* [CVE-2026-89653][IMPORTANT] ceph: reject export_targets ranks >= CEPH_MAX_MDS in mdsmap decode
@ 2026-09-12 3:23 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 3:23 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89653
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ceph: reject export_targets ranks >= CEPH_MAX_MDS in mdsmap decode
Commit: 736adee11af36e407ed902264f8b2fb5cf94b62f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=736adee11af36e407ed902264f8b2fb5cf94b62f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89653
Analysis date: Fri, 11 Sep 2026 23:23:28 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89653 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89653
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89653 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-89653 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;*CWE-20;CWE-120;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '9.8';DESCR 'A memory corruption issue exists in CephFS MDSMap handling because export_targets ranks are controlled by the Ceph monitor and later used as bit numbers in a fixed stack bitmap. A crafted MDSMap with a rank at or above CEPH_MAX_MDS can make set_bit() write past the end of the bitmap during check_new_map, which can corrupt kernel stack memory. For the CVSS the PR:N is used because no privileges are required on the victim host once the attacker can act as or tamper with the Ceph monitor endpoint. The issue is network reachable in Ceph deployments where the client accepts monitor supplied maps over TCP. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact due to an out of bounds kernel stack write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 8) YES DISK SIMPLEFIX LINUS KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=8
ActionableScoreLower=7
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 7
* Conservative score: 7. No floor or cap applied.
* Raw paranoid score: 8
* Paranoid score: 8. No floor or cap applied.
* Final recommended bucket: **Strong Important candidate**::
2. Signal breakdown
Conservative signals:
* +2 Remote reachable / network-triggerable: the malformed MDSMap data is supplied by a Ceph monitor endpoint to a CephFS client. A malicious or compromised monitor, or an attacker able to tamper with monitor traffic, can supply the bad export_targets rank.
* +2 Strong corruption primitive: the patch description directly establishes an attacker-influenced out-of-bounds write, because a monitor-controlled rank is later used as a bit number in a fixed stack bitmap and set_bit() can write past the end of that bitmap.
* +1 Reliable kernel crash / strong DoS: an out-of-bounds write on a kernel stack bitmap can realistically corrupt stack state and crash the kernel.
* +1 Important filesystem/storage path: this is in CephFS MDSMap handling, which is part of filesystem client state and reconnection behavior.
* +1 Weak exploitation concern: the corrupted location is influenced through the rank value used as a bit index, so this is more meaningful than a passive invalid read or NULL dereference, although arbitrary write or controlled full overwrite is not demonstrated.
Paranoid additional signal:
* +1 Integrity plausible: a kernel stack out-of-bounds bit write may corrupt adjacent active stack variables or control-relevant state. This is still not treated as demonstrated arbitrary write or proven privilege escalation.
Non-awarded signals:
* RealUAF not awarded: no stale object or post-lifetime access is established.
* Controlled reclaim not demonstrated.
* Arbitrary full kernel read/write not demonstrated.
* Confidentiality impact not awarded: no disclosure or read-back path is shown.
* Practical exploit chain not awarded: no supplied evidence shows LPE, credential overwrite, callback hijack, or controlled code execution.
* Extra availability not awarded: the crash/DoS effect is counted once.
* Race/timing penalty not applied: this is an input validation/OOB write issue, not a timing race.
* Host-admin penalty not applied: the trigger does not require host-admin privilege on the victim once the CephFS client accepts monitor-supplied maps.
3. Reachability analysis
The direct attacker model is a malicious or compromised Ceph monitor, or an attacker able to tamper with traffic from the monitor endpoint to the CephFS client. The vulnerable input is not ordinary arbitrary Internet packet traffic, but it is network-delivered control data in a Ceph deployment.
Host administrator privileges on the victim are not required for the malformed map to be processed after the client is configured and connected. A prior CephFS mount or deployment setup is required, and that setup is typically administrative, but the vulnerability trigger itself comes from monitor-controlled MDSMap contents.
Namespace-scoped privilege is not relevant here. No user namespace or network namespace capability chain is established or required by the supplied evidence.
No special hardware is required. No race condition is required. The issue depends on a CephFS client receiving and decoding a crafted MDSMap containing export_targets ranks at or above CEPH_MAX_MDS.
4. Primitive and severity interpretation
The exact primitive is an attacker-influenced out-of-bounds bit write on a fixed kernel stack bitmap. The source value is an export_targets rank decoded from the MDSMap. The downstream use in check_new_map() treats that rank as a bit index for set_bit(), so a rank outside the CEPH_MAX_MDS protocol namespace can write beyond the bitmap bounds.
This is not a UAF and not a cleanup-only crash. It is a concrete memory corruption primitive. The write is constrained to setting a bit, but the bit index is attacker-influenced and the target is kernel stack memory adjacent to the fixed bitmap.
PrimitiveClass is CLASS_A because an attacker-influenced kernel stack OOB write is directly established. The demonstrated consequence is at least kernel crash or DoS. The speculative but technically defensible consequence is corruption of adjacent stack state with possible integrity impact. Stronger outcomes such as privilege escalation or arbitrary code execution are not proven from the supplied evidence.
ImportantCandidate is YES because the primitive is concrete kernel memory corruption with network-delivered attacker-controlled input in a filesystem client path. AutoDowngradeAllowed is NO because automatically treating this as ordinary Moderate would discard a real OOB stack write primitive requiring security review.
Likely operational severity is Important-class, with possible Critical concern depending on deployment exposure and whether monitor traffic can be attacker-controlled.
5. One-sentence report phrase
CephFS MDSMap decoding accepted monitor-controlled export_targets ranks outside CEPH_MAX_MDS, allowing a crafted map to make check_new_map() use the rank as a bit index in a fixed stack bitmap and trigger an out-of-bounds kernel stack write that can cause DoS and requires manual review for stronger memory-corruption impact.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: the patch directly establishes a monitor-controlled out-of-bounds kernel stack write. This is a strong corruption primitive and should not be auto-downgraded even though arbitrary write, disclosure, or privilege escalation is not demonstrated.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: ceph: reject export_targets ranks >= CEPH_MAX_MDS in mdsmap decode
Commit: 736adee11af36e407ed902264f8b2fb5cf94b62f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=736adee11af36e407ed902264f8b2fb5cf94b62f
Commit description:
MDSMap export_targets entries are monitor controlled. check_new_map()
uses each entry as a bit number in a fixed stack bitmap, so a rank
outside the protocol namespace can make set_bit() write past the end of
the array.
Reject ranks outside CEPH_MAX_MDS while decoding the map. Do not
validate against possible_max_rank here because maps may legitimately
reference ranks beyond a temporarily reduced max_mds.
Cc: [email protected]
Fixes: d517b39 ("ceph: reconnect to the export targets on new mdsmaps")
Signed-off-by: Jérémy Jean <[email protected]>
Reviewed-by: Alex Markuze <[email protected]>
Signed-off-by: Alex Markuze <[email protected]>
Signed-off-by: Ilya Dryomov <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
fs/ceph/mdsmap.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=736adee11af36e407ed902264f8b2fb5cf94b62f
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89653 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89653
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:L/PR:N/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 9.8
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: 7
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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