[CVE-2026-68121][MODERATE 7.0] pppoe: reload header pointer after dev_hard_header()

[CVE-2026-68121][MODERATE 7.0] pppoe: reload header pointer after dev_hard_header()

[AL-KERNEL]

CVE: CVE-2026-68121
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: pppoe: reload header pointer after dev_hard_header()
Commit: e6493a4d1ee17595766165fa446d45b7e0c318d0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e6493a4d1ee17595766165fa446d45b7e0c318d0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68121
Analysis date: Mon, 10 Aug 2026 10:39:13 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale PPPoE header pointer in `pppoe_sendmsg()` can be used after `dev_hard_header()` reallocates the skb head, causing a small write-after-free into freed kernel skb memory and making local DoS likely with possible LPE concern under favorable allocator and namespace conditions.

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

The original Linux kernel CVE announcement for CVE-2026-68121 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68121

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
======================================================================

CVE-2026-68121	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-825;*CWE-787;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A write-after-free in the PPPoE send path can occur because pppoe_sendmsg keeps a pointer to the PPPoE header before dev_hard_header(). Device header callbacks may expand or reallocate the skb head, which invalidates pointers into the old head. PPPoE then writes the header through the stale pointer into freed skb memory. For the CVSS the PR:L is used because triggering requires a local process able to exercise the PPPoE send path and a practical way to race network device state, for example in a network namespace or delegated network administration context. The issue is not directly network reachable by packet traffic alone. Impact is at least local denial of service via kernel memory corruption. In the worst defensible case it may allow privilege escalation because the primitive is a small kernel use-after-free write rather than a pure crash.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	UAF NETWORK SKB HARDWARE LINUS KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

This is not remote reachable by packet traffic alone, but it is a concrete local kernel memory corruption bug: a stale pointer into an skb head is used for a 6 byte write after `dev_hard_header()` may reallocate/free that head.

## 2. Signal breakdown

Conservative scoring:

* Local unprivileged trigger: +1
  PPPoE send path can be exercised by a local process. The full race also needs network device state changes, which may be reachable via network namespaces or delegated network administration.
* Memory corruption, strong corruption primitive: +2
  The commit explicitly says PPPoE writes six bytes through a stale pointer into the freed skb head.
* Real lifetime corruption: +1
  This is a stale pointer after skb head reallocation/free.
* Reliable kernel crash / strong DoS: +1
  Writing into freed skb memory is a credible kernel crash path.
* Hard or unreliable race / special timing required: -1
  Trigger requires a send blocked in `copy_from_user()` while the first non Ethernet port is added to an empty team device.
* Conservative total: 5

Paranoid delta:

* Privilege escalation plausible: +1 additional over conservative
  The write is small and constrained, but it is still a kernel heap write-after-free. A carefully timed reclaim or allocator interaction could make LPE worth manual review, although no reliable exploit primitive is demonstrated.
* Paranoid total: 6

Not counted:

* Remote reachable / network-triggerable: +0
  The bug is in a local send/control path, not in remote packet receive parsing.
* Common networking core packet path: +0
  PPPoE plus team plus delegated GRE header behavior is specific, not a generic TCP/IP receive path.
* Confidentiality / Integrity direct impact: +0 in conservative score
  The patch shows memory corruption, but not a direct info leak, arbitrary write, or demonstrated privilege escalation.

## 3. Reachability analysis

A local process must exercise `pppoe_sendmsg()` while device header creation can reallocate the skb head. The described trigger also needs concurrent network device reconfiguration involving a team device and a non Ethernet delegated header callback such as GRE.

In typical locked-down deployments, the device setup side likely requires `CAP_NET_ADMIN`. In systems with unprivileged user namespaces or delegated network namespaces, the effective privilege requirement can be closer to local low privilege. This is why the practical triage should not be auto-closed as merely privileged admin-only.

The path is not remotely reachable by ordinary network traffic. Realistic exploitation requires local execution, PPPoE support, team/GRE-like configuration, and a timing window.

Call-site confidence: high. The provided patch shows the exact vulnerable call sequence in `pppoe_sendmsg()`: stale `ph`, call to `dev_hard_header()`, then `memcpy(ph, ...)`.

## 4. Severity interpretation

This behaves as an Actionable Moderate and an Important review candidate. The realistic impact is at least local DoS through kernel memory corruption. The theoretical impact can extend toward LPE because this is a write-after-free into kernel heap memory, but the write size is small, the payload is partly constrained, and the race/device setup is specialized.

So it should not be treated like an ordinary Moderate auto-close issue. It also should not be assumed Critical, because no arbitrary write, controlled object replacement, or demonstrated exploit chain is shown.

## 5. One-sentence report phrase

A stale PPPoE header pointer in `pppoe_sendmsg()` can be used after `dev_hard_header()` reallocates the skb head, causing a small write-after-free into freed kernel skb memory and making local DoS likely with possible LPE concern under favorable allocator and namespace conditions.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.
YES REQUIRES MANUAL CHECK.

Reason: concrete kernel write-after-free, local namespace-dependent reachability, and plausible but unproven privilege escalation potential.

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

Patch: pppoe: reload header pointer after dev_hard_header()
Commit: e6493a4d1ee17595766165fa446d45b7e0c318d0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e6493a4d1ee17595766165fa446d45b7e0c318d0

Commit description:

pppoe_sendmsg() saves a pointer to the PPPoE header before calling
dev_hard_header(). Device header callbacks are allowed to reallocate the
skb head, invalidating pointers into it.

This can happen when a send is blocked in copy_from_user() while the first
non-Ethernet port is added to an empty team device. The team's delegated
GRE header callback then expands the skb head. PPPoE subsequently writes
six bytes through the stale pointer into the freed head.

Reload the PPPoE header through the skb's network-header offset after
device header creation. pskb_expand_head() updates that offset when it
relocates the head.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Asim Viladi Oglu Manizada <manizada@pm.me>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260722093814.3017176-1-manizada@pm.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/ppp/pppoe.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=e6493a4d1ee17595766165fa446d45b7e0c318d0

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

The original Linux kernel CVE announcement for CVE-2026-68121 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68121

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:L/I:L/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: 5
  Paranoid ActionableScore: 6

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: YES
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).