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CVE-2021-47646 (GCVE-0-2021-47646)

Vulnerability from cvelistv5 – Published: 2025-02-26 01:54 – Updated: 2026-08-11 02:05
VLAI
Title
Revert "Revert "block, bfq: honor already-setup queue merges""
Summary
In the Linux kernel, the following vulnerability has been resolved: Revert "Revert "block, bfq: honor already-setup queue merges"" A crash [1] happened to be triggered in conjunction with commit 2d52c58b9c9b ("block, bfq: honor already-setup queue merges"). The latter was then reverted by commit ebc69e897e17 ("Revert "block, bfq: honor already-setup queue merges""). Yet, the reverted commit was not the one introducing the bug. In fact, it actually triggered a UAF introduced by a different commit, and now fixed by commit d29bd41428cf ("block, bfq: reset last_bfqq_created on group change"). So, there is no point in keeping commit 2d52c58b9c9b ("block, bfq: honor already-setup queue merges") out. This commit restores it. [1] https://bugzilla.kernel.org/show_bug.cgi?id=214503
Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
SSVC
Exploitation: none Automatable: no Technical Impact: total
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2025-02-27 17:59 UTC
CWE
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 430a67f9d6169a7b3e328bceb2ef9542e4153c7c , < 65d8a737452e88f251fe5d925371de6d606df613 (git)
Affected: 430a67f9d6169a7b3e328bceb2ef9542e4153c7c , < abc2129e646af7b43025d90a071f83043f1ae76c (git)
Affected: 430a67f9d6169a7b3e328bceb2ef9542e4153c7c , < 4083925bd6dc89216d156474a8076feec904e607 (git)
Affected: 430a67f9d6169a7b3e328bceb2ef9542e4153c7c , < 15729ff8143f8135b03988a100a19e66d7cb7ecd (git)
guessed Create a notification for this product.
Linux Linux Affected: 5.13
Unaffected: 0 , < 5.13 (semver)
Unaffected: 5.15.33 , ≤ 5.15.* (semver)
Unaffected: 5.16.19 , ≤ 5.16.* (semver)
Unaffected: 5.17.2 , ≤ 5.17.* (semver)
Unaffected: 5.18 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
Show details on NVD website
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      "id": "CVE-2021-47646",
      "initial_release_date": "2021-01-01T00:00:00+00:00",
      "product_status:fixed": "166",
      "product_status:known_affected": "70",
      "product_status:known_not_affected": "118",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: Revert \"Revert \"block, bfq: honor already-setup queue merges\"\"",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2021/cve-2021-47646.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "low",
      "current_release_date": "2026-09-03T00:44:46Z",
      "cve": "CVE-2021-47646",
      "id": "CVE-2021-47646",
      "initial_release_date": "2025-02-27T03:14:17Z",
      "product_status:known_affected": "504",
      "product_status:known_not_affected": "231",
      "product_status:recommended": "245",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2021-47646",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2021-47646.json",
      "version": "38"
    },
    "vulnrichment": {
      "containers": {
        "adp": [
          {
            "metrics": [
              {
                "cvssV3_1": {
                  "attackComplexity": "LOW",
                  "attackVector": "LOCAL",
                  "availabilityImpact": "HIGH",
                  "baseScore": 7.8,
                  "baseSeverity": "HIGH",
                  "confidentialityImpact": "HIGH",
                  "integrityImpact": "HIGH",
                  "privilegesRequired": "LOW",
                  "scope": "UNCHANGED",
                  "userInteraction": "NONE",
                  "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
                  "version": "3.1"
                }
              },
              {
                "other": {
                  "content": {
                    "id": "CVE-2021-47646",
                    "options": [
                      {
                        "Exploitation": "none"
                      },
                      {
                        "Automatable": "no"
                      },
                      {
                        "Technical Impact": "total"
                      }
                    ],
                    "role": "CISA Coordinator",
                    "timestamp": "2025-02-27T17:59:39.194997Z",
                    "version": "2.0.3"
                  },
                  "type": "ssvc"
                }
              }
            ],
            "problemTypes": [
              {
                "descriptions": [
                  {
                    "cweId": "CWE-416",
                    "description": "CWE-416 Use After Free",
                    "lang": "en",
                    "type": "CWE"
                  }
                ]
              }
            ],
            "providerMetadata": {
              "dateUpdated": "2025-02-27T17:59:40.880Z",
              "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
              "shortName": "CISA-ADP"
            },
            "title": "CISA ADP Vulnrichment"
          }
        ],
        "cna": {
          "affected": [
            {
              "defaultStatus": "unaffected",
              "product": "Linux",
              "programFiles": [
                "block/bfq-iosched.c"
              ],
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "vendor": "Linux",
              "versions": [
                {
                  "lessThan": "65d8a737452e88f251fe5d925371de6d606df613",
                  "status": "affected",
                  "version": "430a67f9d6169a7b3e328bceb2ef9542e4153c7c",
                  "versionType": "git"
                },
                {
                  "lessThan": "abc2129e646af7b43025d90a071f83043f1ae76c",
                  "status": "affected",
                  "version": "430a67f9d6169a7b3e328bceb2ef9542e4153c7c",
                  "versionType": "git"
                },
                {
                  "lessThan": "4083925bd6dc89216d156474a8076feec904e607",
                  "status": "affected",
                  "version": "430a67f9d6169a7b3e328bceb2ef9542e4153c7c",
                  "versionType": "git"
                },
                {
                  "lessThan": "15729ff8143f8135b03988a100a19e66d7cb7ecd",
                  "status": "affected",
                  "version": "430a67f9d6169a7b3e328bceb2ef9542e4153c7c",
                  "versionType": "git"
                }
              ]
            },
            {
              "defaultStatus": "affected",
              "product": "Linux",
              "programFiles": [
                "block/bfq-iosched.c"
              ],
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "vendor": "Linux",
              "versions": [
                {
