Cloudanix VulnDB

CVE-2025-39779

None

CVSS

—

EPSS

0.2%

Published

Sep 11, 2025

CISA due date

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Cloudanix analysis

Schedule a fix SSVC: Track

A flaw in the Linux file system code can cause the computer to crash when multiple programs write to the same file at the same time on certain storage setups.

Exploitation
Local Privilege
Attacker needs
Local User
Complexity
Moderate
Impact
Denial Of Service
Component
OS kernel
Class
Race Condition
Remediation
Upgrade
Effort
Medium
Patch risk
Low

What it actually does

btrfs_subpage_set_writeback() prematurely clears the PAGECACHE_TAG_TOWRITE tag on folios with partial dirty blocks during writeback in zoned or compressed configurations, violating WB_SYNC_ALL ordering guarantees required by btrfs_wait_ordered_extents and leading to assertion failures in btrfs_finish_ordered_zoned.

Why it matters to the business

Systems using btrfs on zoned storage or with compression may experience unexpected kernel crashes during concurrent file writes, causing downtime, potential data loss on uncommitted operations, and requiring reboots; the issue affects many Linux distributions with btrfs support.

How an attack unfolds

An attacker with local access triggers concurrent writepages calls (one with WB_SYNC_NONE and another with WB_SYNC_ALL) against a btrfs filesystem in zoned mode or using compression; the race between tagging dirty folios and clearing the TOWRITE tag allows a write to complete without waiting on all ordered extents, hitting the assertion in btrfs_finish_ordered_zoned.

How to fix it

Upgrade the Linux kernel to a version containing commit 3d61136945a7008fc90d013c3c67007ce0c96131 or later; rebuild and reboot with the patched btrfs module. For MinimOS packages, update kibana-8, kibana-8-config, and kibana-8-oci-entrypoint to 8.18.3-r0 or newer.

How to confirm the fix worked

Verify the running kernel contains commit 3d61136945a7008fc90d013c3c67007ce0c96131 via git log or uname; confirm absence of the assertion by running fstests generic/464 on zoned btrfs; inspect fs/btrfs/zoned.c for the updated btrfs_subpage_set_writeback_keepwrite logic.

Typical attack path

  1. Mount a btrfs filesystem configured for zoned storage or compression
  2. Run concurrent processes performing partial-page writes and fsync-style operations
  3. Trigger writeback via writepages with mixed WB_SYNC_NONE and WB_SYNC_ALL semantics
  4. Race causes premature TOWRITE tag removal on a partially dirty folio
  5. btrfs_wait_ordered_extents fails to see remaining dirty pages before truncation or completion
  6. Kernel hits assertion failure and oops in btrfs_finish_ordered_zoned

How to detect it

Search kernel logs for 'assertion failed: !list_empty(&ordered->list)' and 'btrfs_finish_ordered_zoned.cold' stack traces; monitor for oops involving btrfs_work_helper and zoned.c on systems with btrfs and zoned storage.

What your security team should do

  • Prioritize patching btrfs-based systems within 30 days given the public fix and crash potential
  • Restrict untrusted local accounts on multi-user systems using btrfs until patched
  • Monitor for btrfs-related kernel oops in logs on zoned or compressed volumes
  • Test the fixed kernel in staging environments with fstests before broad deployment

