfsd: fix use-after-free due to delegation race",
"url": "https://security.access.redhat.com/data/csaf/v2/vex/2021/cve-2021-47506.json",
"version": "3"
},
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"aggregate_severity": "important",
"current_release_date": "2026-09-11T02:37:40Z",
"cve": "CVE-2021-47506",
"id": "CVE-2021-47506",
"initial_release_date": "2024-05-28T03:32:52Z",
"product_status:known_affected": "370",
"product_status:known_not_affected": "101",
"product_status:recommended": "691",
"source": "SUSE CSAF VEX",
"status": "interim",
"title": "SUSE CVE CVE-2021-47506",
"url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2021-47506.json",
"version": "67"
},
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no victim user or administrator action on the server is required.\\nS:U - The use-after-free corrupts kernel slab memory (nfs4_delegation on del_recall_lru) within the host nfsd security authority and does not cross a VM, IOMMU, or other separate boundary.\\nC:H - The unhashed delegation is freed while still linked on del_recall_lru; later laundromat traversal is a classic slab UAF that can be groomed/reallocated to disclose arbitrary kernel memory.\\nI:H - The same freed nfs4_delegation object remains on del_recall_lru so laundromat list operations write into attacker-reclaimable heap, yielding an arbitrary-write / control-flow hijack primitive.\\nA:H - Use-after-free and list corruption of del_recall_lru reliably cause kernel oops/panic in the laundromat thread, fully disrupting availability of the NFS server host.\"}]}], \"affected\": [{\"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\", \"vendor\": \"Linux\", \"product\": \"Linux\", \"versions\": [{\"status\": \"affected\", \"version\": \"dff1399f8addf7129c49bb2227469da79cc30b47\", \"lessThan\": \"04a8d07f3d58308b92630045560799a3faa3ebce\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"dff1399f8addf7129c49bb2227469da79cc30b47\", \"lessThan\": \"348714018139c39533c55661a0c7c990671396b4\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"dff1399f8addf7129c49bb2227469da79cc30b47\", \"lessThan\": \"33645d3e22720cac1e4548f8fef57bf0649536ee\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"dff1399f8addf7129c49bb2227469da79cc30b47\", \"lessThan\": \"2becaa990b93cbd2928292c0b669d3abb6cf06d4\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"dff1399f8addf7129c49bb2227469da79cc30b47\", \"lessThan\": \"e0759696de6851d7536efddfdd2dfed4c4df1f09\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"dff1399f8addf7129c49bb2227469da79cc30b47\", \"lessThan\": \"eeb0711801f5e19ef654371b627682aed3b11373\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"dff1399f8addf7129c49bb2227469da79cc30b47\", \"lessThan\": \"148c816f10fd11df27ca6a9b3238cdd42fa72cd3\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"dff1399f8addf7129c49bb2227469da79cc30b47\", \"lessThan\": \"548ec0805c399c65ed66c6641be467f717833ab5\", \"versionType\": \"git\"}], \"programFiles\": [\"fs/nfsd/nfs4state.c\"], \"defaultStatus\": \"unaffected\"}, {\"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\", \"vendor\": \"Linux\", \"product\": \"Linux\", \"versions\": [{\"status\": \"affected\", \"version\": \"3.17\"}, {\"status\": \"unaffected\", \"version\": \"0\", \"lessThan\": \"3.17\", \"versionType\": \"semver\"}, {\"status\": \"unaffected\", \"version\": \"4.4.296\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"4.4.*\"}, {\"status\": \"unaffected\", \"version\": \"4.9.294\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"4.9.*\"}, {\"status\": \"unaffected\", \"version\": \"4.14.259\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"4.14.*\"}, {\"status\": \"unaffected\", \"version\": \"4.19.222\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"4.19.*\"}, {\"status\": \"unaffected\", \"version\": \"5.4.168\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.4.*\"}, {\"status\": \"unaffected\", \"version\": \"5.10.85\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.10.*\"}, {\"status\": \"unaffected\", \"version\": \"5.15.8\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.15.*\"}, {\"status\": \"unaffected\", \"version\": \"5.16\", \"versionType\": \"original_commit_for_fix\", \"lessThanOrEqual\": \"*\"}], \"programFiles\": [\"fs/nfsd/nfs4state.c\"], \"defaultStatus\": \"affected\"}], \"references\": [{\"url\": \"https://git.kernel.org/stable/c/04a8d07f3d58308b92630045560799a3faa3ebce\"}, {\"url\": \"https://git.kernel.org/stable/c/348714018139c39533c55661a0c7c990671396b4\"}, {\"url\": \"https://git.kernel.org/stable/c/33645d3e22720cac1e4548f8fef57bf0649536ee\"}, {\"url\": \"https://git.kernel.org/stable/c/2becaa990b93cbd2928292c0b669d3abb6cf06d4\"}, {\"url\": \"https://git.kernel.org/stable/c/e0759696de6851d7536efddfdd2dfed4c4df1f09\"}, {\"url\": \"https://git.kernel.org/stable/c/eeb0711801f5e19ef654371b627682aed3b11373\"}, {\"url\": \"https://git.kernel.org/stable/c/148c816f10fd11df27ca6a9b3238cdd42fa72cd3\"}, {\"url\": \"https://git.kernel.org/stable/c/548ec0805c399c65ed66c6641be467f717833ab5\"}], \"x_generator\": {\"engine\": \"bippy-1.2.0\"}, \"descriptions\": [{\"lang\": \"en\", \"value\": \"In the Linux kernel, the following vulnerability has been resolved:\\n\\nnfsd: fix use-after-free due to delegation race\\n\\nA delegation break could arrive as soon as we\u0027ve called vfs_setlease. A\\ndelegation break runs a callback which immediately (in\\nnfsd4_cb_recall_prepare) adds the delegation to del_recall_lru. If we\\nthen exit nfs4_set_delegation without hashing the delegation, it will be\\nfreed as soon as the callback is done with it, without ever being\\nremoved from del_recall_lru.\\n\\nSymptoms show up later as use-after-free or list corruption warnings,\\nusually in the laundromat thread.\\n\\nI suspect aba2072f4523 \\\"nfsd: grant read delegations to clients holding\\nwrites\\\" made this bug easier to hit, but I looked as far back as v3.0\\nand it looks to me it already had the same problem. So I\u0027m not sure\\nwhere the bug was introduced; it may have been there from the beginning.\"}], \"cpeApplicability\": [{\"nodes\": [{\"negate\": false, \"cpeMatch\": [{\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"4.4.296\", \"versionStartIncluding\": \"3.17\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"4.9.294\", \"versionStartIncluding\": \"3.17\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"4.14.259\", \"versionStartIncluding\": \"3.17\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"4.19.222\", \"versionStartIncluding\": \"3.17\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.4.168\", \"versionStartIncluding\": \"3.17\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.10.85\", \"versionStartIncluding\": \"3.17\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.15.8\", \"versionStartIncluding\": \"3.17\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.16\", \"versionStartIncluding\": \"3.17\"}], \"operator\": \"OR\"}]}], \"providerMetadata\": {\"orgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"shortName\": \"Linux\", \"dateUpdated\": \"2026-08-05T08:48:10.240Z\"}}}",
"cveMetadata": "{\"cveId\": \"CVE-2021-47506\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-08-05T08:48:10.240Z\", \"dateReserved\": \"2024-05-22T06:20:56.205Z\", \"assignerOrgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"datePublished\": \"2024-05-24T15:01:52.746Z\", \"assignerShortName\": \"Linux\"}",
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
}
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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