                  "status": "affected",
                  "version": "5.13"
                },
                {
                  "lessThan": "5.13",
                  "status": "unaffected",
                  "version": "0",
                  "versionType": "semver"
                },
                {
                  "lessThanOrEqual": "5.15.*",
                  "status": "unaffected",
                  "version": "5.15.33",
                  "versionType": "semver"
                },
                {
                  "lessThanOrEqual": "5.16.*",
                  "status": "unaffected",
                  "version": "5.16.19",
                  "versionType": "semver"
                },
                {
                  "lessThanOrEqual": "5.17.*",
                  "status": "unaffected",
                  "version": "5.17.2",
                  "versionType": "semver"
                },
                {
                  "lessThanOrEqual": "*",
                  "status": "unaffected",
                  "version": "5.18",
                  "versionType": "original_commit_for_fix"
                }
              ]
            }
          ],
          "cpeApplicability": [
            {
              "nodes": [
                {
                  "cpeMatch": [
                    {
                      "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                      "versionEndExcluding": "5.15.33",
                      "versionStartIncluding": "5.13",
                      "vulnerable": true
                    },
                    {
                      "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                      "versionEndExcluding": "5.16.19",
                      "versionStartIncluding": "5.13",
                      "vulnerable": true
                    },
                    {
                      "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                      "versionEndExcluding": "5.17.2",
                      "versionStartIncluding": "5.13",
                      "vulnerable": true
                    },
                    {
                      "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                      "versionEndExcluding": "5.18",
                      "versionStartIncluding": "5.13",
                      "vulnerable": true
                    }
                  ],
                  "negate": false,
                  "operator": "OR"
                }
              ]
            }
          ],
          "descriptions": [
            {
              "lang": "en",
              "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nRevert \"Revert \"block, bfq: honor already-setup queue merges\"\"\n\nA crash [1] happened to be triggered in conjunction with commit\n2d52c58b9c9b (\"block, bfq: honor already-setup queue merges\"). The\nlatter was then reverted by commit ebc69e897e17 (\"Revert \"block, bfq:\nhonor already-setup queue merges\"\"). Yet, the reverted commit was not\nthe one introducing the bug. In fact, it actually triggered a UAF\nintroduced by a different commit, and now fixed by commit d29bd41428cf\n(\"block, bfq: reset last_bfqq_created on group change\").\n\nSo, there is no point in keeping commit 2d52c58b9c9b (\"block, bfq:\nhonor already-setup queue merges\") out. This commit restores it.\n\n[1] https://bugzilla.kernel.org/show_bug.cgi?id=214503"
            }
          ],
          "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 - The vulnerability is in the BFQ block I/O scheduler and is reached via local syscalls that submit I/O (read/write/fsync/ioctl), through bfq_init_rq \u2192 bfq_get_queue / bfq_setup_cooperator; it is not reachable from network packet processing.\nAC:L - An unprivileged attacker can reliably drive the buggy path by spawning processes that issue concurrent I/O and by migrating between blkcgs so queues are created, merged, moved, and freed under attacker control; success does not depend on victim state beyond BFQ being the active scheduler.\nPR:L - Only an unprivileged local account is required to submit I/O on a BFQ-scheduled device; changing the elevator needs privileges, but BFQ (with group I/O scheduling) is a common/default deployment (e.g. Android), so the attacker need not already be root.\nUI:N - Exploitation requires no action by another user; the attacker triggers the bug solely through their own I/O and process/cgroup activity.\nS:U - Impact is confined to the local kernel\u0027s security authority (memory corruption / privilege escalation on the host); there is no VM escape, IOMMU bypass, or other cross-boundary scope change.\nC:H - The issue is a use-after-free of bfq_queue objects (dangling last_bfqq_created / broken merge state), which per kernel CVSS guidance yields an arbitrary-read primitive via heap reuse of the freed object.\nI:H - The same UAF/memory-corruption condition enables heap spraying and arbitrary-write / control-flow hijacking in kernel context, so integrity impact is High.\nA:H - The bug causes kernel oops/BUG_ON and slab freelist corruption (demonstrated in bugzilla 214503 crashes in bfq_get_queue / bfq_do_early_stable_merge), so availability impact is High."
                }
              ]
            }
          ],
          "providerMetadata": {
            "dateUpdated": "2026-08-11T02:05:32.009Z",
            "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "shortName": "Linux"
          },
          "references": [
            {
              "url": "https://git.kernel.org/stable/c/65d8a737452e88f251fe5d925371de6d606df613"
            },
            {
              "url": "https://git.kernel.org/stable/c/abc2129e646af7b43025d90a071f83043f1ae76c"
            },
            {
              "url": "https://git.kernel.org/stable/c/4083925bd6dc89216d156474a8076feec904e607"
            },
            {
              "url": "https://git.kernel.org/stable/c/15729ff8143f8135b03988a100a19e66d7cb7ecd"
            }
          ],
          "title": "Revert \"Revert \"block, bfq: honor already-setup queue merges\"\"",
          "x_generator": {
            "engine": "bippy-1.2.0"
          }
        }
      },
      "cveMetadata": {
        "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "assignerShortName": "Linux",
        "cveId": "CVE-2021-47646",
        "datePublished": "2025-02-26T01:54:15.122Z",
        "dateReserved": "2025-02-26T01:48:21.520Z",
        "dateUpdated": "2026-08-11T02:05:32.009Z",
        "state": "PUBLISHED"
      },
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}



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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.


Detection rules are retrieved from Rulezet.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.