CVE-2021-47646 [High] | Use-After-Free in Linux Kernel

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

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-04-06	0.05%	0.01%	-0.04%
2	2025-12-23	0.04%	0.05%	+0.01%
3	2025-07-25	—	0.04%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-04-06

0.05%

0.01%

-0.04%

2025-12-23

0.04%

0.05%

+0.01%

2025-07-25

—

0.04%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
7.8	3.1	HIGH	`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H` Click to expand Attack vector (AV:L) They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:L) A normal user session is enough; they don’t have to be admin. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:U) Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems. Confidentiality (C:H) Serious risk that confidential data gets exposed in a big way. Integrity (I:H) They could widely tamper with or forge data—trust in the data is badly hurt. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	1.8	5.9	134c704f-9b21-4f2e-91b3-4a467353bcc0

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

7.8

3.1

HIGH

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H Click to expand

Attack vector (AV:L): They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
Attack complexity (AC:L): Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L): A normal user session is enough; they don’t have to be admin.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U): Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H): Serious risk that confidential data gets exposed in a big way.
Integrity (I:H): They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H): Could take the service down hard or make it unusable for people who depend on it.

1.8

5.9

134c704f-9b21-4f2e-91b3-4a467353bcc0

OS Trackers for CVE-2021-47646

vendor	priority	summary	link
`debian`	not yet assigned	CVE-2021-47646 not yet assigned priority: Debian including 1 source packages (linux), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5.	https://security-tracker.debian.org/tracker/CVE-2021-47646
`redhat`	medium	—	https://access.redhat.com/security/cve/CVE-2021-47646
`suse`	low	—	https://www.suse.com/security/cve/CVE-2021-47646/
`ubuntu`	high	CVE-2021-47646 high priority: Ubuntu including 150 source packages (linux, linux-allwinner-5.19, …), 1487 status rows across 10 suites (bionic, focal, jammy, noble, oracular, plucky, questing, trusty, upstream, xenial): DNE 1097, ignored 146, not-affected 144, released 100.	https://ubuntu.com/security/CVE-2021-47646

Affected software / configurations for CVE-2021-47646

Vendor	Product	Version	Raw CPE
linux	linux_kernel	< 4.19.238	cpe:2.3:o:linux:linux_kernel::::::::
linux	linux_kernel	>= 4.20, < 5.4.189	cpe:2.3:o:linux:linux_kernel::::::::
linux	linux_kernel	>= 5.5, < 5.10.110	cpe:2.3:o:linux:linux_kernel::::::::
linux	linux_kernel	>= 5.11, < 5.15.33	cpe:2.3:o:linux:linux_kernel::::::::
linux	linux_kernel	>= 5.16, < 5.16.19	cpe:2.3:o:linux:linux_kernel::::::::
linux	linux_kernel	>= 5.17, < 5.17.2	cpe:2.3:o:linux:linux_kernel::::::::

References for CVE-2021-47646

URL	Tags
https://git.kernel.org/stable/c/15729ff8143f8135b03988a100a19e66d7cb7ecd	Patch
https://git.kernel.org/stable/c/4083925bd6dc89216d156474a8076feec904e607	Patch
https://git.kernel.org/stable/c/65d8a737452e88f251fe5d925371de6d606df613	Patch
https://git.kernel.org/stable/c/931aff627469a75c77b9fd3823146d0575afffd6	Patch
https://git.kernel.org/stable/c/abc2129e646af7b43025d90a071f83043f1ae76c	Patch
https://git.kernel.org/stable/c/cc051f497eac9d8a0d816cd4bffa3415f2724871	Patch
https://git.kernel.org/stable/c/f990f0985eda59d4f29fc83fcf300c92b1225d39	Patch

URL

CVE-2021-47646 | Revert "Revert "block, bfq: honor already-setup queue merges""

Exploit prediction scoring system (EPSS) score for CVE-2021-47646

Common vulnerability scoring system (CVSS) metrics for CVE-2021-47646

Weakness enumeration for CVE-2021-47646

OS Trackers for CVE-2021-47646

Affected software / configurations for CVE-2021-47646

References for CVE-2021-47646