[CVE-2026-43503][IMPORTANT] net: skbuff: preserve shared-frag marker during coalescing

[CVE-2026-43503][IMPORTANT] net: skbuff: preserve shared-frag marker during coalescing

[AL-KERNEL]

CVE: CVE-2026-43503
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net: skbuff: preserve shared-frag marker during coalescing
Commit: 3599e6b3cc1ada96883d496a50a210d3afbb6987
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3599e6b3cc1ada96883d496a50a210d3afbb6987
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43503
Analysis date: Wed, 03 Jun 2026 14:47:57 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
`skb_try_coalesce() could drop the SKBFL_SHARED_FRAG marker when moving paged skb fragments, allowing ESP input to decrypt in place over shared or page-cache-backed memory and causing network-triggerable data corruption or DoS with possible broader integrity impact.`

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-43503 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43503

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-2026-43503	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-693;*CWE-664;CWE-668;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:H';BEST CVSS score: '8.1';DESCR 'skb_try_coalesce() can move paged fragments from one skb to another without preserving the SKBFL_SHARED_FRAG marker. If the original skb contains externally owned or page-cache-backed fragments, later ESP input may see skb_has_shared_frag() as false and may decrypt in place instead of forcing skb_cow_data(). This can overwrite shared or page-cache-backed memory and can lead to data corruption, denial of service, and potentially broader confidentiality or integrity impact depending on what memory is shared. For the CVSS the PR:N is used because the trigger is network traffic reaching the affected ESP over TCP or related receive path and no local account on the target is required. The issue is network reachable but requires a specific protocol and skb layout, so AC:H is used. Impact is at least remote denial of service or data corruption, and the memory ownership violation makes this suitable for manual security review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	REMOTE SIMPLEFIX SKB LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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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

Conservative signals:

* Remote reachable / network-triggerable: +2
  The affected path can be reached by network traffic processed through TCP receive coalescing and later ESP input, assuming the target has the relevant XFRM/IPsec path active.

* Dirty-Frag-like shared-page / COW ownership violation: +2
  The bug can cause ESP to decrypt in place over externally owned or page-cache-backed skb fragments because `SKBFL_SHARED_FRAG` is lost during `skb_try_coalesce()`.

* Reliable kernel crash / strong DoS or data corruption: +1
  In-place writes into shared/page-cache-backed fragments can corrupt data and may plausibly lead to kernel instability or denial of service.

* Integrity impact plausible: +1
  The core risk is unauthorized modification of data that should not be written in place.

* Common networking core / packet-processing path: +1
  The bug is in `net/core/skbuff.c` and affects skb fragment ownership semantics used by packet-processing paths.

Paranoid additional signal:

* Weak-to-moderate LPE concern: +1
  Shared/page-cache-backed memory corruption is historically security-sensitive. A direct privilege escalation primitive is not demonstrated here, but the ownership/COW bypass is serious enough to require manual review.

Not counted separately to avoid double counting:

* Generic memory corruption +2 and page-cache corruption +2 were not stacked on top of the Dirty-Frag-like primitive. They describe the same underlying issue.

## 3. Reachability analysis

A remote peer may trigger the issue if traffic reaches the affected TCP receive coalescing and ESP input path with shared or page-cache-backed skb fragments. This is not a generic Internet-exposed parser in all deployments, because ESP/XFRM/IPsec configuration and the specific skb layout are required. However, the vulnerable primitive is in the networking data path, not in a local-only ioctl or admin-only control path.

No local user account is inherently required on the victim if the relevant network configuration already exists. Namespaces mainly matter for who can configure XFRM/IPsec state, but triggering the data path can be remote once configured. The path is not default-active on every Linux system, but skb and XFRM/IPsec are common enough that this should not be treated as a rare-only subsystem.

## 4. Severity interpretation

This is not an ordinary Moderate DoS. The patch fixes loss of an ownership marker that protects shared/page-cache-backed fragments from in-place modification. The realistic demonstrated impact is data corruption and possible DoS under specific network and protocol conditions. The theoretical worst case is more serious because shared page-backed writes can cross normal ownership or COW boundaries, similar to Dirty-Frag-style issues.

The evidence does not justify assuming arbitrary code execution. It does justify Important-candidate handling and manual review because the primitive is page-cache/shared-frag corruption in a network-reachable packet path.

## 5. One-sentence report phrase

`skb_try_coalesce() could drop the SKBFL_SHARED_FRAG marker when moving paged skb fragments, allowing ESP input to decrypt in place over shared or page-cache-backed memory and causing network-triggerable data corruption or DoS with possible broader integrity impact.`

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network-reachable in configured environments and involves a shared/page-cache-backed memory ownership violation, not just a crash or validation bug.

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

Patch: net: skbuff: preserve shared-frag marker during coalescing
Commit: 3599e6b3cc1ada96883d496a50a210d3afbb6987
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3599e6b3cc1ada96883d496a50a210d3afbb6987

Commit description:

skb_try_coalesce() can attach paged frags from @from to @to .  If @from
has SKBFL_SHARED_FRAG set, the resulting @to skb can contain the same
externally-owned or page-cache-backed frags, but the shared-frag marker
is currently lost.

That breaks the invariant relied on by later in-place writers.  In
particular, ESP input checks skb_has_shared_frag() before deciding
whether an uncloned nonlinear skb can skip skb_cow_data().  If TCP
receive coalescing has moved shared frags into an unmarked skb, ESP can
see skb_has_shared_frag() as false and decrypt in place over page-cache
backed frags.

Propagate SKBFL_SHARED_FRAG when skb_try_coalesce() transfers paged
frags.  The tailroom copy path does not need the marker because it copies
bytes into @to 's linear data rather than transferring frag descriptors.

Fixes: cef401d ("net: fix possible wrong checksum generation")
Fixes: f4c50a4 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
Signed-off-by: William Bowling <vakzz@zellic.io>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Tested-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://patch.msgid.link/20260513041635.1289541-1-vakzz@zellic.io
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[bwh: Backported to 5.10: Set the SKBTX_SHARED_FRAG flag in
 sk_buff::tx_flags, instead of SKBFL_SHARED_FRAG in sk_buff::flags]
Signed-off-by: Ben Hutchings <benh@debian.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=3599e6b3cc1ada96883d496a50a210d3afbb6987

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

The original Linux kernel CVE announcement for CVE-2026-43503 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43503

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:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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).