---
id: CVE-2021-47646
title: |-
  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…
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…
severity: high
cvss: 7.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H'
cwe:
  - CWE-416
vendor: linux
product: linux_kernel
affected:
  - linux_kernel < 4.19.238
  - 'linux_kernel >= 4.20, < 5.4.189'
  - 'linux_kernel >= 5.5, < 5.10.110'
  - 'linux_kernel >= 5.11, < 5.15.33'
  - 'linux_kernel >= 5.16, < 5.16.19'
  - 'linux_kernel >= 5.17, < 5.17.2'
patched:
  - linux_kernel 5.17.2
published: '2025-02-26'
updated: '2026-08-11'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2021-47646'
references:
  - url: 'https://git.kernel.org/stable/c/15729ff8143f8135b03988a100a19e66d7cb7ecd'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/4083925bd6dc89216d156474a8076feec904e607'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/65d8a737452e88f251fe5d925371de6d606df613'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/abc2129e646af7b43025d90a071f83043f1ae76c'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
tags:
  - nvd
epss: 0.00282
epssPercentile: 0.20921
ingestedAt: '2026-08-11T16:47:03.508Z'
---

## Overview

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

## Affected

- `linux_kernel < 4.19.238`
- `linux_kernel >= 4.20, < 5.4.189`
- `linux_kernel >= 5.5, < 5.10.110`
- `linux_kernel >= 5.11, < 5.15.33`
- `linux_kernel >= 5.16, < 5.16.19`
- `linux_kernel >= 5.17, < 5.17.2`

## Remediation

Upgrade past the affected range:

- `linux_kernel 5.17.2`