[CVE-2026-63803][MODERATE 7.0] hdlc_ppp: sync per-proto timers before freeing hdlc state

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

CVE: CVE-2026-63803
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: hdlc_ppp: sync per-proto timers before freeing hdlc state
Commit: 8308122bc9c065b1f376e081ed300129a2ac9545
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8308122bc9c065b1f376e081ed300129a2ac9545
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63803
Analysis date: Sun, 19 Jul 2026 10:37:12 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in HDLC PPP timer teardown can leave per-protocol timers running after `hdlc->state` is freed, causing a local timer-callback use-after-free that can crash the kernel and may warrant manual review for possible memory-reuse impact.

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

The original Linux kernel CVE announcement for CVE-2026-63803 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63803

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 7.0
Manual review required: YES

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

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-63803	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use after free race exists in the HDLC PPP teardown path because per protocol PPP timers can remain active while detach_hdlc_protocol frees the hdlc state allocation that stores struct ppp. A timer callback that is already running can survive the kfree and later dereference proto state or ppp lock from freed memory, which can crash the kernel and may provide a memory corruption primitive. For the CVSS the PR:L is used for the paranoid score because reliable triggering normally needs local control over HDLC PPP configuration, but that control may be available to a delegated network admin service, container context, or reduced capability root rather than only a full host administrator. The issue is not network reachable in the CVSS sense and requires local control plane actions such as protocol attach, re attach, device teardown, or unregister. Impact is at least local denial of service via kernel crash. In the worst defensible interpretation the UAF may allow confidentiality or integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	INIT UAF NETWORK TIMER HARDWARE LINUS IPV6  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption, weak race UAF primitive: +1
  The patch explicitly describes a use-after-free where a running `ppp_timer` callback can survive `kfree(hdlc->state)` and later dereference freed `struct ppp` memory.
* Real lifetime corruption: +1
  This is a timer lifetime bug where asynchronous timer execution is not synchronized before freeing the owning object.
* Reliable kernel crash / strong DoS: +1
  Dereferencing `proto->state` or `ppp->lock` after the backing allocation is freed can plausibly crash the kernel.
* Hard or unreliable race / special timing required: -1
  Triggering requires a timer callback to overlap with detach or reattach teardown.
* Requires CAP_NET_ADMIN or equivalent in typical deployments: -2
  HDLC protocol attach, reattach, unregister, and device teardown are normally privileged network control-plane operations.
* Rare / difficult-to-reach subsystem: -1
  HDLC PPP WAN usage is not a broad default Linux exposure.

Paranoid additions / differences:

* Weak LPE concern: +1
  The stale timer callback dereferences freed kmalloc memory. No controlled reclaim is shown, but this is more than a NULL dereference and deserves manual review.
* Privileged kernel/device-management lifetime path: +1
  The bug occurs in network device protocol detach and teardown, where wrong lifetime handling affects trusted kernel state.
* Reduced privilege / delegated CAP_NET_ADMIN interpretation: +1 effective adjustment
  In container, service, or delegated network-admin environments, the attacker may not be full host root, so applying the full privileged penalty can understate practical risk.

Call-site confidence: high. The patch and commit describe the relevant allocation, timer setup, detach callback path, and `kfree()` ordering.

## 3. Reachability analysis

The bug is local and control-plane triggered. A realistic trigger needs control over HDLC PPP protocol attachment, reattachment, device teardown, or unregister paths. In normal host deployments this usually requires root or `CAP_NET_ADMIN`, so the conservative score is low-to-borderline.

Namespaces and containers can matter. If a reduced-capability service, container root, or delegated network administrator can manage the affected HDLC device, the privilege requirement is closer to PR:L in practical triage terms rather than full host-root equivalence.

The path is not network-reachable in the CVSS sense. Incoming PPP traffic alone is not enough unless the attacker can also cause the local teardown or reattach timing. The affected subsystem is relatively niche, but the primitive is a real timer UAF rather than a pure resource leak or validation issue.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary low-risk Moderate. The realistic demonstrated impact is local kernel crash or hang from a race-dependent UAF in a non-default subsystem.

The theoretical memory corruption potential is higher because the timer callback can access freed kmalloc memory after object lifetime ended. However, the patch does not show attacker-controlled reclaim, controlled overwrite, type confusion, arbitrary write, or a demonstrated LPE path. Therefore it should not be treated as a strong Important candidate by default, but it should not be auto-closed without review.

## 5. One-sentence report phrase

A race in HDLC PPP timer teardown can leave per-protocol timers running after `hdlc->state` is freed, causing a local timer-callback use-after-free that can crash the kernel and may warrant manual review for possible memory-reuse impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a real kernel UAF race involving asynchronous timer callbacks and freed driver state. Even though exploitation appears privileged and timing-dependent, lifetime bugs of this class can be underestimated, especially in delegated `CAP_NET_ADMIN` or containerized network-management environments.

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

Patch: hdlc_ppp: sync per-proto timers before freeing hdlc state
Commit: 8308122bc9c065b1f376e081ed300129a2ac9545
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8308122bc9c065b1f376e081ed300129a2ac9545

Commit description:

Each PPP control protocol (LCP/IPCP/IPV6CP) embedded in struct ppp
registers a timer via timer_setup(). That struct ppp is the
hdlc->state allocation, which detach_hdlc_protocol() frees with kfree()
in both teardown paths: unregister_hdlc_device() and the re-attach inside
attach_hdlc_protocol().

The ppp proto never registered a .detach callback, so
detach_hdlc_protocol() performs no timer synchronization before the
kfree(). The only cancel, timer_delete(&proto->timer) in ppp_cp_event(),
is partial (it does not wait for a running callback) and only runs on the
->CLOSED transition; ppp_stop()/ppp_close() do not sync either. A
ppp_timer callback already executing (blocked on ppp->lock) survives the
kfree and then dereferences proto->state / ppp->lock in freed memory,
leading to a use-after-free.

Fix this by adding a .detach helper that calls timer_shutdown_sync() on
every per-proto timer. detach_hdlc_protocol() invokes proto->detach(dev)
before kfree(hdlc->state), so timer_shutdown_sync()
now runs on both free paths.
timer_shutdown_sync() is used instead of timer_delete_sync() because the
keepalive path re-arms the timer through add_timer()/mod_timer() and
shutdown blocks any re-activation during teardown.

Initialize the per-protocol timers in ppp_ioctl() when the protocol is
attached, and remove the now-redundant timer_setup() from ppp_start(), so
that the timers are initialized exactly once at attach time and
ppp_timer_release() never operates on uninitialized timer_list
structures. attach_hdlc_protocol() uses kmalloc() (not kzalloc), so
struct ppp's protos[i].timer is uninitialized garbage until the first
timer_setup(); without this init-at-attach, attaching the PPP protocol
without ever bringing the device up would leave timer_shutdown_sync()
operating on uninitialized memory in .detach. Moving the init out of
ppp_start() (which only runs on NETDEV_UP) into the attach path makes the
initialization unconditional and avoids initializing the same timer_list
twice.

This bug was found by static analysis.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Fan Wu <fanwu01@zju.edu.cn>
Link: https://patch.msgid.link/20260617020518.116319-1-fanwu01@zju.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/wan/hdlc_ppp.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=8308122bc9c065b1f376e081ed300129a2ac9545

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

The original Linux kernel CVE announcement for CVE-2026-63803 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63803

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 3
  Paranoid ActionableScore: 5

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 7.0

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).