[CVE-2026-63797][IMPORTANT] rpmsg: char: Fix use-after-free on probe error path

[CVE-2026-63797][IMPORTANT] rpmsg: char: Fix use-after-free on probe error path

[AL-KERNEL]

CVE: CVE-2026-63797
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: rpmsg: char: Fix use-after-free on probe error path
Commit: ddf13f91ca82c94ef7ad9c41a434a03313f8eb1b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ddf13f91ca82c94ef7ad9c41a434a03313f8eb1b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63797
Analysis date: Sun, 19 Jul 2026 09:09:08 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A use-after-free in the rpmsg char probe error path can leave `default_ept->priv` pointing to a freed `eptdev`, allowing an rpmsg peer callback during the failure window to crash the kernel and potentially corrupt memory.

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

The original Linux kernel CVE announcement for CVE-2026-63797 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63797

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-2026-63797	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-755;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'A use after free can occur in the rpmsg char probe error path because rpmsg_chrdev_probe publishes eptdev through default_ept priv before rpmsg_chrdev_eptdev_add has succeeded. If the add operation fails and frees eptdev, the default endpoint may still dispatch message or flow control callbacks with a stale priv pointer. A malicious or compromised rpmsg peer can then reach rpmsg_ept_cb or rpmsg_ept_flow_cb in this window and access freed memory, leading at least to a kernel crash and potentially to memory corruption. For the CVSS the PR:N is used for the paranoid score because the callback input can come from the rpmsg peer once the channel exists and does not require a local Linux user account on the victim. The issue is not Internet network reachable. It is adjacent to the rpmsg transport and is mainly relevant when an attacker can control a remote processor, firmware, or virtual rpmsg endpoint. Impact is denial of service in the practical case, with confidentiality and integrity impact kept high in the paranoid score because the bug class is UAF and the callback path can operate on a freed object.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-63797 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: bc69d10665690492421d926b1cd9a7a36bffd691 YES  NO NO unknown 	MAYBE	ERRORPATH UAF SKB HARDWARE LINUS KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* +1 Firmware-mediated external influence: the trigger is not ordinary Internet networking, but an rpmsg peer, remote processor, firmware, or virtual rpmsg endpoint can influence callbacks.
* +2 Generic memory corruption, strong primitive: the patch explicitly fixes a use-after-free where `default_ept->priv` can point to freed `eptdev`.
* +1 Real lifetime corruption: this is a stale pointer after free on an asynchronous endpoint callback path.
* +1 Reliable kernel crash / strong DoS: callbacks can dereference freed memory, so kernel crash is realistic.
* +1 Weak LPE concern: the flow-control callback writes fields through the stale `eptdev` pointer, but no controlled reclaim or object replacement is demonstrated.
* +1 Integrity impact plausible: write-after-free behavior in `rpmsg_ept_flow_cb()` can modify freed/reused memory in a limited but real way.
* -1 Hard or unreliable race / special timing required: exploitation depends on `rpmsg_chrdev_eptdev_add()` failing while callbacks are still possible in the probe window.

Paranoid delta:

* +1 Confidentiality impact plausible: because this is a real UAF on a callback path, limited disclosure or object-state exposure cannot be excluded without deeper allocator/reclaim analysis.

Not counted:

* No +2 remote reachable: this is not IP/network reachable in the usual sense.
* No +2 strong LPE: there is no demonstrated attacker-controlled reclaim, arbitrary write, function-pointer control, or reliable object replacement.
* No rare-subsystem penalty: rpmsg is platform-specific, but on affected systems the path is a normal device/firmware communication mechanism.

Call-site confidence: high. The patch shows both the publishing site in `rpmsg_chrdev_probe()` and the affected callbacks `rpmsg_ept_cb()` and `rpmsg_ept_flow_cb()`.

## 3. Reachability analysis

The realistic attacker is a malicious or compromised rpmsg peer, remote processor firmware, or virtual rpmsg endpoint able to deliver messages or flow-control updates during the probe error window. A local Linux user account is not necessarily required if the hostile input comes from the peer side of the rpmsg transport.

Namespaces and containers do not materially lower or raise the main trigger, because the vulnerable path is tied to device probing and endpoint callbacks rather than a normal namespaced userspace API. On general-purpose servers without rpmsg hardware or virtual rpmsg devices, exposure is low. On embedded, SoC, Android-like, remoteproc, or virtualization environments using rpmsg channels, exposure is more relevant.

The issue is not default-exposed over the public Internet. Exploitation requires the affected rpmsg char channel, a probe failure in `rpmsg_chrdev_eptdev_add()`, and callback delivery in the vulnerable window.

## 4. Severity interpretation

This behaves above an ordinary Moderate because the patch fixes a real UAF, not just a NULL dereference or validation bug. The practical impact is most clearly local or firmware-mediated denial of service via kernel crash.

The theoretical memory corruption potential is meaningful because one callback reads and uses the freed `eptdev`, while the flow-control callback writes through it. However, the patch does not demonstrate controlled reclaim, type confusion, arbitrary write, or a reliable privilege-escalation path. Therefore the conservative result is Actionable Moderate, while the paranoid result reaches Strong Important candidate for manual review sensitivity.

## 5. One-sentence report phrase

A use-after-free in the rpmsg char probe error path can leave `default_ept->priv` pointing to a freed `eptdev`, allowing an rpmsg peer callback during the failure window to crash the kernel and potentially corrupt memory.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is an explicit UAF on an asynchronous callback path with at least one write-through-stale-pointer case, and the true impact depends on allocator reuse, timing, and whether the rpmsg peer can be attacker controlled in the target deployment.

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

Patch: rpmsg: char: Fix use-after-free on probe error path
Commit: ddf13f91ca82c94ef7ad9c41a434a03313f8eb1b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ddf13f91ca82c94ef7ad9c41a434a03313f8eb1b

Commit description:

rpmsg_chrdev_probe() stores the newly allocated eptdev in the default
endpoint's priv pointer before calling rpmsg_chrdev_eptdev_add(). If
rpmsg_chrdev_eptdev_add() then fails, its error path frees eptdev while
the default endpoint may still dispatch callbacks with the stale priv
pointer.

Avoid publishing eptdev through the default endpoint until
rpmsg_chrdev_eptdev_add() succeeds. Messages received before the priv
pointer is published should be ignored by rpmsg_ept_cb(). Flow-control
updates can hit rpmsg_ept_flow_cb() in the same window, so make both
callbacks return success when priv is NULL.

Fixes: bc69d10 ("rpmsg: char: Introduce the "rpmsg-raw" channel")
Signed-off-by: Yuho Choi <dbgh9129@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20260601183247.1962010-1-dbgh9129@gmail.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/rpmsg/rpmsg_char.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=ddf13f91ca82c94ef7ad9c41a434a03313f8eb1b

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

The original Linux kernel CVE announcement for CVE-2026-63797 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63797

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:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 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).