[CVE-2026-64450][MODERATE 7.0] tipc: fix out-of-bounds read in broadcast Gap ACK blocks

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-64450
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tipc: fix out-of-bounds read in broadcast Gap ACK blocks
Commit: 055663d21dc4336f67933ab26bef3c5934be6324
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=055663d21dc4336f67933ab26bef3c5934be6324
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64450
Analysis date: Sat, 25 Jul 2026 09:03:53 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A TIPC neighbour can send a malformed broadcast Gap ACK block that passes internal length consistency checks but exceeds the message data area, causing an out-of-bounds kernel heap read in `kmemdup()` and potentially a kernel crash.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64450 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64450

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: MODERATE 7.0
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-2026-64450	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';CWE-125;CWE-126;*CWE-20;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.6';DESCR 'A network adjacent out-of-bounds read exists in the TIPC broadcast Gap ACK blocks handling path. tipc_get_gap_ack_blks only validated that the Gap ACK record length matched the ugack_cnt and bgack_cnt fields, but the broadcast receiver did not verify that the record fits inside msg_data_sz. A TIPC neighbour that negotiated TIPC_GAP_ACK_BLOCK can send a crafted broadcast STATE_MSG with a short data area and a large but internally consistent Gap ACK record, causing kmemdup to read beyond the receive skb. For the CVSS the PR:N because no privileges are required on the victim once the attacker can participate as a TIPC neighbour or reach the TIPC broadcast domain. The issue is network adjacent rather than Internet wide in typical deployments because TIPC is usually used inside clustered or trusted internal networks. Impact is at least denial of service through kernel crash. For the paranoid score, limited confidentiality and integrity impact are kept because the primitive is an out-of-bounds kernel heap read whose copied bytes are later consumed as protocol state, but the patch does not show an arbitrary write or a proven privilege escalation path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	READ KASAN OOB TIPC SKB IMPROVEONLY LINUS  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  Trigger is a crafted TIPC broadcast STATE_MSG from a TIPC neighbour. This is network-adjacent rather than public Internet reachable.
* Reliable kernel crash / strong DoS: +1
  The commit includes a KASAN slab-out-of-bounds read in `kmemdup()` from the receive skb path.
* Availability impact realistic: +1
  A single malformed Gap ACK record can reach the faulty copy path after negotiation, making DoS realistic.
* Rare AND difficult-to-reach subsystem/configuration: -1
  TIPC is not normally exposed broadly and requires a configured TIPC domain plus neighbour capability negotiation.

Paranoid additional signals:

* Confidentiality impact plausible: +1
  The primitive is an out-of-bounds kernel heap read, although no direct user-visible disclosure path is demonstrated.
* Integrity impact plausible: +1
  Copied out-of-bounds bytes are later consumed as Gap ACK state, so limited protocol-state corruption is defensible, but not arbitrary kernel write.

Not counted:

* No strong generic memory corruption bonus. This is an OOB read, not OOB write, UAF, double-free, or controlled overwrite.
* No privilege escalation bonus. The patch does not show reclaim, object replacement, arbitrary write, callback control, or type confusion.
* No Dirty-Pipe-like or page-cache ownership bypass signal.

## 3. Reachability analysis

A TIPC neighbour that negotiated `TIPC_GAP_ACK_BLOCK` can trigger the bug by sending a malformed broadcast `STATE_MSG` with a short data area but an internally consistent Gap ACK length/count pair. No local privileges are required on the victim once the attacker can participate in the TIPC broadcast domain. This is not a general Internet remote bug in typical deployments, because TIPC is usually used inside clustered or trusted internal networks and must be configured. Namespaces may matter only if an attacker can gain access to a TIPC-capable network namespace or cluster segment.

Call-site confidence: high. The provided patch shows the receive path from `tipc_rcv()` through `tipc_node_bc_sync_rcv()` and `tipc_bcast_sync_rcv()`, and the commit describes the downstream `kmemdup()` OOB read in `tipc_link_advance_transmq()`.

## 4. Severity interpretation

This behaves stronger than an ordinary Moderate because it is network-adjacent and has a concrete kernel OOB read with a demonstrated KASAN crash. It is not a Strong Important candidate by default because the demonstrated primitive is read-only OOB access from skb memory, with no shown information disclosure channel, write primitive, object lifetime corruption, or LPE path. The conservative interpretation is network-adjacent DoS. The paranoid interpretation is Actionable Moderate because limited C/I impact is technically defensible from kernel heap OOB read plus protocol-state consumption.

