[CVE-2026-98215][MODERATE 7.0] selinux: preserve user SID across nested backing files

[CVE-2026-98215][MODERATE 7.0] selinux: preserve user SID across nested backing files

[AL-KERNEL]

CVE: CVE-2026-98215
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: selinux: preserve user SID across nested backing files
Commit: caa1b913d90d5dc07073733326d2af0f0288f089
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=caa1b913d90d5dc07073733326d2af0f0288f089
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98215
Analysis date: Fri, 09 Oct 2026 05:54:08 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98215 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98215

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
======================================================================

CVE-2026-98215	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98215 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC)	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:L';CWE-863;CWE-284;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '6.5';DESCR 'SELinux can perform an fd use authorization check against the wrong target SID when mmap operates through nested backing files such as overlayfs over overlayfs or FUSE passthrough backed by overlayfs. The saved top level user SID can be replaced by the SID of the process that opened an intermediate backing file. This can incorrectly deny mprotect without a domain transition and can produce an incorrect authorization decision after a transition. For the CVSS the PR:L reflects that a local process must reach the affected nested backing file and invoke mmap or mprotect, while administrative privileges are not necessarily required if the mount topology already exists. AC:H reflects the need for a specific nested filesystem configuration, SELinux enforcing mode, and a policy or domain transition that makes the incorrect SID relevant. The issue is not directly network reachable. Impact includes a confirmed local access denial and a plausible SELinux policy bypass with confidentiality or integrity impact depending on the installed policy and the mapped file.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	READ IMPROVEONLY LINUS TEST  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Applied conservative safety floor: 4 -> 5 because the patch establishes loss of the correct SELinux authorization identity and evaluation against the wrong SID.
* No cap was applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative positive signals:

* +2 Concrete authorization decision defect. The `fd { use }` decision can be evaluated against the intermediate mounter SID instead of the top-level user SID. With a domain transition and differing policy permissions, this can change the result of a mandatory SELinux decision.
* +1 Security-state lifetime mismatch. The top-level user SID is lost while the mapping remains available for a later `mprotect()` authorization check. The fix carries that SID across every backing-file layer.
* +1 Local below-host-admin trigger. Given the affected nested backing-file topology, an ordinary process that can access the file can invoke `mmap()` and `mprotect()` without host-admin privilege.

Paranoid-only signal:

* +1 Plausible policy-bypass consequence. After a domain transition, checking the wrong SID can permit `fd { use }` where checking the preserved top-level SID would deny it. This is technically supported by the SID-selection mechanics, although the supplied test demonstrates false denial rather than a successful unauthorized allow.

Important non-awarded signals:

* B5-R is not applicable. There is no heap OOB write, retry-accumulated index, corrupted descriptor consumption, or memory-corruption primitive.
* No UAF, stale freed object, repeated release, refcount mutation, controlled reclaim, or arbitrary kernel write is established.
* No sensitive file, credential, or privileged FD theft is demonstrated.
* No root privilege escalation or arbitrary privileged execution is demonstrated.
* Confidentiality and integrity were not separately scored because no particular protected mapping or successful unauthorized access is supplied.
* No reliable kernel crash or host-wide availability impact is established.
* Remote reachability is not established.
* The false-denial result is not separately counted as strong DoS because it affects the relevant mapping operation rather than system-wide availability.

3. Reachability analysis

The vulnerable operation is reached through local `mmap()` and `mprotect()` activity on a file whose backing path contains nested overlayfs layers or FUSE passthrough backed by overlayfs. SELinux enforcing mode and a policy in which the top-level user SID differs meaningfully from the intermediate mounter SID are required.

Creating the filesystem topology may require prior administrative setup, but the triggering process does not necessarily need host-admin privilege once that topology is mounted and accessible. The supplied prior analysis also identifies PR:L rather than PR:H. Therefore `HostAdminRequired=NO`.

The evidence does not establish that creation of the complete nested backing-file topology and all relevant SELinux policy conditions can be performed inside an attacker-created namespace. Therefore `NamespaceScopedPrivilege=UNKNOWN`.

There is no race or timing requirement. No special hardware is required. The issue is local and is not directly triggered by network traffic.

4. Primitive and severity interpretation

The exact primitive is incorrect SELinux authorization identity propagation. At the first backing layer, the top-level file SID is saved in `uf_sid`. At a nested layer, the vulnerable code reads the ordinary SID of the already-backed intermediate file and overwrites the original authorization identity with the mounter SID. A later `mprotect()` operation consequently checks `fd { use }` against the wrong security domain.

This is not memory corruption and not a lifetime violation involving freed memory. It is a security-state lifetime and propagation mismatch that affects a mandatory access-control decision.

The demonstrated consequence is an incorrect denial of `mprotect(PROT_READ)`. The patch-supported security concern beyond that denial is that, following a domain transition, permissions may differ between the real top-level user SID and the intermediate mounter SID. The wrong-target check can therefore produce an incorrect allow as well as an incorrect deny.

`PrimitiveClass=CLASS_A` is used because the defect affects an externally meaningful SELinux authorization gate rather than passive label bookkeeping. A downstream sensitive-resource compromise or LPE is not established, so the score remains 5 rather than being promoted to a strong 7-8 Important score.

`ImportantCandidate=YES`, `ManualReview=REQUIRED`, and `AutoDowngradeAllowed=NO` are appropriate because automatically treating the issue as a harmless false denial could miss a policy-dependent SELinux bypass after a domain transition.

5. One-sentence report phrase

Nested backing files can replace the saved top-level user SID with an intermediate mounter SID, causing SELinux to evaluate mprotect fd use authorization against the wrong security domain and potentially produce an incorrect allow or deny decision.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Review should determine whether affected deployments expose nested backing files to unprivileged processes and whether their SELinux policies contain domain transitions where permission against the mounter SID is weaker than permission against the preserved top-level user SID.

7. Call-site confidence

Call-site confidence: high

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

Patch: selinux: preserve user SID across nested backing files
Commit: caa1b913d90d5dc07073733326d2af0f0288f089
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=caa1b913d90d5dc07073733326d2af0f0288f089

Commit description:

SELinux saves the user file SID in a backing-file security blob so it
remains available after mmap() replaces vma->vm_file with a backing file.

For nested backing files (overlayfs over overlayfs, or FUSE passthrough
backed by overlayfs), user_file may itself be a backing file.  Its
fsec->sid is the SID of the mounter that opened it, rather than the user
that opened the top-level file.  mprotect() then checks fd { use } against
the mounter SID.  This can incorrectly deny access without a domain
transition, or check the wrong target SID after one.

Copy the saved user SID when user_file is a backing file.  Keep using the
regular file SID for the first backing layer.

With two nested overlayfs mounts and SELinux enforcing,
mprotect(PROT_READ) returns EACCES with an fd { use } denial against the
mounter SID.  With this change, mprotect() succeeds.

Tested on arm64 QEMU with a small BusyBox initramfs and a purpose-built
SELinux policy.  The original test was also repeated with Fedora Cloud
Base 44 userspace and gave the same result.

Cc: stable@vger.kernel.org
Fixes: 82544d3 ("selinux: fix overlayfs mmap() and mprotect() access checks")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  security/selinux/hooks.c
  security/selinux/include/objsec.h

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=caa1b913d90d5dc07073733326d2af0f0288f089

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

The original Linux kernel CVE announcement for CVE-2026-98215 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98215

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L
  The Best / paranoid CVSS score: 6.5

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: unknown
  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).