# CVE-2025-21754: btrfs: fix assertion failure when splitting ordered extent after transaction abort

> A local attacker can trigger a kernel assertion failure and crash the system by performing direct IO writes during a transaction abort.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Feb 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-21754, DEBIAN-CVE-2025-21754, GHSA-rv8h-hrgv-p5cw, ROOT-OS-UBUNTU-2404-CVE-2025-21754, UBUNTU-CVE-2025-21754

## What is CVE-2025-21754?

CVE-2025-21754 is a medium-severity denial of service vulnerability in Linux kernel, published on 27 Feb 2025. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix assertion failure when splitting ordered extent after transaction abort

If while we are doing a direct IO write a transaction abort happens, we
mark all existing ordered extents with the BTRFS_ORDERED_IOERR flag (done
at btrfs_destroy_ordered_extents()), and then after that if we enter
btrfs_split_ordered_extent() and the ordered extent has bytes left
(meaning we have a bio that doesn't cover the whole ordered extent, see
details at btrfs_extract_ordered_extent()), we will fail on the following
assertion at btrfs_split_ordered_extent():

   ASSERT(!(flags & ~BTRFS_ORDERED_TYPE_FLAGS));

because the BTRFS_ORDERED_IOERR flag is set and the definition of
BTRFS_ORDERED_TYPE_FLAGS is just the union of all flags that identify the
type of write (regular, nocow, prealloc, compressed, direct IO, encoded).

Fix this by returning an error from btrfs_extract_ordered_extent() if we
find the BTRFS_ORDERED_IOERR flag in the ordered extent. The error will
be the error that resulted in the transaction abort or -EIO if no
transaction abort happened.

This was recently reported by syzbot with the following trace:

   FAULT_INJECTION: forcing a failure.
   name failslab, interval 1, probability 0, space 0, times 1
   CPU: 0 UID: 0 PID: 5321 Comm: syz.0.0 Not tainted 6.13.0-rc5-syzkaller #0
   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
   Call Trace:
    <TASK>
    __dump_stack lib/dump_stack.c:94 [inline]
    dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
    fail_dump lib/fault-inject.c:53 [inline]
    should_fail_ex+0x3b0/0x4e0 lib/fault-inject.c:154
    should_failslab+0xac/0x100 mm/failslab.c:46
    slab_pre_alloc_hook mm/slub.c:4072 [inline]
    slab_alloc_node mm/slub.c:4148 [inline]
    __do_kmalloc_node mm/slub.c:4297 [inline]
    __kmalloc_noprof+0xdd/0x4c0 mm/slub.c:4310
    kmalloc_noprof include/linux/slab.h:905 [inline]
    kzalloc_noprof include/linux/slab.h:1037 [inline]
    btrfs_chunk_alloc_add_chunk_item+0x244/0x1100 fs/btrfs/volumes.c:5742
    reserve_chunk_space+0x1ca/0x2c0 fs/btrfs/block-group.c:4292
    check_system_chunk fs/btrfs/block-group.c:4319 [inline]
    do_chunk_alloc fs/btrfs/block-group.c:3891 [inline]
    btrfs_chunk_alloc+0x77b/0xf80 fs/btrfs/block-group.c:4187
    find_free_extent_update_loop fs/btrfs/extent-tree.c:4166 [inline]
    find_free_extent+0x42d1/0x5810 fs/btrfs/extent-tree.c:4579
    btrfs_reserve_extent+0x422/0x810 fs/btrfs/extent-tree.c:4672
    btrfs_new_extent_direct fs/btrfs/direct-io.c:186 [inline]
    btrfs_get_blocks_direct_write+0x706/0xfa0 fs/btrfs/direct-io.c:321
    btrfs_dio_iomap_begin+0xbb7/0x1180 fs/btrfs/direct-io.c:525
    iomap_iter+0x697/0xf60 fs/iomap/iter.c:90
    __iomap_dio_rw+0xeb9/0x25b0 fs/iomap/direct-io.c:702
    btrfs_dio_write fs/btrfs/direct-io.c:775 [inline]
    btrfs_direct_write+0x610/0xa30 fs/btrfs/direct-io.c:880
    btrfs_do_write_iter+0x2a0/0x760 fs/btrfs/file.c:1397
    do_iter_readv_writev+0x600/0x880
    vfs_writev+0x376/0xba0 fs/read_write.c:1050
    do_pwritev fs/read_write.c:1146 [inline]
    __do_sys_pwritev2 fs/read_write.c:1204 [inline]
    __se_sys_pwritev2+0x196/0x2b0 fs/read_write.c:1195
    do_syscall_x64 arch/x86/entry/common.c:52 [inline]
    do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
    entry_SYSCALL_64_after_hwframe+0x77/0x7f
   RIP: 0033:0x7f1281f85d29
   RSP: 002b:00007f12819fe038 EFLAGS: 00000246 ORIG_RAX: 0000000000000148
   RAX: ffffffffffffffda RBX: 00007f1282176080 RCX: 00007f1281f85d29
   RDX: 0000000000000001 RSI: 0000000020000240 RDI: 0000000000000005
   RBP: 00007f12819fe090 R08: 0000000000000000 R09: 0000000000000003
   R10: 0000000000007000 R11: 0000000000000246 R12: 0000000000000002
   R13: 0000000000000000 R14: 00007f1282176080 R15: 00007ffcb9e23328
    </TASK>
   BTRFS error (device loop0 state A): Transaction aborted (error -12)
   BTRFS: error (device loop0 state A
---truncated---

Weakness: CWE-617 (Reachable Assertion)

## Which versions are affected by CVE-2025-21754?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21754?

Apply the vendor's patch.

## Is CVE-2025-21754 being exploited?

There is no known exploitation of CVE-2025-21754 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-21754 and fix it if it is affected.

What it is: CVE-2025-21754 is a medium-severity denial of service vulnerability in Linux kernel, published on 27 Feb 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-21754 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-21754/ (Markdown: https://insights.sbomify.com/CVE-2025-21754/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21754
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21754
- OSV: https://osv.dev/vulnerability/CVE-2025-21754

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21754/
Summaries are generated and may contain mistakes; the official description above is the record.