[CVE-2026-74433][MODERATE 7.0] rxrpc: Fix UAF in rxgk_issue_challenge()

[CVE-2026-74433][MODERATE 7.0] rxrpc: Fix UAF in rxgk_issue_challenge()

[AL-KERNEL]

CVE: CVE-2026-74433
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: rxrpc: Fix UAF in rxgk_issue_challenge()
Commit: 844b8525ce503405c462ad67f750bec648720397
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=844b8525ce503405c462ad67f750bec648720397
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74433
Analysis date: Sun, 16 Aug 2026 02:20:29 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A use-after-free in `rxgk_issue_challenge()` can occur because the challenge page is freed before `trace_rxrpc_tx_packet()` uses `whdr` pointing into that page, making this a network-adjacent RxRPC lifetime bug with realistic DoS impact and limited but manual-review-worthy corruption concern.

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

The original Linux kernel CVE announcement for CVE-2026-74433 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74433

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: NO

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-2026-74433	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-825;*CWE-672;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '7';DESCR 'A use-after-free can occur in rxgk_issue_challenge() because the page holding the challenge packet is freed before trace_rxrpc_tx_packet() uses whdr that points into that page. The visible primitive is a stale pointer read in the tracepoint path rather than an obvious write primitive, so the realistic impact is limited denial of service or diagnostic UAF when tracing or debug checks are active. For the CVSS the PR:N is used for the paranoid network case because a remote peer may trigger challenge generation without local user access when the RxRPC rxgk service and tracing path are reachable. The issue is network reachable in affected RxRPC deployments, but practical triggering depends on non-default rxgk usage and tracepoint or debug visibility. Impact is at least availability loss and in the paranoid case may include limited confidentiality or integrity impact due to the use-after-free class, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-74433 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 9d1d2b59341f58126a69b51f9f5f8ccb9f12e54a YES  NO NO unknown 	MAYBE	INIT UAF 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: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  `rxgk_issue_challenge()` is in RxRPC challenge generation and can plausibly be reached due to remote peer activity when RxRPC rxgk is in use.

* Memory corruption, weak/indirect corruption candidate: +1
  The patch fixes a real UAF, but the visible primitive is a stale pointer passed to a tracepoint and apparently read for tracing. No write-after-free, reclaim control, callback control, or type confusion is shown.

* Real lifetime corruption: +1
  `whdr` points into a page that was freed before `trace_rxrpc_tx_packet()` used it.

* Reliable kernel crash / strong DoS: +1
  A UAF read in tracing/debug paths can plausibly trigger KASAN reports, warnings, or kernel crashes when the tracepoint/debug configuration is active.

* Rare AND difficult-to-reach subsystem/configuration: -1
  The affected path depends on RxRPC rxgk/GSSAPI usage and the problematic dereference appears tied to tracepoint consumption, which is not a broad default packet path.

Paranoid adjustment:

* Paranoid score removes the rare/configuration penalty for environments where RxRPC rxgk and tracing are intentionally enabled and reachable. This raises the practical triage score from 4 to 5.

Not awarded:

* Strong LPE plausibility: +0
  The patch does not show attacker-controlled reclaim, object replacement, write-after-free, type confusion, callback dispatch, or refcount takeover.

* Confidentiality / Integrity impact: +0 in the conservative score
  The demonstrated primitive is stale read/use by a tracepoint, not a user-visible info leak or attacker-controlled write.

## 3. Reachability analysis

A remote RxRPC peer may be able to cause challenge generation if the target uses the rxgk security class. However, rxgk/GSSAPI RxRPC is not a common default Internet-facing configuration. The UAF is most relevant when the tracepoint path actually consumes `whdr`; if tracing is disabled, practical impact may be much lower.

No local privileges are required in the network-triggered interpretation, but realistic triggering requires a reachable affected RxRPC deployment. Namespaces and containers do not materially improve reachability unless they expose or route access to the relevant RxRPC service.

Call-site confidence: medium. The vulnerable function and immediate use-after-free ordering are visible, but broader deployment and tracepoint activation details are not fully shown in the supplied patch.

## 4. Severity interpretation

This is not an ordinary low-priority cleanup because it is a real UAF in network-adjacent kernel code. At the same time, the shown primitive is weak: a stale pointer is used by a tracepoint after the backing page is freed, with no demonstrated attacker-controlled reuse or write primitive.

Realistically, this behaves like an actionable Moderate issue: network-reachable in affected deployments, memory-lifetime bug class, likely DoS/debug UAF impact. It is not a strong Important candidate unless additional evidence shows attacker-controlled reclaim, disclosure through trace buffers available to lower-privileged users, or a stronger corruption path.

## 5. One-sentence report phrase

A use-after-free in `rxgk_issue_challenge()` can occur because the challenge page is freed before `trace_rxrpc_tx_packet()` uses `whdr` pointing into that page, making this a network-adjacent RxRPC lifetime bug with realistic DoS impact and limited but manual-review-worthy corruption concern.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a real UAF in a network-adjacent RxRPC path, but the visible primitive is limited to tracepoint use after free and does not currently demonstrate LPE, arbitrary write, or attacker-controlled object reuse.

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

Patch: rxrpc: Fix UAF in rxgk_issue_challenge()
Commit: 844b8525ce503405c462ad67f750bec648720397
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=844b8525ce503405c462ad67f750bec648720397

Commit description:

Fix rxgk_issue_challenge() to free the page containing the challenge
content after invoking the tracepoint as the whdr passed to the tracepoint
points into the page just freed.

Fixes: 9d1d2b5 ("rxrpc: rxgk: Implement the yfs-rxgk security class (GSSAPI)")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Link: https://patch.msgid.link/20260609140911.838677-4-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/rxrpc/rxgk.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=844b8525ce503405c462ad67f750bec648720397

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

The original Linux kernel CVE announcement for CVE-2026-74433 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74433

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