## 5. One-sentence report phrase

A TIPC neighbour can send a malformed broadcast Gap ACK block that passes internal length consistency checks but exceeds the message data area, causing an out-of-bounds kernel heap read in `kmemdup()` and potentially a kernel crash.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is network-adjacent, includes a concrete OOB kernel read primitive, and has high availability impact. It should not be auto-closed even though practical exploitation appears limited to configured TIPC environments and no proven privilege escalation path is shown.

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

Patch: tipc: fix out-of-bounds read in broadcast Gap ACK blocks
Commit: 055663d21dc4336f67933ab26bef3c5934be6324
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=055663d21dc4336f67933ab26bef3c5934be6324

Commit description:

A broadcast PROTOCOL/STATE_MSG can carry a Gap ACK blocks record in its
data area. tipc_get_gap_ack_blks() only verifies that the record's len
field is self-consistent with its ugack_cnt/bgack_cnt counts
(sz == struct_size(p, gacks, ugack_cnt + bgack_cnt)); it does not check
that the record actually fits in the message data area, msg_data_sz().

The unicast caller tipc_link_proto_rcv() bounds it ("if (glen > dlen)
break;"), but the broadcast caller tipc_bcast_sync_rcv() discards the
returned size, so tipc_link_advance_transmq() copies the record off the
receive skb with an attacker-controlled count:

	this_ga = kmemdup(ga, struct_size(ga, gacks, ga->bgack_cnt),
			  GFP_ATOMIC);

A TIPC neighbour that negotiated TIPC_GAP_ACK_BLOCK triggers it with one
ordinary broadcast STATE_MSG (msg_bc_ack_invalid() clear), sized so its
data area is short, carrying a Gap ACK record with len = 0x400,
bgack_cnt = 0xff and ugack_cnt = 0. len then equals
struct_size(p, gacks, 255), so the consistency check passes and ga is
non-NULL; kmemdup() reads struct_size(ga, gacks, 255) = 1024 bytes out
of the much smaller skb:

  BUG: KASAN: slab-out-of-bounds in kmemdup_noprof+0x48/0x60
  Read of size 1024 at addr ffff0000c7030d38 by task poc864/69
  Call trace:
   kmemdup_noprof+0x48/0x60
   tipc_link_advance_transmq+0x86c/0xb80
   tipc_link_bc_ack_rcv+0x19c/0x1e0
   tipc_bcast_sync_rcv+0x1c4/0x2c4
   tipc_rcv+0x85c/0x1340
   tipc_l2_rcv_msg+0xac/0x104
  The buggy address belongs to the object at ffff0000c7030d00
   which belongs to the cache skbuff_small_head of size 704
  The buggy address is located 56 bytes inside of
   allocated 704-byte region [ffff0000c7030d00, ffff0000c7030fc0)

The copied-out bytes are subsequently consumed as gap/ack values, but
the read is already out of bounds at the kmemdup() regardless of how
they are used.

The unicast STATE path drops such a message: "if (glen > dlen) break;"
skips the rest of STATE_MSG handling and the skb is freed. Make the
broadcast path drop it too. tipc_bcast_sync_rcv() now bounds the record
against msg_data_sz() and, when it does not fit, reports it back through
tipc_node_bc_sync_rcv() to tipc_rcv() so the skb is discarded rather than
processed. ga is not cleared on this path: ga == NULL already means
"legacy peer without Selective ACK", a distinct legitimate state.

Fixes: d7626b5 ("tipc: introduce Gap ACK blocks for broadcast link")
Cc: stable@vger.kernel.org
Signed-off-by: Samuel Page <sam@bynar.io>
Reviewed-by: Tung Nguyen <tung.quang.nguyen@est.tech>
Link: https://patch.msgid.link/20260625143815.1525412-1-sam@bynar.io
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/tipc/bcast.c
  net/tipc/bcast.h
  net/tipc/node.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=055663d21dc4336f67933ab26bef3c5934be6324

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

The original Linux kernel CVE announcement for CVE-2026-64450 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64450

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:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7.6

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: 3
  Paranoid ActionableScore: 5

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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE 7.0

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.
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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).