[CVE-2026-72042][IMPORTANT] ipmi: Fix user refcount underflow in event delivery

[CVE-2026-72042][IMPORTANT] ipmi: Fix user refcount underflow in event delivery

[AL-KERNEL]

CVE: CVE-2026-72042
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipmi: Fix user refcount underflow in event delivery
Commit: ddbb6e3dc9bb4743de686aa1598c31e745cee76b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ddbb6e3dc9bb4743de686aa1598c31e745cee76b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72042
Analysis date: Sat, 15 Aug 2026 15:13:11 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A reference-counting bug in the IPMI event delivery rollback path can prematurely free an `ipmi_user` that remains linked on `intf->users`, allowing later event delivery to dereference freed memory and potentially causing a local kernel crash or, in the worst case, a UAF-based privilege escalation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72042 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72042

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-2026-72042	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN 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-416;CWE-911;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A reference counting bug in the IPMI event delivery rollback path can free an ipmi_user too early. ipmi_free_recv_msg already drops the temporary user reference owned by the receive message, but the rollback path also performed an extra kref_put on the same user. The freed user may remain linked on intf users, so later event delivery can dereference freed memory or hit refcount_t add on zero checks. For the CVSS the PR:L value is used for the paranoid score because a local process with access to the IPMI device interface may be enough in deployments where /dev/ipmi is delegated to a service account or hardware management group. The issue is not directly network reachable from ordinary IP traffic. Impact is at least local denial of service via kernel crash or warning, and in the worst still defensible case may include privilege escalation due to a kernel use after free primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-72042 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB UAF LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local trigger with limited privileges: +1
  A local process with access to the IPMI userspace interface can plausibly participate in event delivery paths. This is not ordinary remote network reachability.
* Generic memory corruption, UAF/refcount lifetime class: +2
  The extra `kref_put()` can free an `ipmi_user` object too early while it remains linked on `intf->users`.
* Real lifetime corruption: +1
  This is a concrete reference-counting lifetime bug, not just bookkeeping.
* Reliable kernel crash / strong DoS: +1
  Later event delivery may dereference a freed user or trigger `refcount_t` add-on-zero warnings.
* Rare/configuration-dependent subsystem: -1
  IPMI is common on servers but not a universal default local attack surface.

Paranoid additional interpretation:

* Privilege escalation plausible: +2 instead of weak concern
  Because the bug is a kernel UAF candidate involving a freed object that remains reachable from a global interface user list. There is no demonstrated reclaim primitive in the patch, but the refcount misuse and stale list membership make manual LPE review justified.

## 3. Reachability analysis

The issue is local, not directly reachable from ordinary IP traffic. IPMI-over-LAN to a BMC should not be treated as direct access to this host kernel `ipmi_msghandler.c` path. The practical trigger requires local access to the IPMI device or a service account that can interact with the kernel IPMI interface.

In typical deployments `/dev/ipmi*` may be restricted to root or a hardware-management group. That makes the conservative score lower than a fully unprivileged local bug. However, many server environments delegate IPMI access to monitoring, inventory, or management daemons, so treating this as PR:L for paranoid triage is defensible. Namespaces do not obviously make this broadly reachable unless the device node is explicitly passed through.

Call-site confidence: medium-high. The patch and commit message clearly describe the ownership transfer through `ipmi_alloc_recv_msg()` and `ipmi_free_recv_msg()`, and the rollback path is visible.

## 4. Severity interpretation

This is more than an ordinary Moderate cleanup because it fixes a concrete reference-counting lifetime error that can leave a freed kernel object linked for later use. The realistic demonstrated impact is local DoS through warning, oops, or crash. The theoretical but defensible higher concern is local privilege escalation from a kernel UAF primitive, although the patch does not show attacker-controlled reclaim, controlled overwrite, callback control, or a proven exploit chain.

Therefore the conservative bucket is Actionable Moderate. The paranoid bucket is Strong Important candidate for manual review.

## 5. One-sentence report phrase

A reference-counting bug in the IPMI event delivery rollback path can prematurely free an `ipmi_user` that remains linked on `intf->users`, allowing later event delivery to dereference freed memory and potentially causing a local kernel crash or, in the worst case, a UAF-based privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a kernel refcount underflow and UAF candidate with stale list reachability. Even though exploitation beyond DoS is not demonstrated, the lifetime primitive is strong enough that it should not be auto-closed.

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

Patch: ipmi: Fix user refcount underflow in event delivery
Commit: ddbb6e3dc9bb4743de686aa1598c31e745cee76b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ddbb6e3dc9bb4743de686aa1598c31e745cee76b

Commit description:

ipmi_alloc_recv_msg(user) takes the temporary user reference owned by the
receive message, and ipmi_free_recv_msg() drops it again. If event delivery
fails after allocating receive messages for earlier users,
handle_read_event_rsp() rolls those messages back with
ipmi_free_recv_msg().

That rollback path still drops user->refcount explicitly after freeing each
message. The extra put can free a user that remains linked on intf->users,
so later event delivery may dereference a freed user or trip refcount_t's
addition-on-zero warning when ipmi_alloc_recv_msg() tries to acquire
another reference.

Remove the stale explicit put and the now-dead user assignment. Keep the
list_del() and ipmi_free_recv_msg() calls; they are the required rollback
operations.

Fixes: b52da40 ("ipmi: Rework user message limit handling")
Cc: stable@vger.kernel.org # v6.18+
Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
Message-ID: <20260521130628.3641050-1-matt@readmodwrite.com>
Signed-off-by: Corey Minyard <corey@minyard.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/char/ipmi/ipmi_msghandler.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=ddbb6e3dc9bb4743de686aa1598c31e745cee76b

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

The original Linux kernel CVE announcement for CVE-2026-72042 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72042

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:N/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: 5
  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: 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).