[CVE-2026-74513][MODERATE 7.0] dibs: fix use-after-free of dmb_node in loopback attach/detach/unregister

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

CVE: CVE-2026-74513
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: dibs: fix use-after-free of dmb_node in loopback attach/detach/unregister
Commit: c0837aeace96152d14b17fdd19d70102b6631a7d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0837aeace96152d14b17fdd19d70102b6631a7d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74513
Analysis date: Sat, 15 Aug 2026 15:07:11 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in DIBS loopback DMB attach detach and unregister can free `dmb_node` between hash lookup and refcount update, causing a local UAF with realistic kernel crash impact and limited but defensible refcount corruption concern.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74513 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74513

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-2026-74513	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in the DIBS loopback DMB management path can cause a use after free of dmb_node. The affected attach detach and unregister paths looked up a node under dmb_ht_lock but then dropped the lock before changing the node refcount, allowing another concurrent path to remove the node from the hash table and free it with kfree. A local attacker able to exercise SMC-D or DIBS loopback DMB operations may race these paths and trigger a kernel crash. For the CVSS the PR:L is used for the paranoid score because reliable triggering requires a local user or process context that can reach the loopback DMB operations, but not necessarily full administrator privileges. The issue is not directly network reachable because the affected object is in the loopback DIBS path. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel heap use after free.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-74513 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	LOCK INIT RACE UAF HARDWARE LINUS KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. The affected SMC-D / DIBS loopback operations are local and plausibly reachable by a local process using the relevant SMC path.
* Memory corruption, weak primitive: +1. The patch explicitly describes UAF of `dmb_node`, but reliable attacker-controlled reclaim or object replacement is not demonstrated.
* Real lifetime corruption: +1. The bug is a refcount and hash-table lifetime race where an object can be freed between lookup and refcount operation.
* Reliable kernel crash / strong DoS: +1. A stale `dmb_node` can be dereferenced or modified after `kfree()`, making kernel crash realistic.
* Hard or unreliable race: -1. Exploitation requires winning a concurrent attach/detach/unregister timing window.
* Privileged kernel/device-management lifetime path: +1. The corruption occurs in trusted DIBS DMB object lifetime management.

Paranoid adjustment:

* Stronger UAF/refcount concern: +1 over conservative. The stale access is not only a read. `refcount_inc_not_zero()` and `refcount_dec_and_test()` can perform atomic writes to freed memory, so a refcount-based corruption concern is defensible even though no full reclaim primitive is shown.

## 3. Reachability analysis

A local user or local process that can exercise SMC-D / DIBS loopback DMB operations may trigger concurrent attach, detach, and unregister paths. This is not directly network reachable because the affected code is in the loopback DIBS path rather than a remote packet parser. Namespace and container impact depends on whether the environment exposes SMC/DIBS functionality to reduced-privilege workloads. If only host administrators can configure or reach this path, the practical score is closer to the conservative value. The path is specialized and not as broadly exposed as TCP/IP or netfilter, but the bug is a real kernel object lifetime UAF.

## 4. Severity interpretation

This is more than an ordinary Moderate correctness issue because the patch fixes an explicit UAF involving hash-table membership and refcount transitions. Realistic impact is local DoS via kernel crash. Privilege escalation is not demonstrated, but the stale refcount operation can write to freed memory, so the issue should remain actionable and manually reviewed. The conservative interpretation treats it as race-dependent UAF without proven reclaim. The paranoid interpretation treats the refcount write-after-free as enough to justify Actionable Moderate handling.

## 5. One-sentence report phrase

A race in DIBS loopback DMB attach detach and unregister can free `dmb_node` between hash lookup and refcount update, causing a local UAF with realistic kernel crash impact and limited but defensible refcount corruption concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: explicit UAF plus refcount writes to freed memory. No remote vector or proven LPE primitive is shown, but this is a real lifetime corruption bug and should not be auto-closed.

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

Patch: dibs: fix use-after-free of dmb_node in loopback attach/detach/unregister
Commit: c0837aeace96152d14b17fdd19d70102b6631a7d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0837aeace96152d14b17fdd19d70102b6631a7d

Commit description:

dibs_lo_attach_dmb(), dibs_lo_detach_dmb() and dibs_lo_unregister_dmb()
look up the dmb_node under dmb_ht_lock, drop the lock and only then
operate on the node's refcount. Nothing keeps the node alive across
that window: __dibs_lo_unregister_dmb() removes the node from the hash
table under the write lock and immediately frees it.

A concurrent final put can therefore free the node between the lookup
and the refcount operation:

CPU0 (attach)                     CPU1 (owner unregisters)

read_lock_bh(&dmb_ht_lock)
find dmb_node (refcnt == 1)
read_unlock_bh(&dmb_ht_lock)
                                  refcount_dec_and_test() 1 -> 0
                                  write_lock_bh(&dmb_ht_lock)
                                  hash_del(&dmb_node->list)
                                  write_unlock_bh(&dmb_ht_lock)
                                  kfree(dmb_node)
refcount_inc_not_zero(&dmb_node->refcnt)  <-- use-after-free

The same window exists for the refcount_dec_and_test() calls in the
detach and unregister paths.

Close the race structurally by making hash table membership and the
refcount transitions atomic with respect to each other:

- Perform the final refcount_dec_and_test() and hash_del() in a single
  dmb_ht_lock write-side critical section, in both the unregister and
  the detach path. Freeing the node still happens after the lock is
  dropped, which is safe because a node whose refcount reached zero has
  left the hash table and can no longer be found.

- This establishes the invariant that any node found in the hash table
  holds at least one reference, and that the final reference can only
  be dropped under the write lock. dibs_lo_attach_dmb() can thus take
  its reference with a plain refcount_inc() while still holding the
  read lock; refcount_inc_not_zero() is no longer needed.

__dibs_lo_unregister_dmb() no longer touches the hash table and is
renamed to dibs_lo_free_dmb() accordingly.

Note: commit cc21191 ("dibs: Move data path to dibs layer") moved
the code to its current location; the race was introduced earlier by
commit c3a910f ("net/smc: implement DMB-merged operations of
loopback-ism").

Tested SMC-D via ISM and dibs loopback.

Cc: stable@vger.kernel.org
Fixes: c3a910f ("net/smc: implement DMB-merged operations of loopback-ism")
Reported-by: Rahul Chandelkar <rc@rexion.ai>
Signed-off-by: Hidayath Khan <hidayath@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://patch.msgid.link/20260727093530.968834-1-hidayath@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/dibs/dibs_loopback.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=c0837aeace96152d14b17fdd19d70102b6631a7d

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

The original Linux kernel CVE announcement for CVE-2026-74513 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74513

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:L/I:L/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: 4
  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: YES
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.
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