CVE-2025-21634 [Medium] | Security Vulnerability in Linux Kernel

In the Linux kernel, the following vulnerability has been resolved: cgroup/cpuset: remove kernfs active break A warning was found: WARNING: CPU: 10 PID: 3486953 at fs/kernfs/file.c:828 CPU: 10 PID: 3486953 Comm: rmdir Kdump: loaded Tainted: G RIP: 0010:kernfs_should_drain_open_files+0x1a1/0x1b0 RSP: 0018:ffff8881107ef9e0 EFLAGS: 00010202 RAX: 0000000080000002 RBX: ffff888154738c00 RCX: dffffc0000000000 RDX: 0000000000000007 RSI: 0000000000000004 RDI: ffff888154738c04 RBP: ffff888154738c04 R08: ffffffffaf27fa15 R09: ffffed102a8e7180 R10: ffff888154738c07 R11: 0000000000000000 R12: ffff888154738c08 R13: ffff888750f8c000 R14: ffff888750f8c0e8 R15: ffff888154738ca0 FS: 00007f84cd0be740(0000) GS:ffff8887ddc00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000555f9fbe00c8 CR3: 0000000153eec001 CR4: 0000000000370ee0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: kernfs_drain+0x15e/0x2f0 __kernfs_remove+0x165/0x300 kernfs_remove_by_name_ns+0x7b/0xc0 cgroup_rm_file+0x154/0x1c0 cgroup_addrm_files+0x1c2/0x1f0 css_clear_dir+0x77/0x110 kill_css+0x4c/0x1b0 cgroup_destroy_locked+0x194/0x380 cgroup_rmdir+0x2a/0x140 It can be explained by: rmdir echo 1 > cpuset.cpus kernfs_fop_write_iter // active=0 cgroup_rm_file kernfs_remove_by_name_ns kernfs_get_active // active=1 __kernfs_remove // active=0x80000002 kernfs_drain cpuset_write_resmask wait_event //waiting (active == 0x80000001) kernfs_break_active_protection // active = 0x80000001 // continue kernfs_unbreak_active_protection // active = 0x80000002 ... kernfs_should_drain_open_files // warning occurs kernfs_put_active This warning is caused by 'kernfs_break_active_protection' when it is writing to cpuset.cpus, and the cgroup is removed concurrently. The commit 3a5a6d0c2b03 ("cpuset: don't nest cgroup_mutex inside get_online_cpus()") made cpuset_hotplug_workfn asynchronous, This change involves calling flush_work(), which can create a multiple processes circular locking dependency that involve cgroup_mutex, potentially leading to a deadlock. To avoid deadlock. the commit 76bb5ab8f6e3 ("cpuset: break kernfs active protection in cpuset_write_resmask()") added 'kernfs_break_active_protection' in the cpuset_write_resmask. This could lead to this warning. After the commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug processing synchronous"), the cpuset_write_resmask no longer needs to wait the hotplug to finish, which means that concurrent hotplug and cpuset operations are no longer possible. Therefore, the deadlock doesn't exist anymore and it does not have to 'break active protection' now. To fix this warning, just remove kernfs_break_active_protection operation in the 'cpuset_write_resmask'.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2025-11-21	0.05%	0.04%	-0.01%
2	2025-11-18	0.04%	0.05%	+0.01%
3	2025-01-20	—	0.04%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2025-11-21

0.05%

0.04%

-0.01%

2025-11-18

0.04%

0.05%

+0.01%

2025-01-20

—

0.04%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
5.5	3.1	MEDIUM	`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:N) Doesn’t really leak secrets in a meaningful way. Integrity (I:N) Data isn’t meaningfully altered or forged. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	1.8	3.6	[email protected]
5.5	3.1	MEDIUM	`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:N) Doesn’t really leak secrets in a meaningful way. Integrity (I:N) Data isn’t meaningfully altered or forged. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	1.8	3.6	134c704f-9b21-4f2e-91b3-4a467353bcc0

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

5.5

3.1

MEDIUM

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:N): Doesn’t really leak secrets in a meaningful way.
Integrity (I:N): Data isn’t meaningfully altered or forged.
Availability (A:H): Could take the service down hard or make it unusable for people who depend on it.

1.8

3.6

[email protected]

5.5

3.1

MEDIUM

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:N): Doesn’t really leak secrets in a meaningful way.
Integrity (I:N): Data isn’t meaningfully altered or forged.
Availability (A:H): Could take the service down hard or make it unusable for people who depend on it.

1.8

3.6

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

OS Trackers for CVE-2025-21634

vendor	priority	summary	link
`debian`	not yet assigned	CVE-2025-21634 not yet assigned priority: Debian including 1 source packages (linux), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 3, open 2.	https://security-tracker.debian.org/tracker/CVE-2025-21634
`redhat`	low	—	https://access.redhat.com/security/cve/CVE-2025-21634
`suse`	medium	—	https://www.suse.com/security/cve/CVE-2025-21634/
`ubuntu`	medium	CVE-2025-21634 medium priority: Ubuntu including 158 source packages (linux, linux-allwinner-5.19, …), 1551 status rows across 10 suites (bionic, focal, jammy, noble, oracular, plucky, questing, trusty, upstream, xenial): DNE 1145, ignored 150, released 136, needed 74, not-affected 46.	https://ubuntu.com/security/CVE-2025-21634

Affected software / configurations for CVE-2025-21634

Vendor	Product	Version	Raw CPE
linux	linux_kernel	>= 3.16, < 6.12.10	cpe:2.3:o:linux:linux_kernel::::::::
linux	linux_kernel	6.13	cpe:2.3:o:linux:linux_kernel:6.13:rc1::::::
linux	linux_kernel	6.13	cpe:2.3:o:linux:linux_kernel:6.13:rc2::::::
linux	linux_kernel	6.13	cpe:2.3:o:linux:linux_kernel:6.13:rc3::::::
linux	linux_kernel	6.13	cpe:2.3:o:linux:linux_kernel:6.13:rc4::::::
linux	linux_kernel	6.13	cpe:2.3:o:linux:linux_kernel:6.13:rc5::::::
linux	linux_kernel	6.13	cpe:2.3:o:linux:linux_kernel:6.13:rc6::::::

References for CVE-2025-21634

URL	Tags
https://git.kernel.org/stable/c/11cb1d643a74665a4e14749414f48f82cbc15c64	Patch
https://git.kernel.org/stable/c/3cb97a927fffe443e1e7e8eddbfebfdb062e86ed	Patch

URL

CVE-2025-21634 | cgroup/cpuset: remove kernfs active break

Exploit prediction scoring system (EPSS) score for CVE-2025-21634

Common vulnerability scoring system (CVSS) metrics for CVE-2025-21634

Weakness enumeration for CVE-2025-21634

OS Trackers for CVE-2025-21634

Affected software / configurations for CVE-2025-21634

References for CVE-2025-21634