VulnerabilityPublished 1 Apr 2025Updated 4 Nov 2025

CVE-2025-21942: Linux kernel · btrfs: zoned: fix extent range end unlock in cow_file_range()

Affects Linux kernelSource GCVE Database

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Authoritative record:CVE-2025-21942· Medium

Not known to be exploited · CVSS 5.5 Medium.

Data as of 30 Oct 2026, 15:49 UTC

CVSS 3.1 base score

5.5Medium

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

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

Attack Vector: Local
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Local: The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., tricking a legitimate user into opening a malicious document).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 1 Apr 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Affected software

Product	Affected	Fixed in
Ubuntu 24.04 LTSlinux-raspi-realtime`pkg:deb/ubuntu/linux-raspi-realtime?arch=source&distro=noble`Other threats	Affected =6.8.0-2019.20	Fixed in–
Ubuntu 24.04 LTSlinux-realtime`pkg:deb/ubuntu/linux-realtime?arch=source&distro=noble`Other threats	Affected =6.8.1-1015.16	Fixed in–
Ubuntu 22.04 LTSlinux-allwinner-5.19`pkg:deb/ubuntu/linux-allwinner-5.19?arch=source&distro=jammy`Other threats	Affected 5.19.0-1007.7~22.04.1 – 5.19.0-1015.15~22.04.1 8 affected builds	Fixed in–
Ubuntu 22.04 LTSlinux-aws-5.19`pkg:deb/ubuntu/linux-aws-5.19?arch=source&distro=jammy`Other threats	Affected 5.19.0-1019.20~22.04.1 – 5.19.0-1029.30~22.04.1 10 affected builds	Fixed in–
Ubuntu 22.04 LTSlinux-aws-6.2`pkg:deb/ubuntu/linux-aws-6.2?arch=source&distro=jammy`Other threats	Affected 6.2.0-1005.5~22.04.1 – 6.2.0-1018.18~22.04.1 14 affected builds	Fixed in–
Ubuntu 22.04 LTSlinux-aws-6.5`pkg:deb/ubuntu/linux-aws-6.5?arch=source&distro=jammy`Other threats	Affected 6.5.0-1008.8~22.04.1 – 6.5.0-1024.24~22.04.1 14 affected builds	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

In the Linux kernel, the following vulnerability has been resolved: btrfs: zoned: fix extent range end unlock in cow_file_range() Running generic/751 on the for-next branch often results in a hang like below. They are both stack by locking an extent. This suggests someone forget to unlock an extent. INFO: task kworker/u128:1:12 blocked for more than 323 seconds. Not tainted 6.13.0-BTRFS-ZNS+ #503 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:kworker/u128:1 state:D stack:0 pid:12 tgid:12 ppid:2 flags:0x00004000 Workqueue: btrfs-fixup btrfs_work_helper [btrfs] Call Trace: <TASK> __schedule+0x534/0xdd0 schedule+0x39/0x140 __lock_extent+0x31b/0x380 [btrfs] ? __pfx_autoremove_wake_function+0x10/0x10 btrfs_writepage_fixup_worker+0xf1/0x3a0 [btrfs] btrfs_work_helper+0xff/0x480 [btrfs] ? lock_release+0x178/0x2c0 process_one_work+0x1ee/0x570 ? srso_return_thunk+0x5/0x5f worker_thread+0x1d1/0x3b0 ? __pfx_worker_thread+0x10/0x10 kthread+0x10b/0x230 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x30/0x50 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1a/0x30 </TASK> INFO: task kworker/u134:0:184 blocked for more than 323 seconds. Not tainted 6.13.0-BTRFS-ZNS+ #503 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:kworker/u134:0 state:D stack:0 pid:184 tgid:184 ppid:2 flags:0x00004000 Workqueue: writeback wb_workfn (flush-btrfs-4) Call Trace: <TASK> __schedule+0x534/0xdd0 schedule+0x39/0x140 __lock_extent+0x31b/0x380 [btrfs] ? __pfx_autoremove_wake_function+0x10/0x10 find_lock_delalloc_range+0xdb/0x260 [btrfs] writepage_delalloc+0x12f/0x500 [btrfs] ? srso_return_thunk+0x5/0x5f extent_write_cache_pages+0x232/0x840 [btrfs] btrfs_writepages+0x72/0x130 [btrfs] do_writepages+0xe7/0x260 ? srso_return_thunk+0x5/0x5f ? lock_acquire+0xd2/0x300 ? srso_return_thunk+0x5/0x5f ? find_held_lock+0x2b/0x80 ? wbc_attach_and_unlock_inode.part.0+0x102/0x250 ? wbc_attach_and_unlock_inode.part.0+0x102/0x250 __writeback_single_inode+0x5c/0x4b0 writeback_sb_inodes+0x22d/0x550 __writeback_inodes_wb+0x4c/0xe0 wb_writeback+0x2f6/0x3f0 wb_workfn+0x32a/0x510 process_one_work+0x1ee/0x570 ? srso_return_thunk+0x5/0x5f worker_thread+0x1d1/0x3b0 ? __pfx_worker_thread+0x10/0x10 kthread+0x10b/0x230 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x30/0x50 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1a/0x30 </TASK> This happens because we have another success path for the zoned mode. When there is no active zone available, btrfs_reserve_extent() returns -EAGAIN. In this case, we have two reactions. (1) If the given range is never allocated, we can only wait for someone to finish a zone, so wait on BTRFS_FS_NEED_ZONE_FINISH bit and retry afterward. (2) Or, if some allocations are already done, we must bail out and let the caller to send IOs for the allocation. This is because these IOs may be necessary to finish a zone. The commit 06f364284794 ("btrfs: do proper folio cleanup when cow_file_range() failed") moved the unlock code from the inside of the loop to the outside. So, previously, the allocated extents are unlocked just after the allocation and so before returning from the function. However, they are no longer unlocked on the case (2) above. That caused the hang issue. Fix the issue by modifying the 'end' to the end of the allocated range. Then, we can exit the loop and the same unlock code can properly handle the case.

References

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