# CVE-2021-47038: Bluetooth: avoid deadlock between hci_dev->lock and socket lock

> A deadlock in the Linux kernel Bluetooth subsystem can cause a denial of service when specific socket operations are performed.

- **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:** 28 Feb 2024
- **Updated:** 11 May 2026
- **Also known as:** DEBIAN-CVE-2021-47038, GHSA-3jvq-9rg9-g8rp, UBUNTU-CVE-2021-47038

## What is CVE-2021-47038?

CVE-2021-47038 is a medium-severity vulnerability in Linux kernel, published on 28 Feb 2024. 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:

Bluetooth: avoid deadlock between hci_dev->lock and socket lock

Commit eab2404ba798 ("Bluetooth: Add BT_PHY socket option") added a
dependency between socket lock and hci_dev->lock that could lead to
deadlock.

It turns out that hci_conn_get_phy() is not in any way relying on hdev
being immutable during the runtime of this function, neither does it even
look at any of the members of hdev, and as such there is no need to hold
that lock.

This fixes the lockdep splat below:

 ======================================================
 WARNING: possible circular locking dependency detected
 5.12.0-rc1-00026-g73d464503354 #10 Not tainted
 ------------------------------------------------------
 bluetoothd/1118 is trying to acquire lock:
 ffff8f078383c078 (&hdev->lock){+.+.}-{3:3}, at: hci_conn_get_phy+0x1c/0x150 [bluetooth]

 but task is already holding lock:
 ffff8f07e831d920 (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}-{0:0}, at: l2cap_sock_getsockopt+0x8b/0x610

 which lock already depends on the new lock.

 the existing dependency chain (in reverse order) is:

 -> #3 (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}-{0:0}:
        lock_sock_nested+0x72/0xa0
        l2cap_sock_ready_cb+0x18/0x70 [bluetooth]
        l2cap_config_rsp+0x27a/0x520 [bluetooth]
        l2cap_sig_channel+0x658/0x1330 [bluetooth]
        l2cap_recv_frame+0x1ba/0x310 [bluetooth]
        hci_rx_work+0x1cc/0x640 [bluetooth]
        process_one_work+0x244/0x5f0
        worker_thread+0x3c/0x380
        kthread+0x13e/0x160
        ret_from_fork+0x22/0x30

 -> #2 (&chan->lock#2/1){+.+.}-{3:3}:
        __mutex_lock+0xa3/0xa10
        l2cap_chan_connect+0x33a/0x940 [bluetooth]
        l2cap_sock_connect+0x141/0x2a0 [bluetooth]
        __sys_connect+0x9b/0xc0
        __x64_sys_connect+0x16/0x20
        do_syscall_64+0x33/0x80
        entry_SYSCALL_64_after_hwframe+0x44/0xae

 -> #1 (&conn->chan_lock){+.+.}-{3:3}:
        __mutex_lock+0xa3/0xa10
        l2cap_chan_connect+0x322/0x940 [bluetooth]
        l2cap_sock_connect+0x141/0x2a0 [bluetooth]
        __sys_connect+0x9b/0xc0
        __x64_sys_connect+0x16/0x20
        do_syscall_64+0x33/0x80
        entry_SYSCALL_64_after_hwframe+0x44/0xae

 -> #0 (&hdev->lock){+.+.}-{3:3}:
        __lock_acquire+0x147a/0x1a50
        lock_acquire+0x277/0x3d0
        __mutex_lock+0xa3/0xa10
        hci_conn_get_phy+0x1c/0x150 [bluetooth]
        l2cap_sock_getsockopt+0x5a9/0x610 [bluetooth]
        __sys_getsockopt+0xcc/0x200
        __x64_sys_getsockopt+0x20/0x30
        do_syscall_64+0x33/0x80
        entry_SYSCALL_64_after_hwframe+0x44/0xae

 other info that might help us debug this:

 Chain exists of:
   &hdev->lock --> &chan->lock#2/1 --> sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP

  Possible unsafe locking scenario:

        CPU0                    CPU1
        ----                    ----
   lock(sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP);
                                lock(&chan->lock#2/1);
                                lock(sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP);
   lock(&hdev->lock);

  *** DEADLOCK ***

 1 lock held by bluetoothd/1118:
  #0: ffff8f07e831d920 (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}-{0:0}, at: l2cap_sock_getsockopt+0x8b/0x610 [bluetooth]

 stack backtrace:
 CPU: 3 PID: 1118 Comm: bluetoothd Not tainted 5.12.0-rc1-00026-g73d464503354 #10
 Hardware name: LENOVO 20K5S22R00/20K5S22R00, BIOS R0IET38W (1.16 ) 05/31/2017
 Call Trace:
  dump_stack+0x7f/0xa1
  check_noncircular+0x105/0x120
  ? __lock_acquire+0x147a/0x1a50
  __lock_acquire+0x147a/0x1a50
  lock_acquire+0x277/0x3d0
  ? hci_conn_get_phy+0x1c/0x150 [bluetooth]
  ? __lock_acquire+0x2e1/0x1a50
  ? lock_is_held_type+0xb4/0x120
  ? hci_conn_get_phy+0x1c/0x150 [bluetooth]
  __mutex_lock+0xa3/0xa10
  ? hci_conn_get_phy+0x1c/0x150 [bluetooth]
  ? lock_acquire+0x277/0x3d0
  ? mark_held_locks+0x49/0x70
  ? mark_held_locks+0x49/0x70
  ? hci_conn_get_phy+0x1c/0x150 [bluetooth]
  hci_conn_get_phy+0x
---truncated---

Weakness: CWE-667 (Improper Locking)

## Which versions are affected by CVE-2021-47038?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47038?

Apply the vendor's patch.

## Is CVE-2021-47038 being exploited?

There is no known exploitation of CVE-2021-47038 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-2021-47038 and fix it if it is affected.

What it is: CVE-2021-47038 is a medium-severity vulnerability in Linux kernel, published on 28 Feb 2024. 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-2021-47038 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-2021-47038/ (Markdown: https://insights.sbomify.com/CVE-2021-47038/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47038
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47038
- OSV: https://osv.dev/vulnerability/CVE-2021-47038

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47038/
Summaries are generated and may contain mistakes; the official description above is the record.