[CVE-2026-46300][IMPORTANT] net: skbuff: propagate shared-frag marker through frag-transfer helpers

[CVE-2026-46300][IMPORTANT] net: skbuff: propagate shared-frag marker through frag-transfer helpers

[AL-KERNEL]

CVE: CVE-2026-46300
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net: skbuff: propagate shared-frag marker through frag-transfer helpers
Commit: fbeab9555564a1b98e8582cd106dfe46c4606991
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fbeab9555564a1b98e8582cd106dfe46c4606991
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46300
Analysis date: Sat, 23 May 2026 08:52:27 -0400
ActionableScore: 9
ActionableScore lower bound: 8
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
Missing propagation of `SKBFL_SHARED_FRAG` in SKB frag-transfer helpers can let later in-place network writers corrupt externally owned or page-cache-backed pages, including root-owned read-only file cache data, creating a plausible local privilege escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-46300 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46300

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-2026-46300	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-664;CWE-863;CWE-284;CWE-668;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'SKB frag transfer helpers failed to preserve SKBFL_SHARED_FRAG when moving page backed fragments between skbs. This can make the destination skb keep references to externally owned or page cache backed pages while skb_has_shared_frag returns false, so later in place writers such as ESP input may modify pages that should have been copied first. A local unprivileged user may be able to corrupt the page cache of a root owned read only file in affected configurations, which makes this more than a simple crash bug and creates a plausible local privilege escalation path. For the CVSS the PR:L is used because the attacker needs local user level ability to create or drive the relevant skb data path, but does not need administrative privileges on the victim. The issue is not scored as directly network reachable because remote packet delivery alone does not normally provide the page cache backed skb fragments needed for the described write primitive. Impact is high integrity impact and potentially high confidentiality and availability impact if page cache corruption is used to affect privileged code or security sensitive files.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)	YES	REMOTE SKB HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	guess

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

ActionableScore=9
ActionableScoreLower=8

## 1. ActionableScore

* Conservative score: 8
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  The commit explicitly states that an unprivileged user can reach the bad path in affected configurations.

* Strong LPE plausibility: +2
  The described primitive allows writes into the page cache of a root-owned read-only file, which is a plausible local privilege escalation path.

* Shared-page / page-cache / COW ownership corruption: +4 capped combined signal
  The same underlying primitive qualifies as strong memory corruption, Dirty-Pipe-like ownership bypass, and page-cache corruption. These are not fully stacked to avoid double counting, but the combined primitive is high risk.

* Practical exploit chain strongly implied: +1
  The commit gives a concrete chain involving `nft dup to local`, `nf_dup_ipv4()` / `xt_TEE`, ESP input, and authencesn-ESN stray writes.

* Confidentiality impact plausible: +1
  If privileged file-backed data or security-sensitive cache state can be corrupted and later consumed, confidentiality impact is plausible through follow-on privilege or policy bypass.

* Integrity impact plausible: +1
  Direct page-cache corruption of root-owned read-only files is a strong integrity impact.

* Common networking core / packet-processing path: +1
  The bug is in central SKB frag-transfer helpers and affects GRO, segmentation, skb shifting, and TCP payload cloning paths.

No subtraction applied:

* No hard race requirement is central to the primitive.
* No CAP_SYS_ADMIN / CAP_NET_ADMIN requirement is assumed for the attacker, because the commit describes an unprivileged user corruption path when the affected system configuration exists.
* No rare-subsystem penalty is applied because SKB, XFRM / ESP, netfilter duplication, and page-cache interaction are specifically called out as paths that should not be dismissed solely as niche.

## 3. Reachability analysis

The realistic attacker is a local unprivileged user on a system where the relevant networking and XFRM / ESP path is configured. The bug is not scored as directly remote reachable, because remote packets alone normally do not provide the page-cache-backed SKB fragments needed for the write primitive. Namespaces or containers may matter if an unprivileged workload can generate the relevant traffic or interact with a preconfigured duplication / ESP path, but the key issue is that the attacker does not need full host-root privileges once the vulnerable path is present.

The path is configuration-dependent, but not merely theoretical. The commit explicitly describes a concrete route where stripped `SKBFL_SHARED_FRAG` state lets ESP input perform in-place writes into externally owned or page-cache-backed pages.

## 4. Severity interpretation

This behaves more like an Important-class kernel vulnerability than an ordinary Moderate DoS. The issue is not just a crash, warning, or metadata leak. It is an ownership / COW boundary violation where SKB helpers lose the shared-frag marker, causing later writers to treat shared page-backed fragments as private.

The realistic evidence supports at least Actionable Moderate and likely Important handling because the patch context describes unprivileged page-cache corruption of root-owned read-only file data. The paranoid score is higher because Dirty-Pipe-like primitives have historically been underestimated during early triage, but the score is still kept below a remote Critical-style bucket because direct network-only exploitation is not established.