The record

In the Linux kernel, the following vulnerability has been resolved: btrfs: subpage: keep TOWRITE tag until folio is cleaned btrfs_subpage_set_writeback() calls folio_start_writeback() the first time a folio is written back, and it also clears the PAGECACHE_TAG_TOWRITE tag even if there are still dirty blocks in the folio. This can break ordering guarantees, such as those required by btrfs_wait_ordered_extents(). That ordering breakage leads to a real failure. For example, running generic/464 on a zoned setup will hit the following ASSERT. This happens because the broken ordering fails to flush existing dirty pages before the file size is truncated. assertion failed: !list_empty(&ordered->list) :: 0, in fs/btrfs/zoned.c:1899 ------------[ cut here ]------------ kernel BUG at fs/btrfs/zoned.c:1899! Oops: invalid opcode: 0000 [#1] SMP NOPTI CPU: 2 UID: 0 PID: 1906169 Comm: kworker/u130:2 Kdump: loaded Not tainted 6.16.0-rc6-BTRFS-ZNS+ #554 PREEMPT(voluntary) Hardware name: Supermicro Super Server/H12SSL-NT, BIOS 2.0 02/22/2021 Workqueue: btrfs-endio-write btrfs_work_helper [btrfs] RIP: 0010:btrfs_finish_ordered_zoned.cold+0x50/0x52 [btrfs] RSP: 0018:ffffc9002efdbd60 EFLAGS: 00010246 RAX: 000000000000004c RBX: ffff88811923c4e0 RCX: 0000000000000000 RDX: 0000000000000000 RSI: ffffffff827e38b1 RDI: 00000000ffffffff RBP: ffff88810005d000 R08: 00000000ffffdfff R09: ffffffff831051c8 R10: ffffffff83055220 R11: 0000000000000000 R12: ffff8881c2458c00 R13: ffff88811923c540 R14: ffff88811923c5e8 R15: ffff8881c1bd9680 FS: 0000000000000000(0000) GS:ffff88a04acd0000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f907c7a918c CR3: 0000000004024000 CR4: 0000000000350ef0 Call Trace: <TASK> ? srso_return_thunk+0x5/0x5f btrfs_finish_ordered_io+0x4a/0x60 [btrfs] btrfs_work_helper+0xf9/0x490 [btrfs] process_one_work+0x204/0x590 ? srso_return_thunk+0x5/0x5f worker_thread+0x1d6/0x3d0 ? __pfx_worker_thread+0x10/0x10 kthread+0x118/0x230 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x205/0x260 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1a/0x30 </TASK> Consider process A calling writepages() with WB_SYNC_NONE. In zoned mode or for compressed writes, it locks several folios for delalloc and starts writing them out. Let's call the last locked folio folio X. Suppose the write range only partially covers folio X, leaving some pages dirty. Process A calls btrfs_subpage_set_writeback() when building a bio. This function call clears the TOWRITE tag of folio X, whose size = 8K and the block size = 4K. It is following state. 0 4K 8K |/////|/////| (flag: DIRTY, tag: DIRTY) <-----> Process A will write this range. Now suppose process B concurrently calls writepages() with WB_SYNC_ALL. It calls tag_pages_for_writeback() to tag dirty folios with PAGECACHE_TAG_TOWRITE. Since folio X is still dirty, it gets tagged. Then, B collects tagged folios using filemap_get_folios_tag() and must wait for folio X to be written before returning from writepages(). 0 4K 8K |/////|/////| (flag: DIRTY, tag: DIRTY|TOWRITE) However, between tagging and collecting, process A may call btrfs_subpage_set_writeback() and clear folio X's TOWRITE tag. 0 4K 8K | |/////| (flag: DIRTY|WRITEBACK, tag: DIRTY) As a result, process B won't see folio X in its batch, and returns without waiting for it. This breaks the WB_SYNC_ALL ordering requirement. Fix this by using btrfs_subpage_set_writeback_keepwrite(), which retains the TOWRITE tag. We now manually clear the tag only after the folio becomes clean, via the xas operation.

Affected packages

Ecosystem Package Introduced Fixed
Debian:12 linux 0 No fix
Debian:13 linux 0 No fix
Debian:14 linux 0 No fix
Debian:14 linux 0 6.16.5-1
Linux Kernel 5.13.0 6.12.44
openSUSE:Leap 16.0 dtb-aarch64 0 6.12.0-160000.7.1
openSUSE:Leap 16.0 kernel-64kb 0 6.12.0-160000.7.1
openSUSE:Leap 16.0 kernel-default 0 6.12.0-160000.7.1
openSUSE:Leap 16.0 kernel-default-base 0 6.12.0-160000.6.1.160000.2.4
openSUSE:Leap 16.0 kernel-docs 0 6.12.0-160000.7.1
openSUSE:Leap 16.0 kernel-kvmsmall 0 6.12.0-160000.7.1
openSUSE:Leap 16.0 kernel-obs-build 0 6.12.0-160000.7.1
openSUSE:Leap 16.0 kernel-obs-qa 0 6.12.0-160000.7.1
openSUSE:Leap 16.0 kernel-rt 0 6.12.0-160000.7.1
openSUSE:Leap 16.0 kernel-source 0 6.12.0-160000.7.1
openSUSE:Leap 16.0 kernel-syms 0 6.12.0-160000.7.1
openSUSE:Leap 16.0 kernel-zfcpdump 0 6.12.0-160000.7.1

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