# CVE-2026-64177: phonet/pep: disable BH around forwarded sk_receive_skb()

> A race condition in the Linux kernel Phonet protocol allows a local attacker to cause a system deadlock and 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:** 19 Jul 2026
- **Updated:** 19 Jul 2026
- **Also known as:** BELL-CVE-2026-64177, DEBIAN-CVE-2026-64177, GHSA-g3fg-4885-2jhq, UBUNTU-CVE-2026-64177

## What is CVE-2026-64177?

CVE-2026-64177 is a medium-severity vulnerability in Linux kernel, published on 19 Jul 2026. 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:

phonet/pep: disable BH around forwarded sk_receive_skb()

The networking receive path is usually run from softirq context, but
protocols that take the socket lock may have packets stored in the
backlog and processed later from process context. In that case
release_sock() -> __release_sock() drops the slock with spin_unlock_bh()
and then calls sk->sk_backlog_rcv() with bottom halves enabled.

Typical sk_backlog_rcv handlers process the socket whose backlog is
being drained, so the BH state at entry is irrelevant for the slocks
they touch. pep_do_rcv() is different: when the inbound skb targets an
existing PEP pipe, it forwards the skb to a different *child* socket
via sk_receive_skb(). That helper takes the child slock with
bh_lock_sock_nested(), which is just spin_lock_nested() and assumes BH
is already off. The same child slock therefore ends up acquired with
BH on (process path) and with BH off (softirq path):

  process context                   softirq context
  ---------------                   ---------------
  release_sock(listener)            __netif_receive_skb()
   __release_sock()                  phonet_rcv()
    spin_unlock_bh()                  __sk_receive_skb(listener)
    [BH now ENABLED]                  [BH already disabled]
    sk_backlog_rcv:                   sk_backlog_rcv:
     pep_do_rcv()                      pep_do_rcv()
      sk_receive_skb(child)             sk_receive_skb(child)
       bh_lock_sock_nested(child)        bh_lock_sock_nested(child)
       => SOFTIRQ-ON-W                   => IN-SOFTIRQ-W

Lockdep flags this as inconsistent lock state, and it can become a real
self-deadlock if a softirq on the same CPU tries to receive to the same
child socket while its slock is held in the BH-enabled path:

  WARNING: inconsistent lock state
  inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
   (slock-AF_PHONET/1){+.?.}-{3:3}, at: __sk_receive_skb+0x1cf/0x900
    __sk_receive_skb              net/core/sock.c:563
    sk_receive_skb                include/net/sock.h:2022 [inline]
    pep_do_rcv                    net/phonet/pep.c:675
    sk_backlog_rcv                include/net/sock.h:1190
    __release_sock                net/core/sock.c:3216
    release_sock                  net/core/sock.c:3815
    pep_sock_accept               net/phonet/pep.c:879

Wrap the forwarded sk_receive_skb() in local_bh_disable() /
local_bh_enable() so the child slock is always acquired with BH off.
local_bh_disable() nests safely on the softirq path.

Discovered via in-house syzkaller fuzzing; the same root cause also
on the linux-6.1.y syzbot dashboard as extid 44f0626dd6284f02663c.
Reproduced under KASAN + LOCKDEP + PROVE_LOCKING, reproducer:
https://pastebin.com/A3t8xzCR

## Which versions are affected by CVE-2026-64177?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-64177?

Apply the vendor's patch.

## Is CVE-2026-64177 being exploited?

There is no known exploitation of CVE-2026-64177 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-2026-64177 and fix it if it is affected.

What it is: CVE-2026-64177 is a medium-severity vulnerability in Linux kernel, published on 19 Jul 2026. 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-2026-64177 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-2026-64177/ (Markdown: https://insights.sbomify.com/CVE-2026-64177/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64177
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64177
- OSV: https://osv.dev/vulnerability/CVE-2026-64177

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64177/
Summaries are generated and may contain mistakes; the official description above is the record.