btrfs: zoned: fix extent range end unlock in cow_file_range()
CVSS score
5.5
EPSS probability
%0.2
Risk score
0.0
Published
1 y ago
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Vulnerability description
DayBreach CVE AI Research
CVE-2025-21942 has vendor, distribution, patch and security-source records on file. We can compare them and prepare an explanation linked to the evidence.
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.
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SCORING: CVSS from Multiple Sources
CVSS Scores
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| SCORE | VERSION | SEVERITY | VECTOR | SOURCE |
|---|---|---|---|---|
| 5.5 | CVSS V31 | MEDIUM | CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H | NVD |
Patch and Remediation Guide
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Public exploit and active exploitation status
Verified exploit intelligence
No verified public PoC, exploit repository or Metasploit module has been found for this CVE yet.
Weakness Classification and Attack Patterns (CWE & CAPEC)
CWE - Common Weakness Enumeration
While CVE identifies specific instances of vulnerabilities, CWE categorises the common flaws or weaknesses that can lead to vulnerabilities. CVE-2025-21942 is associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification (CAPEC) stores attack patterns, which are descriptions of the common attributes and approaches employed by adversaries to exploit the CVE-2025-21942 weaknesses.
Attack Vector and Accessibility
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