[CVE-2026-53363][IMPORTANT] xfrm: iptfs: preserve shared-frag marker in iptfs_consume_frags()

[CVE-2026-53363][IMPORTANT] xfrm: iptfs: preserve shared-frag marker in iptfs_consume_frags()

[AL-KERNEL]

CVE: CVE-2026-53363
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: xfrm: iptfs: preserve shared-frag marker in iptfs_consume_frags()
Commit: dd66f7f6e360ee82cd905517726f8e9091265de5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dd66f7f6e360ee82cd905517726f8e9091265de5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53363
Analysis date: Fri, 10 Jul 2026 08:43:45 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
IP-TFS may drop the SKBFL_SHARED_FRAG marker while moving skb paged fragments, allowing ESP to perform unsafe in-place encryption on shared or read-only page-cache backed data and creating a Dirty-Frag-like integrity corruption risk.

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

The original Linux kernel CVE announcement for CVE-2026-53363 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53363

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.

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

CVE-2026-53363	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	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-664;CWE-471;CWE-668;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H';BEST CVSS score: '7';DESCR 'IP-TFS can move paged skb fragments from one skb to another without preserving SKBFL_SHARED_FRAG. If fragments backed by read-only page-cache pages lose this marker, ESP may treat in-place encryption as safe and can modify shared data that should not be modified. The realistic trigger is local traffic generation through an IPsec IP-TFS path with suitable shared page-cache backed skb fragments. For the CVSS the PR:L is used because a local unprivileged process may be enough to generate the affected traffic pattern when the tunnel is configured. The issue is not rated AV:N because the patch does not show that a remote packet alone can create the required local page-cache backed fragments. Impact is at least denial of service and integrity corruption, with paranoid confidentiality and privilege escalation risk due to shared page-cache corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)  SKIP CVE-2026-53363 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	SIMPLEFIX SKB LINUS PACKET  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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

ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

* Local unprivileged trigger: +1
  A local process may be able to generate outbound traffic through an already configured IPsec/IP-TFS path.

* Page-cache or shared-page corruption primitive: +2
  The lost `SKBFL_SHARED_FRAG` marker can make ESP treat shared or read-only page-cache backed skb fragments as safe for in-place encryption.

* Privilege escalation plausible: +2
  Page-cache/shared-frag corruption is a Dirty-Frag-like ownership violation and can plausibly affect data that should not be writable by the triggering context.

* Integrity impact plausible: +1
  The core risk is unintended modification of shared or read-only backed data.

* Common packet-processing / XFRM path relevance: +1
  XFRM/IPsec is a security-sensitive networking datapath, even though IP-TFS itself requires specific configuration.

* Paranoid additional availability/confidentiality concern: +1
  If corrupted shared/file-backed data affects privileged objects or causes kernel instability, impact may extend beyond integrity-only corruption.

No full remote +2 is applied because the patch does not show that an external packet alone can create the required local shared page-cache backed fragments. No admin penalty is applied to the triggering step because once the tunnel is configured, traffic generation may be possible by an unprivileged local user.

## 3. Reachability analysis

The realistic attacker is a local user on a system where IPsec/IP-TFS is already configured. The attacker does not necessarily need to configure XFRM state or policy, but does need traffic to traverse the affected IP-TFS path with suitable skb paged fragments. Namespaces may matter if untrusted workloads can send traffic through host-configured IPsec paths, but configuring IP-TFS itself is normally privileged. This is not a default-enabled universal path, but XFRM/IPsec should not be treated as an obscure-only subsystem.

Call-site confidence: medium. The patch clearly affects skb fragment ownership metadata in the XFRM/IP-TFS consume path, but the full ESP in-place encryption call chain is not included in the diff.

## 4. Severity interpretation

This behaves more like an Important candidate than an ordinary Moderate issue. The practical exploitability depends on IP-TFS deployment and fragment construction, so it is not a broad remote network bug. However, the primitive is not merely a crash or bookkeeping mismatch. Losing `SKBFL_SHARED_FRAG` can turn a shared/read-only page-backed fragment into an unsafe in-place encryption target, which is a high-risk shared-page corruption class.

Theoretical worst case includes privilege escalation through page-cache or shared-page corruption. Realistic evidence supports at least actionable integrity impact and mandatory manual review.

## 5. One-sentence report phrase

IP-TFS may drop the SKBFL_SHARED_FRAG marker while moving skb paged fragments, allowing ESP to perform unsafe in-place encryption on shared or read-only page-cache backed data and creating a Dirty-Frag-like integrity corruption risk.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is shared-fragment metadata loss in a security-sensitive XFRM/ESP path and is related to a known skb shared-page corruption class, so it should not be auto-closed as ordinary DoS.

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

Patch: xfrm: iptfs: preserve shared-frag marker in iptfs_consume_frags()
Commit: dd66f7f6e360ee82cd905517726f8e9091265de5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dd66f7f6e360ee82cd905517726f8e9091265de5

Commit description:

iptfs_consume_frags() transfers paged fragments from one socket buffer
to another but fails to propagate the SKBFL_SHARED_FRAG flag. This is
the same class of bug that was fixed in skb_try_coalesce() for
CVE-2026-46300 : when fragments backed by read-only page-cache pages are
merged, the marker indicating their shared nature must be preserved so
that ESP can decide correctly whether in-place encryption is safe.

Apply the same two-line fix used in skb_try_coalesce() to
iptfs_consume_frags().

Fixes: b96ba31 ("xfrm: iptfs: share page fragments of inner packets")
Cc: stable@vger.kernel.org # 6.14+
Signed-off-by: Takao Sato <takaosato1997@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/xfrm/xfrm_iptfs.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=dd66f7f6e360ee82cd905517726f8e9091265de5

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

The original Linux kernel CVE announcement for CVE-2026-53363 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53363

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: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: 7
  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).