# CVE-2021-47242: mptcp: fix soft lookup in subflow_error_report()

> A soft lockup in the Linux kernel's MPTCP subflow error reporting can cause a denial of service by hanging the CPU.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 21 May 2024
- **Updated:** 5 Aug 2026
- **Also known as:** GHSA-gc8r-vh42-27g4, UBUNTU-CVE-2021-47242

## What is CVE-2021-47242?

CVE-2021-47242 is a high-severity denial of service vulnerability in Linux kernel, published on 21 May 2024. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

mptcp: fix soft lookup in subflow_error_report()

Maxim reported a soft lookup in subflow_error_report():

 watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [swapper/0:0]
 RIP: 0010:native_queued_spin_lock_slowpath
 RSP: 0018:ffffa859c0003bc0 EFLAGS: 00000202
 RAX: 0000000000000101 RBX: 0000000000000001 RCX: 0000000000000000
 RDX: ffff9195c2772d88 RSI: 0000000000000000 RDI: ffff9195c2772d88
 RBP: ffff9195c2772d00 R08: 00000000000067b0 R09: c6e31da9eb1e44f4
 R10: ffff9195ef379700 R11: ffff9195edb50710 R12: ffff9195c2772d88
 R13: ffff9195f500e3d0 R14: ffff9195ef379700 R15: ffff9195ef379700
 FS:  0000000000000000(0000) GS:ffff91961f400000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 000000c000407000 CR3: 0000000002988000 CR4: 00000000000006f0
 Call Trace:
  <IRQ>
 _raw_spin_lock_bh
 subflow_error_report
 mptcp_subflow_data_available
 __mptcp_move_skbs_from_subflow
 mptcp_data_ready
 tcp_data_queue
 tcp_rcv_established
 tcp_v4_do_rcv
 tcp_v4_rcv
 ip_protocol_deliver_rcu
 ip_local_deliver_finish
 __netif_receive_skb_one_core
 netif_receive_skb
 rtl8139_poll 8139too
 __napi_poll
 net_rx_action
 __do_softirq
 __irq_exit_rcu
 common_interrupt
  </IRQ>

The calling function - mptcp_subflow_data_available() - can be invoked
from different contexts:
- plain ssk socket lock
- ssk socket lock + mptcp_data_lock
- ssk socket lock + mptcp_data_lock + msk socket lock.

Since subflow_error_report() tries to acquire the mptcp_data_lock, the
latter two call chains will cause soft lookup.

This change addresses the issue moving the error reporting call to
outer functions, where the held locks list is known and the we can
acquire only the needed one.

Weakness: CWE-667 (Improper Locking)

## Which versions are affected by CVE-2021-47242?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47242?

No fix has been published for CVE-2021-47242 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-47242 being exploited?

There is no known exploitation of CVE-2021-47242 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-2021-47242 and fix it if it is affected.

What it is: CVE-2021-47242 is a high-severity denial of service vulnerability in Linux kernel, published on 21 May 2024. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-47242 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-2021-47242/ (Markdown: https://insights.sbomify.com/CVE-2021-47242/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47242
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47242
- OSV: https://osv.dev/vulnerability/CVE-2021-47242

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47242/
Summaries are generated and may contain mistakes; the official description above is the record.