## 5. One-sentence report phrase

Missing propagation of `SKBFL_SHARED_FRAG` in SKB frag-transfer helpers can let later in-place network writers corrupt externally owned or page-cache-backed pages, including root-owned read-only file cache data, creating a plausible local privilege escalation primitive.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because the patch describes a concrete page-cache corruption primitive reachable by an unprivileged local user in affected configurations, with plausible local privilege escalation impact rather than DoS-only behavior.

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

Patch: net: skbuff: propagate shared-frag marker through frag-transfer helpers
Commit: fbeab9555564a1b98e8582cd106dfe46c4606991
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fbeab9555564a1b98e8582cd106dfe46c4606991

Commit description:

Two frag-transfer helpers (__pskb_copy_fclone() and skb_shift()) fail
to propagate the SKBFL_SHARED_FRAG bit in skb_shinfo()->flags when
moving frags from source to destination.  __pskb_copy_fclone() defers
the rest of the shinfo metadata to skb_copy_header() after copying
frag descriptors, but that helper only carries over gso_{size,segs,
type} and never touches skb_shinfo()->flags; skb_shift() moves frag
descriptors directly and leaves flags untouched.  As a result, the
destination skb keeps a reference to the same externally-owned or
page-cache-backed pages while reporting skb_has_shared_frag() as
false.

The mismatch is harmful in any in-place writer that uses
skb_has_shared_frag() to decide whether shared pages must be detoured
through skb_cow_data().  ESP input is one such writer (esp4.c,
esp6.c), and a single nft 'dup to <local>' rule -- or any other
nf_dup_ipv4() / xt_TEE caller -- is enough to land a pskb_copy()'d
skb in esp_input() with the marker stripped, letting an unprivileged
user write into the page cache of a root-owned read-only file via
authencesn-ESN stray writes.

Set SKBFL_SHARED_FRAG on the destination whenever frag descriptors
were actually moved from the source.  skb_copy() and skb_copy_expand()
share skb_copy_header() too but linearize all paged data into freshly
allocated head storage and emerge with nr_frags == 0, so
skb_has_shared_frag() returns false on its own; they need no change.

The same omission exists in skb_gro_receive() and skb_gro_receive_list().
The former moves the incoming skb's frag descriptors into the
accumulator's last sub-skb via two paths (a direct frag-move loop and
the head_frag + memcpy path); the latter chains the incoming skb whole
onto p's frag_list.  Downstream skb_segment() reads only
skb_shinfo(p)->flags, and skb_segment_list() reuses each sub-skb's
shinfo as the nskb -- both p and lp must carry the marker.

The same omission also exists in tcp_clone_payload(), which builds an
MTU probe skb by moving frag descriptors from skbs on sk_write_queue
into a freshly allocated nskb.  The helper falls into the same family
and warrants the same fix for consistency; no TCP TX-side in-place
writer is currently known to reach a user page through this gap, but
a future consumer depending on the marker would regress silently.

The same omission exists in skb_segment(): the per-iteration flag
merge takes only head_skb's flag, and the inner switch that rebinds
frag_skb to list_skb on head_skb-frags exhaustion does not fold the
new frag_skb's flag into nskb.  Fold frag_skb's flag at both sites
so segments drawing frags from frag_list members carry the marker.

Fixes: cef401d ("net: fix possible wrong checksum generation")
Fixes: f4c50a4 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
Suggested-by: Sabrina Dubroca <sd@queasysnail.net>
Suggested-by: Sultan Alsawaf <sultan@kerneltoast.com>
Suggested-by: Ben Hutchings <ben@decadent.org.uk>
Suggested-by: Lin Ma <malin89@huawei.com>
Suggested-by: Jingguo Tan <tanjingguo@huawei.com>
Suggested-by: Aaron Esau <aaron1esau@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Tested-by: Rajat Gupta <rajat.gupta@oss.qualcomm.com>
Link: https://patch.msgid.link/ageeJfJHwgzmKXbh@v4bel
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
[bwh: Backported to 5.10:
 - Set the SKBTX_SHARED_FRAG flag in skb_shared_info::tx_flags,
   instead of SKBFL_SHARED_FRAG in skb_shared_info::flags
 - skb_gro_receive() and skb_gro_receive_list() are in skbuff.c here
 - Drop change to tcp_clone_payload(), which does not exist here
 - Adjust context in skb_shift()
]
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=fbeab9555564a1b98e8582cd106dfe46c4606991

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

The original Linux kernel CVE announcement for CVE-2026-46300 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46300

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:H/I:H/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 8
  Paranoid ActionableScore: 9

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).