[CVE-2026-99284][IMPORTANT] net: skbuff: propagate shared-frag marker in skb_zerocopy()

[CVE-2026-99284][IMPORTANT] net: skbuff: propagate shared-frag marker in skb_zerocopy()

[AL-KERNEL]

CVE: CVE-2026-99284
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: net: skbuff: propagate shared-frag marker in skb_zerocopy()
Commit: 1c623e3cbf43f12dd4a72712079bfa247a9b8ba3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1c623e3cbf43f12dd4a72712079bfa247a9b8ba3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99284
Analysis date: Sat, 07 Nov 2026 22:05:31 -0400
ActionableScore: 7
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
Missing shared-fragment propagation lets an unprivileged local user drive ESP in-place decryption into a root-owned read-only file’s page-cache backing.

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

The original Linux kernel CVE announcement for CVE-2026-99284 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99284

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

CVE-2026-99284	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-99284 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT IMPORTANT AEGIS_AS=5 LEGACY=IMPORTANT RESULT=SAME (final reconciliation agrees; DECISION: both 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-123;CWE-284;CWE-668;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L';BEST CVSS score: '7';DESCR 'skb_zerocopy can transfer page cache backed fragments without SKBFL_SHARED_FRAG, so skb_has_shared_frag returns false for the destination skb. ESP input may then skip skb_cow_data and decrypt the nonlinear skb in place after nfnetlink_queue or an openvswitch upcall, giving an unprivileged local user a controlled write into page cache pages belonging to a root owned read only file. For the CVSS the PR:L value is selected because reliable triggering requires a local unprivileged process and access to the relevant packet handling path. This is not directly network reachable. The demonstrated primitive is unauthorized file content modification with plausible privilege escalation and service disruption, while direct information disclosure is not established. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 7 with recommendation Strong Important candidate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to IMPORTANT because the ActionableScore threshold rule was applied. KP and FD specialist checks stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied. independent legacy evidence preservation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied. final reconciliation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied and KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 and INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 and B3_PENDING_NONKP_INTEGRITY_PRESERVE and MANUAL_REVIEW_REQUIRED and LEGACY_B3_INDEPENDENT_IMPORTANT and MANUAL_REVIEW_REQUIRED and B3_PENDING_SHARED_BACKING_IMPORTANT_PRESERVE and MANUAL_REVIEW_REQUIRED and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 4.7 and ActionableScore 5 with primitive class CLASS_A and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	MAYBE	REMOTE SIMPLEFIX LEAK SKB LINUS INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 B3_PENDING_NONKP_INTEGRITY_PRESERVE MANUAL_REVIEW_REQUIRED LEGACY_B3_INDEPENDENT_IMPORTANT MANUAL_REVIEW_REQUIRED B3_PENDING_SHARED_BACKING_IMPORTANT_PRESERVE MANUAL_REVIEW_REQUIRED KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=7
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=CONFIRMED
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 7.
* Conservative score: 7; no floor or cap applied.
* Raw paranoid score: 7; no additional speculative effects awarded.
* Paranoid score: 7.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

* +2 Dirty-Pipe-like ownership/COW breach: lost `SKBFL_SHARED_FRAG` lets ESP decrypt directly into page-cache-backed fragments belonging to a root-owned read-only file.
* +1 Integrity impact: protected file contents are modified without write permission.
* +1 Local unprivileged trigger: the commit explicitly identifies an unprivileged local user as the attacker.
* +1 Local high-control data-plane path: the attacker materially controls the file-backed fragment and packet-processing sequence, not merely object creation.
* +1 Practical exploit chain: `skb_zerocopy()` loses the marker, ESP skips `skb_cow_data()`, and in-place decryption changes the protected page-cache page.
* +1 Important storage path: corruption reaches page-cache-backed file contents.
* No confidentiality, kernel panic, arbitrary-target write, credential overwrite, or completed LPE point is awarded. The ownership breach is scored once rather than also adding generic page-cache corruption.

3. Reachability analysis

The supplied commit grounds a below-host-admin local trigger through skb zerocopy used by NFQUEUE or Open vSwitch upcalls, followed by ESP input. It explicitly states that an unprivileged user can modify a root-owned read-only file, so host UID 0 and host-administrator powers are not required. The evidence does not establish that the complete chain is available solely through attacker-created user/network namespaces, so namespace-scoped privilege remains unknown. This is not direct remote reachability, and configuration/topology prerequisites are not fully detailed.

4. Primitive and severity reading

The primitive is a controlled cross-ownership write into shared page-cache backing. `skb_zerocopy()` copies fragment references without propagating their shared-fragment state; ESP therefore treats the destination as privately writable and consumes the corrupted ownership metadata by decrypting in place. This is active shared-backing corruption, not a UAF or merely an in-skb edit.

The protected victim and below-host-admin boundary are expressly identified, making this stronger than B3-PENDING: the cross-authority file-content impact is confirmed. It is CLASS_A and Important-preserving. LPE and persistent on-disk corruption remain plausible but unproven. Auto-downgrade is disallowed because the demonstrated primitive crosses file ownership and write-permission boundaries.

B4-PENDING does not apply: no authorization predicate changes DENY to ALLOW; the failure is writable/shared-state preservation.

5. One-sentence report phrase

Missing shared-fragment propagation lets an unprivileged local user drive ESP in-place decryption into a root-owned read-only file’s page-cache backing.

6. Manual review advice

MANUAL CHECK REQUIRED

The demonstrated protected shared-page write is an Important-class integrity primitive even without proven LPE or arbitrary kernel write.

7. Call-site confidence

Call-site confidence: high

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

Patch: net: skbuff: propagate shared-frag marker in skb_zerocopy()
Commit: 1c623e3cbf43f12dd4a72712079bfa247a9b8ba3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1c623e3cbf43f12dd4a72712079bfa247a9b8ba3

Commit description:

skb_zerocopy() moves frag descriptors from @from into @to and takes a
reference on each frag, but it does not copy skb_shinfo(from)->flags into
skb_shinfo(to)->flags.  A destination skb holding the same page-cache
backed frags as the source therefore reports skb_has_shared_frag() as
false.

That breaks the invariant relied on by later in-place writers.  ESP input
checks skb_has_shared_frag() before deciding whether an uncloned
nonlinear skb can skip skb_cow_data(); with the marker lost, an skb that
skb_zerocopy() built for nfnetlink_queue or for an openvswitch upcall is
decrypted in place, letting an unprivileged user write into the page
cache of a root-owned read-only file.

Propagate SKBFL_SHARED_FRAG when frag descriptors are copied, as done for
the other frag-transfer helpers.

Fixes: af2806f ("net: Export skb_zerocopy() to zerocopy from one skb to another")
Fixes: f4c50a4 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
Cc: stable@kernel.org
Signed-off-by: Zizhen Liu <pointerliu@126.com>
Reviewed-by: Eric Dumazet <edumazet@kernel.org>
Link: https://patch.msgid.link/20261022104217.1852191-1-pointerliu@126.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ sashal: Adjusted context in skb_zerocopy() due to missing from->unreadable propagation. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/core/skbuff.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=1c623e3cbf43f12dd4a72712079bfa247a9b8ba3

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

The original Linux kernel CVE announcement for CVE-2026-99284 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99284

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:H/A:L
  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: unknown
  Paranoid ActionableScore: 7

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: NO
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).