# CVE-2025-21886: RDMA/mlx5: Fix implicit ODP hang on parent deregistration

> A reference counting bug in the Linux kernel's mlx5 RDMA driver causes a system hang during memory deregistration, leading to denial of service.

- **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 Mar 2025
- **Updated:** 21 Oct 2026
- **Also known as:** BELL-CVE-2025-21886, DEBIAN-CVE-2025-21886, GHSA-4q32-287f-wf74, UBUNTU-CVE-2025-21886

## What is CVE-2025-21886?

CVE-2025-21886 is a medium-severity denial of service vulnerability in Linux kernel, published on 27 Mar 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:

RDMA/mlx5: Fix implicit ODP hang on parent deregistration

Fix the destroy_unused_implicit_child_mr() to prevent hanging during
parent deregistration as of below [1].

Upon entering destroy_unused_implicit_child_mr(), the reference count
for the implicit MR parent is incremented using:
refcount_inc_not_zero().

A corresponding decrement must be performed if
free_implicit_child_mr_work() is not called.

The code has been updated to properly manage the reference count that
was incremented.

[1]
INFO: task python3:2157 blocked for more than 120 seconds.
Not tainted 6.12.0-rc7+ #1633
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:python3         state:D stack:0     pid:2157 tgid:2157  ppid:1685   flags:0x00000000
Call Trace:
<TASK>
__schedule+0x420/0xd30
schedule+0x47/0x130
__mlx5_ib_dereg_mr+0x379/0x5d0 [mlx5_ib]
? __pfx_autoremove_wake_function+0x10/0x10
ib_dereg_mr_user+0x5f/0x120 [ib_core]
? lock_release+0xc6/0x280
destroy_hw_idr_uobject+0x1d/0x60 [ib_uverbs]
uverbs_destroy_uobject+0x58/0x1d0 [ib_uverbs]
uobj_destroy+0x3f/0x70 [ib_uverbs]
ib_uverbs_cmd_verbs+0x3e4/0xbb0 [ib_uverbs]
? __pfx_uverbs_destroy_def_handler+0x10/0x10 [ib_uverbs]
? lock_acquire+0xc1/0x2f0
? ib_uverbs_ioctl+0xcb/0x170 [ib_uverbs]
? ib_uverbs_ioctl+0x116/0x170 [ib_uverbs]
? lock_release+0xc6/0x280
ib_uverbs_ioctl+0xe7/0x170 [ib_uverbs]
? ib_uverbs_ioctl+0xcb/0x170 [ib_uverbs]
 __x64_sys_ioctl+0x1b0/0xa70
? kmem_cache_free+0x221/0x400
do_syscall_64+0x6b/0x140
entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f20f21f017b
RSP: 002b:00007ffcfc4a77c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007ffcfc4a78d8 RCX: 00007f20f21f017b
RDX: 00007ffcfc4a78c0 RSI: 00000000c0181b01 RDI: 0000000000000003
RBP: 00007ffcfc4a78a0 R08: 000056147d125190 R09: 00007f20f1f14c60
R10: 0000000000000001 R11: 0000000000000246 R12: 00007ffcfc4a7890
R13: 000000000000001c R14: 000056147d100fc0 R15: 00007f20e365c9d0
</TASK>

## Which versions are affected by CVE-2025-21886?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21886?

Apply the vendor's patch.

## Is CVE-2025-21886 being exploited?

There is no known exploitation of CVE-2025-21886 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-2025-21886 and fix it if it is affected.

What it is: CVE-2025-21886 is a medium-severity denial of service vulnerability in Linux kernel, published on 27 Mar 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-2025-21886 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-2025-21886/ (Markdown: https://insights.sbomify.com/CVE-2025-21886/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21886
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21886
- OSV: https://osv.dev/vulnerability/CVE-2025-21886

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21886/
Summaries are generated and may contain mistakes; the official description above is the record.