# CVE-2025-21970: net/mlx5: Bridge, fix the crash caused by LAG state check

> A null pointer dereference in the Linux kernel mlx5 bridge driver causes a system crash when removing a LAG device from a bridge.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/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:** 1 Apr 2025
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2025-21970, DEBIAN-CVE-2025-21970, GHSA-m5q5-9mr9-g93x, ROOT-OS-UBUNTU-2404-CVE-2025-21970, UBUNTU-CVE-2025-21970

## What is CVE-2025-21970?

CVE-2025-21970 is a high-severity vulnerability in Linux kernel, published on 1 Apr 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:

net/mlx5: Bridge, fix the crash caused by LAG state check

When removing LAG device from bridge, NETDEV_CHANGEUPPER event is
triggered. Driver finds the lower devices (PFs) to flush all the
offloaded entries. And mlx5_lag_is_shared_fdb is checked, it returns
false if one of PF is unloaded. In such case,
mlx5_esw_bridge_lag_rep_get() and its caller return NULL, instead of
the alive PF, and the flush is skipped.

Besides, the bridge fdb entry's lastuse is updated in mlx5 bridge
event handler. But this SWITCHDEV_FDB_ADD_TO_BRIDGE event can be
ignored in this case because the upper interface for bond is deleted,
and the entry will never be aged because lastuse is never updated.

To make things worse, as the entry is alive, mlx5 bridge workqueue
keeps sending that event, which is then handled by kernel bridge
notifier. It causes the following crash when accessing the passed bond
netdev which is already destroyed.

To fix this issue, remove such checks. LAG state is already checked in
commit 15f8f168952f ("net/mlx5: Bridge, verify LAG state when adding
bond to bridge"), driver still need to skip offload if LAG becomes
invalid state after initialization.

 Oops: stack segment: 0000 [#1] SMP
 CPU: 3 UID: 0 PID: 23695 Comm: kworker/u40:3 Tainted: G           OE      6.11.0_mlnx #1
 Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
 Workqueue: mlx5_bridge_wq mlx5_esw_bridge_update_work [mlx5_core]
 RIP: 0010:br_switchdev_event+0x2c/0x110 [bridge]
 Code: 44 00 00 48 8b 02 48 f7 00 00 02 00 00 74 69 41 54 55 53 48 83 ec 08 48 8b a8 08 01 00 00 48 85 ed 74 4a 48 83 fe 02 48 89 d3 <4c> 8b 65 00 74 23 76 49 48 83 fe 05 74 7e 48 83 fe 06 75 2f 0f b7
 RSP: 0018:ffffc900092cfda0 EFLAGS: 00010297
 RAX: ffff888123bfe000 RBX: ffffc900092cfe08 RCX: 00000000ffffffff
 RDX: ffffc900092cfe08 RSI: 0000000000000001 RDI: ffffffffa0c585f0
 RBP: 6669746f6e690a30 R08: 0000000000000000 R09: ffff888123ae92c8
 R10: 0000000000000000 R11: fefefefefefefeff R12: ffff888123ae9c60
 R13: 0000000000000001 R14: ffffc900092cfe08 R15: 0000000000000000
 FS:  0000000000000000(0000) GS:ffff88852c980000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007f15914c8734 CR3: 0000000002830005 CR4: 0000000000770ef0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 PKRU: 55555554
 Call Trace:
  <TASK>
  ? __die_body+0x1a/0x60
  ? die+0x38/0x60
  ? do_trap+0x10b/0x120
  ? do_error_trap+0x64/0xa0
  ? exc_stack_segment+0x33/0x50
  ? asm_exc_stack_segment+0x22/0x30
  ? br_switchdev_event+0x2c/0x110 [bridge]
  ? sched_balance_newidle.isra.149+0x248/0x390
  notifier_call_chain+0x4b/0xa0
  atomic_notifier_call_chain+0x16/0x20
  mlx5_esw_bridge_update+0xec/0x170 [mlx5_core]
  mlx5_esw_bridge_update_work+0x19/0x40 [mlx5_core]
  process_scheduled_works+0x81/0x390
  worker_thread+0x106/0x250
  ? bh_worker+0x110/0x110
  kthread+0xb7/0xe0
  ? kthread_park+0x80/0x80
  ret_from_fork+0x2d/0x50
  ? kthread_park+0x80/0x80
  ret_from_fork_asm+0x11/0x20
  </TASK>

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2025-21970?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21970?

Apply the vendor's patch.

## Is CVE-2025-21970 being exploited?

There is no known exploitation of CVE-2025-21970 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-2025-21970 and fix it if it is affected.

What it is: CVE-2025-21970 is a high-severity vulnerability in Linux kernel, published on 1 Apr 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-2025-21970 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-2025-21970/ (Markdown: https://insights.sbomify.com/CVE-2025-21970/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21970
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21970
- OSV: https://osv.dev/vulnerability/CVE-2025-21970

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21970/
Summaries are generated and may contain mistakes; the official description above is the record.