* [CVE-2026-90278][MODERATE 7.0] md: wait for behind writes before destroying bitmap [ Upstream
@ 2026-09-17 19:37 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-17 19:37 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90278
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: md: wait for behind writes before destroying bitmap [ Upstream
Commit: 73881ff7a75913f919a1ce9d9571bfaab8e8588d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=73881ff7a75913f919a1ce9d9571bfaab8e8588d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90278
Analysis date: Thu, 17 Sep 2026 15:37:38 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90278 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90278
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-90278 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90278 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-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'md __md_stop() could destroy the bitmap before mddev_detach() had a chance to wait for RAID1 behind write completions. With llbitmap, the wait is implemented in wait_behind_writes() while destroy() tears down the llbitmap storage, so completions may run after the storage has been freed. This is a local race and use-after-free candidate in the md storage path and can at least cause denial of service via kernel crash or corrupted md bitmap state. For the CVSS the PR:L is used for the paranoid score because md control may be delegated to a storage management service, reduced capability root, or another local process with limited but relevant administrative access. The issue is not directly network reachable and requires local control of md stop or detach behavior plus outstanding behind writes. Impact is at least local denial of service. In the worst still defensible interpretation, freed llbitmap storage may be accessed by later completions and could affect confidentiality or integrity, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 4) YES DISK SIMPLEFIX LINUS KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=3
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3. No floor or cap applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Conservative signals:
* +2 Real UAF / lifetime violation. The commit directly states that RAID1 behind-write completions could still run after llbitmap storage had been freed.
* +1 Reliable kernel crash / strong DoS. A completion path accessing freed bitmap storage can realistically crash the kernel or corrupt md bitmap state.
* +1 Important storage path. The bug is in md RAID bitmap teardown, which is an important storage consistency path.
* +1 Active-state retirement / synchronization bypass. The old ordering destroyed bitmap storage before the detach path could run wait_behind_writes(), bypassing the required synchronization protocol.
* -2 Host-admin-like md control in the direct kernel interface model. Stopping or detaching md devices is normally an administrative storage-management operation.
Paranoid signals:
* +2 Real UAF / lifetime violation.
* +1 Reliable kernel crash / strong DoS.
* +1 Important storage path.
* +1 Active-state retirement / synchronization bypass.
* +1 Structured post-free access concern. The stale operation is not merely a passive diagnostic read: behind-write completions may continue operating on freed llbitmap storage, leaving unresolved post-lifetime corruption risk.
* -1 Reduced or delegated storage-management privilege model. The supplied context suggests md control may be delegated to a storage service, reduced-capability root, or another local process with relevant administrative access, so the paranoid score does not assume full host-admin only.
Non-awarded signals:
* Controlled reclaim is not demonstrated.
* Arbitrary kernel read or write is not demonstrated.
* Privilege escalation is not demonstrated.
* Remote reachability is not established.
* Concrete repeated release/refcount mutation is not established. This is async post-free completion use, not a visible second put/free/refcount operation on the exact same reference.
* Extra availability was not awarded because the same crash/corruption consequence is already counted once.
* C/I impact is not awarded as a direct demonstrated consequence, but remains a manual-review concern for the paranoid interpretation.
Repeated release/refcount check:
* First release can end lifetime: yes, md_bitmap_destroy() tears down llbitmap storage.
* Exact same reference survives to a second release/refcount operation: not established.
* Second operation is a real lifetime/refcount mutation: not established.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 from repeated-release rule applied: no.
3. Reachability analysis
The direct trigger appears to require local control over md device stop or detach behavior while RAID1 behind writes are still outstanding. In normal Linux deployments, md array stop/removal is an administrative storage-management operation, usually performed by root or a privileged service. The supplied evidence does not establish that an ordinary unprivileged local user can complete the trigger.
Namespace-scoped privilege is not established. md block device management is not shown to be fully user-namespace or container-namespace scoped. A container or namespace alone would not be enough to trigger this without access to the relevant md control path.
Prior setup is likely required in practice: an md RAID1 setup using llbitmap and behind writes must exist. This is not a network-triggered bug. No special rare hardware is required beyond use of the md RAID stack, but the vulnerable feature depends on llbitmap and RAID1 behind-write behavior.
The timing aspect is asynchronous completion ordering, but the bug is primarily an ordering/synchronization error in teardown. I did not apply a hard-race penalty because the commit describes a missing wait before destroy, not merely a narrow accidental race window.
4. Primitive and severity interpretation
The exact primitive is a post-lifetime access to llbitmap storage by RAID1 behind-write completions after md_bitmap_destroy() has already torn the storage down. This is a real Class B lifetime/UAF primitive in kernel storage code.
This is not a simple ordinary stale cleanup dereference only. The stale users are asynchronous behind-write completion paths that should have been waited for by wait_behind_writes() before bitmap destruction. However, it is also not Class A because the patch does not establish controlled reclaim, controlled replacement, arbitrary write, callback control, credential manipulation, or demonstrated LPE.
Demonstrated consequence is local denial of service or storage-state corruption through post-free completion access. Speculative consequence is stronger memory corruption if freed llbitmap storage is reused in a useful way, but that is not proven from the supplied patch. ImportantCandidate is YES because a real async post-free storage-path primitive remains unresolved beyond a plain NULL dereference. AutoDowngradeAllowed is NO because automatically closing this as ordinary crash-only would discard a concrete kernel lifetime violation in an important storage path.
Likely operational severity is actionable Moderate, with Strong Important review warranted if additional evidence shows below-host-admin reachability, controlled reuse, or meaningful post-free writes into attacker-influenced objects.
5. One-sentence report phrase
md __md_stop() could destroy llbitmap storage before mddev_detach() waited for RAID1 behind-write completions, allowing asynchronous completions to access freed bitmap storage and potentially causing a local kernel crash or unresolved post-free corruption.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a real async use-after-free candidate in md RAID bitmap teardown, and the post-free completion behavior may involve more than an immediate crash even though controlled exploitation is not demonstrated.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: md: wait for behind writes before destroying bitmap [ Upstream
Commit: 73881ff7a75913f919a1ce9d9571bfaab8e8588d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=73881ff7a75913f919a1ce9d9571bfaab8e8588d
Changed files:
drivers/md/md.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=73881ff7a75913f919a1ce9d9571bfaab8e8588d
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90278 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90278
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:H/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: 3
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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