[CVE-2026-68136][IMPORTANT] net: gro: fix double aggregation of flush-marked skbs

[CVE-2026-68136][IMPORTANT] net: gro: fix double aggregation of flush-marked skbs

[AL-KERNEL]

CVE: CVE-2026-68136
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net: gro: fix double aggregation of flush-marked skbs
Commit: 107e1a469f53a2a70874f3f12bf6fcd23925da1d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=107e1a469f53a2a70874f3f12bf6fcd23925da1d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68136
Analysis date: Mon, 10 Aug 2026 12:20:21 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A missing flush check in skb_gro_receive_list can allow flush-marked already aggregated skbs to be re-aggregated in forwarding paths, corrupting frag_list state and causing a kernel panic in skb_segment, with paranoid triage treating the skb list corruption as a memory corruption candidate.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68136 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68136

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: IMPORTANT
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-2026-68136	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-20;**CWE-476;*CWE-664;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'skb_gro_receive_list can re aggregate an skb that is already marked with the GRO flush flag. In a forwarding or tethering path this can allow an already aggregated skb with an existing frag_list to enter a second GRO aggregation session, corrupting the frag_list chain and later crashing in skb_segment during GSO segmentation. A network attacker does not need a local account if they can send traffic that reaches the affected GRO forwarding path, but practical triggering depends on driver and device behavior such as LRO or GRO frag_list chaining and on traffic being forwarded to another device. For the CVSS the PR:N is used because the attacker only needs adjacent or internal network reachability to the affected packet path and does not need privileges on the target host. Impact is at least denial of service via kernel panic. Because the bug corrupts skb frag_list structure and may involve dangling references or corrupted list traversal, the paranoid score treats it as a memory corruption candidate that should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	REMOTE DANGER INIT SKB QDISC KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. Traffic from an adjacent or internal network can reach the affected GRO forwarding path without local code execution.
* Memory corruption, weak/indirect corruption candidate: +1. The patch context describes corrupted skb frag_list structure, malformed list traversal, and NULL or corrupted pointer dereference, but not a controlled overwrite or reclaim primitive.
* Reliable kernel crash / strong DoS: +1. The described downstream effect is a kernel panic in skb_segment during GSO segmentation.
* Broad/default/common subsystem: +1. GRO/GSO and skb handling are common Linux networking mechanisms.
* Common networking core path or packet-processing path: +1. The affected code is in net/core/gro.c and impacts packet aggregation and forwarding behavior.

Paranoid additional interpretation:

* Memory corruption, strong corruption primitive instead of weak: +1 net increase. The frag_list chain corruption may involve dangling references or corrupted skb list structure in a central networking path.
* Real lifetime corruption: +1. The description explicitly mentions possible dangling refs in the malformed frag_list chain.

Not counted:

* Availability impact realistic was not added separately to avoid double-counting the same kernel panic already counted as strong DoS.
* No local unprivileged trigger was added because the main trigger is network packet delivery to the affected forwarding path, not local API use.
* No Dirty-Pipe-like or page-cache primitive is indicated.

## 3. Reachability analysis

A remote or adjacent-network sender may trigger the bug if their packets traverse an affected device forwarding or tethering path where LRO/GRO produces an already aggregated skb with frag_list and the skb is later reprocessed by GRO despite the flush flag. The attacker does not need a local account on the target host in the described scenario.

The path is not a generic public Internet exposure in the same sense as an ordinary TCP listener. It depends on deployment details such as forwarding, tethering, driver behavior, LRO/GRO fraglist support, and a TX path that later calls skb_segment. Namespaces do not materially reduce the risk for the network-triggered case, but local namespace control is not required by the described scenario.

Call-site confidence: high. The input provides the changed helper, the missing flush check, the downstream skb_segment crash path, and the concrete forwarding scenario.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is network-triggerable in a core packet-processing path and corrupts skb frag_list state before a kernel panic. The conservative interpretation treats it as Actionable Moderate because the demonstrated impact is reliable DoS and the corruption primitive is not shown to be attacker-controlled enough for confirmed privilege escalation.

The paranoid interpretation is a Strong Important candidate because corrupted skb list structure and possible dangling references in the networking core can be an underestimated memory-corruption class. There is not enough evidence to claim arbitrary code execution or practical privilege escalation, but the bug should not be auto-closed without manual review.

## 5. One-sentence report phrase

A missing flush check in skb_gro_receive_list can allow flush-marked already aggregated skbs to be re-aggregated in forwarding paths, corrupting frag_list state and causing a kernel panic in skb_segment, with paranoid triage treating the skb list corruption as a memory corruption candidate.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: network-reachable skb frag_list corruption in a common networking core path is enough for manual review even though the demonstrated impact is kernel panic and the practical exploitation requirements depend on GRO/LRO forwarding conditions.

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

Patch: net: gro: fix double aggregation of flush-marked skbs
Commit: 107e1a469f53a2a70874f3f12bf6fcd23925da1d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=107e1a469f53a2a70874f3f12bf6fcd23925da1d

Commit description:

Commit 0ab03f3 ("net-gro: Fix GRO flush when receiving a GSO
packet.") added a flush check to skb_gro_receive(), but
skb_gro_receive_list() lacks the same validation.

As a result, packets marked with NAPI_GRO_CB(skb)->flush may still be
re-aggregated.

This allows already-GRO'd packets with existing frag_list to be
re-aggregated into a new GRO session, corrupting the frag_list chain
structure. When skb_segment() attempts to unpack these malformed packets,
it encounters invalid state and triggers a kernel panic.

Scenario (Tethering/Device forwarding):
  1. Driver: Generated aggregated packet P1 via LRO with frag_list
  2. Dev A: Receives aggregated fraglist packet and flush flag set
  3. Dev A: Re-enters GRO, skb_gro_receive_list() is called
  4. Missing flush check allows re-aggregation despite flush flag
  5. Frag_list chain becomes corrupted (loops or dangling refs)
  6. Dev B: TX path calls skb_segment(), crashes on corrupted frag_list

Root cause in skb_segment():
  The check at line ~4891:
    if (hsize <= 0 && i >= nfrags && skb_headlen(list_skb) &&
        (skb_headlen(list_skb) == len || sg)) {

  When frag_list is corrupted by double aggregation, when list_skb is
  a NULL pointer from skb->next, skb_headlen(list_skb) dereference
  NULL/corrupted pointers occurs.

Call Trace:
 skb_headlen(NULL skb)
 skb_segment
 tcp_gso_segment
 tcp4_gso_segment
 inet_gso_segment
 skb_mac_gso_segment
 __skb_gso_segment
 skb_gso_segment
 validate_xmit_skb
 validate_xmit_skb_list
 sch_direct_xmit
 qdisc_restart
 __qdisc_run
 qdisc_run
 net_tx_action

Fix: Add NAPI_GRO_CB(skb)->flush validation to the early-return check in
skb_gro_receive_list(), matching the defensive programming pattern of
skb_gro_receive().

Fixes: 3a1296a ("net: Support GRO/GSO fraglist chaining.")
Cc: stable@vger.kernel.org
Signed-off-by: Shiming Cheng <shiming.cheng@mediatek.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260709014704.3625-1-shiming.cheng@mediatek.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/core/gro.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=107e1a469f53a2a70874f3f12bf6fcd23925da1d

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

The original Linux kernel CVE announcement for CVE-2026-68136 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68136

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:A/AC:H/PR:N/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:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 6
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).