[CVE-2026-46181][IMPORTANT] RDMA/mlx4: Fix mis-use of RCU in mlx4_srq_event()

[CVE-2026-46181][IMPORTANT] RDMA/mlx4: Fix mis-use of RCU in mlx4_srq_event()

[AL-KERNEL]

CVE: CVE-2026-46181
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: RDMA/mlx4: Fix mis-use of RCU in mlx4_srq_event()
Commit: 1e2a44875b6afb4add1115f7f3351dcbeb6f273d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1e2a44875b6afb4add1115f7f3351dcbeb6f273d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46181
Analysis date: Thu, 28 May 2026 10:36:49 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
`mlx4_srq_event()` used an unsafe RCU lookup and unchecked refcount increment for SRQ objects, allowing async events to race with SRQ creation or teardown and potentially access freed or partially initialized kernel memory.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-46181 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46181

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-2026-46181	IMPORTANT	CHECK	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-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'mlx4_srq_event used an RCU lookup for SRQ objects even though the SRQ object lifetime was not protected by RCU, and it also incremented the refcount without checking whether the object was already being destroyed or not fully initialized. A local user with access to the RDMA device may create or destroy SRQs while asynchronous device events are delivered, which can lead to use after free or access to a partially initialized kernel object. For the CVSS the PR:L is used because RDMA uverbs or device access can be enough in environments where the device is delegated to users or containers. The issue is not directly network reachable by itself, although device or firmware delivered async events are part of the trigger. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel lifetime corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8) 	MAYBE	REMOTE LOCK DANGER SIMPLEFIX NETWORK KERNEL_PANIC_PLUS_UAF KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user with RDMA/uverbs access may create or destroy SRQs.
* Generic memory corruption: +2. The bug is a possible use-after-free or access to an invalid SRQ object.
* Real lifetime corruption: +1. RCU was used for lookup, but the `mlx4_srq` object lifetime was not RCU-protected.
* Reliable kernel crash / strong DoS: +1. Async events racing with SRQ creation or teardown can crash the kernel.
* Local high-control device API: +1. RDMA userspace APIs allow users to shape SRQ object lifetime.
* Hard race / timing required: -1. Triggering depends on async events overlapping SRQ allocation or destruction.
* Constrained primitive: -1. The patch does not show controlled reclaim, controlled overwrite, or arbitrary write.

Paranoid additional signals:

* Weak to strong LPE concern: +1. UAF/refcount lifetime bugs in kernel object management can sometimes become exploitable if object reuse is influenceable.
* Privileged device-management memory corruption path: +1. The corruption occurs in a trusted RDMA device async event path.

## 3. Reachability analysis

The issue is local and depends on access to mlx4 RDMA device functionality. A user with uverbs access may influence SRQ creation and destruction, while async hardware or firmware events provide the racing event path.

This is not directly network reachable by ordinary packets. However, device-delivered async events are part of the trigger, and exposure increases in environments where RDMA devices are delegated to untrusted users or containers.

The affected hardware and driver are not universal, but where mlx4 RDMA is deployed the path is reachable through normal RDMA object management.

## 4. Severity interpretation

This behaves like a Strong Important candidate under paranoid triage and at least an actionable Moderate. The realistic demonstrated impact is local DoS through kernel crash.

The theoretical higher impact is possible privilege escalation because the primitive is a kernel lifetime race with UAF potential. No concrete arbitrary write or exploit chain is shown, so the higher score is for manual-review sensitivity, not proven LPE.

## 5. One-sentence report phrase

`mlx4_srq_event()` used an unsafe RCU lookup and unchecked refcount increment for SRQ objects, allowing async events to race with SRQ creation or teardown and potentially access freed or partially initialized kernel memory.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a kernel lifetime race with possible UAF in an RDMA async event path, and practical severity depends on RDMA device exposure, async event controllability, and allocator reuse behavior.

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

Patch: RDMA/mlx4: Fix mis-use of RCU in mlx4_srq_event()
Commit: 1e2a44875b6afb4add1115f7f3351dcbeb6f273d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1e2a44875b6afb4add1115f7f3351dcbeb6f273d

Commit description:

Sashiko points out the radix_tree itself is RCU safe, but nothing ever
frees the mlx4_srq struct with RCU, and it isn't even accessed within the
RCU critical section. It also will crash if an event is delivered before
the srq object is finished initializing.

Use the spinlock since it isn't easy to make RCU work, use
refcount_inc_not_zero() to protect against partially initialized objects,
and order the refcount_set() to be after the srq is fully initialized.

Cc: stable@vger.kernel.org
Fixes: 30353bf ("net/mlx4_core: Use RCU to perform radix tree lookup for SRQ")
Link: https://sashiko.dev/#/patchset/0-v2-1c49eeb88c48%2B91-rdma_udata_rep_jgg%40nvidia.com?part=5
Link: https://patch.msgid.link/r/12-v1-41f3135e5565+9d2-rdma_ai_fixes1_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/ethernet/mellanox/mlx4/srq.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=1e2a44875b6afb4add1115f7f3351dcbeb6f273d

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

The original Linux kernel CVE announcement for CVE-2026-46181 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46181

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: 6
  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).