[CVE-2026-64177][MODERATE REGULAR] phonet/pep: disable BH around forwarded sk_receive_skb()

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-64177
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: phonet/pep: disable BH around forwarded sk_receive_skb()
Commit: f08c45076e4fd8b0adbc5eb186d6e6a3e7350d7b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f08c45076e4fd8b0adbc5eb186d6e6a3e7350d7b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64177
Analysis date: Sun, 19 Jul 2026 13:22:37 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Borderline manual review recommended
Manual review required: YES

Summary:
A locking race in phonet PEP can acquire the same child socket lock with inconsistent BH state from process and softirq receive paths, allowing a local or adjacent attacker in configured Phonet environments to cause a self-deadlock and denial of service.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64177 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64177

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE REGULAR
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-64177	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-667;CWE-362;*CWE-833;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'A locking race in phonet pep can make pep_do_rcv forward an skb to a child socket through sk_receive_skb() while bottom halves are enabled on the process backlog path. The same child socket lock can then be acquired with BH on in process context and with BH off in softirq context, which can produce an inconsistent lock state and a real self deadlock if softirq receive processing targets the same child socket on the same CPU. For the CVSS the PR:N is used for the paranoid score because an attacker may not need local privileges if Phonet traffic can be injected to an already configured PEP endpoint from an adjacent network or device channel. The realistic local score uses PR:L because reliable triggering usually requires local AF_PHONET socket activity and backlog state manipulation. Impact is denial of service through deadlock or soft lockup. There is no clear UAF, OOB access, or arbitrary write primitive in this patch context, so confidentiality and integrity are not increased for CVSS.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Borderline manual review recommended (with actual score 2)  SKIP CVE-2026-64177 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	KASAN LOCK DEADLOCK WARNONLY SKB HARDWARE LINUS SYZBOT PACKET  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Borderline manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process may be able to exercise AF_PHONET or PEP socket behavior if the protocol is enabled and exposed.
* Reliable kernel crash / strong DoS: +1. The commit describes a real self-deadlock possibility, not only a lockdep cosmetic warning.
* Hard or unreliable race / special timing required: -1. The deadlock depends on overlap between process backlog handling and softirq receive handling on the same child socket.
* Rare and difficult-to-reach subsystem/configuration: -1. Phonet and PEP are uncommon in typical Linux deployments.

Paranoid additional interpretation:

* Remote or adjacent network-triggerable: +2. If an existing PEP endpoint is reachable through Phonet traffic from an adjacent device or network channel, the packet receive side may be externally influenced without a local account.
* Availability impact realistic: +1. A real self-deadlock in the socket receive path can persist and disrupt service or system responsiveness.
* Same race and rarity deductions apply: -2 total.

No memory corruption signals are awarded. The patch shows a locking context mismatch, not UAF, OOB access, double free, type confusion, arbitrary write, or refcount takeover.

## 3. Reachability analysis

A realistic trigger usually requires local AF_PHONET socket activity, an established PEP pipe, backlog processing via release_sock(), and concurrent softirq receive activity targeting the same child socket. In that conservative model, the attacker is local and likely unprivileged if AF_PHONET sockets are accessible.

The paranoid model treats this as adjacent-network influenced if Phonet traffic can be injected into an already configured PEP endpoint. This is not ordinary Internet remote exposure and should not be treated like TCP/IP core parsing. Containers and namespaces may matter if AF_PHONET is available inside a delegated network namespace, but this depends on kernel configuration and deployment policy.

Call-site confidence: high. The commit message and patch show the relevant call path directly: pep_do_rcv() forwards to sk_receive_skb(), which takes the child socket lock through bh_lock_sock_nested().

## 4. Severity interpretation

This behaves like an ordinary Moderate DoS bug with a manual-review reason, not an Important memory-corruption issue. The theoretical impact is deadlock or soft lockup. There is no supported privilege escalation primitive and no confidentiality or integrity impact from the patch context.

The main reason not to auto-close blindly is that the bug is in a network protocol receive path and may be adjacent-triggerable in systems that actually use Phonet or PEP.

## 5. One-sentence report phrase

A locking race in phonet PEP can acquire the same child socket lock with inconsistent BH state from process and softirq receive paths, allowing a local or adjacent attacker in configured Phonet environments to cause a self-deadlock and denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review should confirm whether CONFIG_PHONET is enabled, whether AF_PHONET or PEP sockets are reachable by untrusted users, and whether adjacent Phonet traffic can reach an existing PEP endpoint.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: phonet/pep: disable BH around forwarded sk_receive_skb()
Commit: f08c45076e4fd8b0adbc5eb186d6e6a3e7350d7b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f08c45076e4fd8b0adbc5eb186d6e6a3e7350d7b

Commit description:

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

Fixes: 9641458 ("Phonet: Pipe End Point for Phonet Pipes protocol")
Link: https://syzkaller.appspot.com/bug?extid=44f0626dd6284f02663c
Cc: stable@vger.kernel.org
Signed-off-by: Zijing Yin <yzjaurora@gmail.com>
Acked-by: Rémi Denis-Courmont <remi@remlab.net>
Reported-by: syzbot+9f4a135646b66c509935@syzkaller.appspotmail.com
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260519172635.86304-1-yzjaurora@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/core/sock.c
  include/net/sock.h
  net/phonet/pep.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f08c45076e4fd8b0adbc5eb186d6e6a3e7350d7b

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64177 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64177

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.3

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 1
  Paranoid ActionableScore: 3

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).