# CVE-2025-40119: ext4: fix potential null deref in ext4_mb_init()

> A local attacker with mount privileges can cause a kernel crash via null pointer dereference in ext4 filesystem initialization.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 12 Nov 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-40119, GHSA-336v-j3x2-qmh8, UBUNTU-CVE-2025-40119

## What is CVE-2025-40119?

CVE-2025-40119 is a vulnerability in Linux, published on 12 Nov 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:

ext4: fix potential null deref in ext4_mb_init()

In ext4_mb_init(), ext4_mb_avg_fragment_size_destroy() may be called
when sbi->s_mb_avg_fragment_size remains uninitialized (e.g., if groupinfo
slab cache allocation fails). Since ext4_mb_avg_fragment_size_destroy()
lacks null pointer checking, this leads to a null pointer dereference.

==================================================================
EXT4-fs: no memory for groupinfo slab cache
BUG: kernel NULL pointer dereference, address: 0000000000000000
PGD 0 P4D 0
Oops: Oops: 0002 [#1] SMP PTI
CPU:2 UID: 0 PID: 87 Comm:mount Not tainted 6.17.0-rc2 #1134 PREEMPT(none)
RIP: 0010:_raw_spin_lock_irqsave+0x1b/0x40
Call Trace:
 <TASK>
 xa_destroy+0x61/0x130
 ext4_mb_init+0x483/0x540
 __ext4_fill_super+0x116d/0x17b0
 ext4_fill_super+0xd3/0x280
 get_tree_bdev_flags+0x132/0x1d0
 vfs_get_tree+0x29/0xd0
 do_new_mount+0x197/0x300
 __x64_sys_mount+0x116/0x150
 do_syscall_64+0x50/0x1c0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
==================================================================

Therefore, add necessary null check to ext4_mb_avg_fragment_size_destroy()
to prevent this issue. The same fix is also applied to
ext4_mb_largest_free_orders_destroy().

## Which versions are affected by CVE-2025-40119?


## How do I fix CVE-2025-40119?

Apply the vendor's patch.

## Is CVE-2025-40119 being exploited?

There is no known exploitation of CVE-2025-40119 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40119 and fix it if it is affected.

What it is: CVE-2025-40119 is a vulnerability in Linux, published on 12 Nov 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-2025-40119 in the wild and no public exploit code.

Affected product: Linux. 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-2025-40119/ (Markdown: https://insights.sbomify.com/CVE-2025-40119/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40119
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40119
- OSV: https://osv.dev/vulnerability/CVE-2025-40119

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40119/
Summaries are generated and may contain mistakes; the official description above is the record.