---
id: CVE-2025-21658
title: 'btrfs: avoid NULL pointer dereference if no valid extent tree'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  btrfs: avoid NULL pointer dereference if no valid extent tree

  [BUG]
  Syzbot reported a crash with the following call trace:

    BTRFS info (device loop0): scrub: started…
severity: medium
cvss: 5.5
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'
cvssSource: adp
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 42437a6386ffeaaf200731e73d723ea491f3fe7d <
    1fb37b7840fdcbf69d062a79d14afbf566b11873
  - >-
    Linux >= 42437a6386ffeaaf200731e73d723ea491f3fe7d <
    24b85a8b0310e0144da9ab30be42e87e6476638a
  - >-
    Linux >= 42437a6386ffeaaf200731e73d723ea491f3fe7d <
    aee5f69f3e6cd82bfefaca1b70b40b6cd8f3f784
  - >-
    Linux >= 42437a6386ffeaaf200731e73d723ea491f3fe7d <
    6aecd91a5c5b68939cf4169e32bc49f3cd2dd329
  - Linux 5.11
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2025-10-01T19:52:37.080379Z'
published: '2025-01-21'
updated: '2026-10-30'
sourceUpdated: '2026-10-30T12:20:41.665Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2025-21658'
references:
  - url: 'https://git.kernel.org/stable/c/1fb37b7840fdcbf69d062a79d14afbf566b11873'
  - url: 'https://git.kernel.org/stable/c/24b85a8b0310e0144da9ab30be42e87e6476638a'
  - url: 'https://git.kernel.org/stable/c/aee5f69f3e6cd82bfefaca1b70b40b6cd8f3f784'
  - url: 'https://git.kernel.org/stable/c/6aecd91a5c5b68939cf4169e32bc49f3cd2dd329'
tags:
  - cve.org
epss: 0.00203
epssPercentile: 0.10319
ingestedAt: '2026-11-03T15:48:07.696Z'
---

## Overview

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

btrfs: avoid NULL pointer dereference if no valid extent tree

[BUG]
Syzbot reported a crash with the following call trace:

  BTRFS info (device loop0): scrub: started on devid 1
  BUG: kernel NULL pointer dereference, address: 0000000000000208
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 106e70067 P4D 106e70067 PUD 107143067 PMD 0
  Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
  CPU: 1 UID: 0 PID: 689 Comm: repro Kdump: loaded Tainted: G           O       6.13.0-rc4-custom+ #206
  Tainted: [O]=OOT_MODULE
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022
  RIP: 0010:find_first_extent_item+0x26/0x1f0 [btrfs]
  Call Trace:
   <TASK>
   scrub_find_fill_first_stripe+0x13d/0x3b0 [btrfs]
   scrub_simple_mirror+0x175/0x260 [btrfs]
   scrub_stripe+0x5d4/0x6c0 [btrfs]
   scrub_chunk+0xbb/0x170 [btrfs]
   scrub_enumerate_chunks+0x2f4/0x5f0 [btrfs]
   btrfs_scrub_dev+0x240/0x600 [btrfs]
   btrfs_ioctl+0x1dc8/0x2fa0 [btrfs]
   ? do_sys_openat2+0xa5/0xf0
   __x64_sys_ioctl+0x97/0xc0
   do_syscall_64+0x4f/0x120
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
   </TASK>

[CAUSE]
The reproducer is using a corrupted image where extent tree root is
corrupted, thus forcing to use "rescue=all,ro" mount option to mount the
image.

Then it triggered a scrub, but since scrub relies on extent tree to find
where the data/metadata extents are, scrub_find_fill_first_stripe()
relies on an non-empty extent root.

But unfortunately scrub_find_fill_first_stripe() doesn't really expect
an NULL pointer for extent root, it use extent_root to grab fs_info and
triggered a NULL pointer dereference.

[FIX]
Add an extra check for a valid extent root at the beginning of
scrub_find_fill_first_stripe().

The new error path is introduced by 42437a6386ff ("btrfs: introduce
mount option rescue=ignorebadroots"), but that's pretty old, and later
commit b979547513ff ("btrfs: scrub: introduce helper to find and fill
sector info for a scrub_stripe") changed how we do scrub.

So for kernels older than 6.6, the fix will need manual backport.

## Affected

- `Linux >= 42437a6386ffeaaf200731e73d723ea491f3fe7d < 1fb37b7840fdcbf69d062a79d14afbf566b11873`
- `Linux >= 42437a6386ffeaaf200731e73d723ea491f3fe7d < 24b85a8b0310e0144da9ab30be42e87e6476638a`
- `Linux >= 42437a6386ffeaaf200731e73d723ea491f3fe7d < aee5f69f3e6cd82bfefaca1b70b40b6cd8f3f784`
- `Linux >= 42437a6386ffeaaf200731e73d723ea491f3fe7d < 6aecd91a5c5b68939cf4169e32bc49f3cd2dd329`
- `Linux 5.11`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.