[CVE-2026-89647][LOW] ceph: do not repeat ceph_trim_dentries() if no progress possible

[CVE-2026-89647][LOW] ceph: do not repeat ceph_trim_dentries() if no progress possible

[AL-KERNEL]

CVE: CVE-2026-89647
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: ceph: do not repeat ceph_trim_dentries() if no progress possible
Commit: 5a541eb401acb89d791da41189d7a79220164b93
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5a541eb401acb89d791da41189d7a79220164b93
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89647
Analysis date: Sat, 12 Sep 2026 04:31:19 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89647 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89647

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89647	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89647 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-400;*CWE-835;CWE-770;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'CephFS cap reclaim work could repeatedly requeue itself when ceph_trim_dentries returned EAGAIN even though no dentries were freed and no cap pressure existed. The dir lease walk made this worse because valid leases could be touched and moved to the tail of the list, refreshing timestamps and draining the scan budget without reclaim progress. For the CVSS the PR:L value is used because exploitation requires a local user or local process able to exercise an already mounted CephFS client, not just remote packet reachability. The issue is not directly network reachable as an unauthenticated remote attack, although it only affects systems using CephFS and depends on Ceph client workload and lease state. Impact is denial of service through CPU consumption and lock contention, with no supported evidence of memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	DISK UAF HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Local below-host-admin reachability. A local user or local process can exercise an already mounted CephFS client through normal filesystem workload, assuming the CephFS mount already exists.
* +1 Resource exhaustion / performance DoS. The patch and commit establish a repeated ceph_trim_dentries() requeue loop with high CPU usage and heavy mdsc->dentry_list_lock contention.

Non-awarded signals:

* Real UAF not awarded. No freed object, stale pointer, double release, or post-lifetime use is established.
* Concrete repeated release/refcount not awarded. There is no first release that may end object lifetime followed by a second lifetime mutation on the same dangling reference.
* Concrete persistent resource/reference leak not awarded. The issue is repeated futile work, not a missing put/release that permanently loses a kernel reference.
* Reliable kernel crash not awarded. The demonstrated effect is CPU and lock contention, not panic or fatal kernel fault.
* Confidentiality and integrity not awarded. The patch does not establish disclosure, unauthorized modification, memory corruption, or privilege escalation.
* Remote reachability not awarded. The issue is in the local CephFS client reclaim path and is not directly triggered by unauthenticated network packets.
* Important-grade leverage not awarded. No controlled reclaim, arbitrary write, security-boundary bypass, COW violation, or ownership violation is shown.

Negative signals:

* No negative privilege penalty applied. Host-admin setup may be needed to mount CephFS initially, but the trigger can occur through ordinary use of an already mounted filesystem.
* No race/timing penalty applied. This is a deterministic busy-loop/progress logic bug rather than a narrow race.
* No niche-subsystem penalty applied. CephFS is optional, but the supplied evidence describes real production impact and the scoring should not subtract merely because the subsystem is specialized.

3. Reachability analysis

The evaluated attacker is a local user or local process on a system where CephFS is already mounted. Such a process can plausibly create or exercise dentry and directory lease activity that reaches the CephFS client reclaim path.

The issue is not directly network reachable as an unauthenticated remote attack. A Ceph server or network peer is not shown to directly send a packet that forces the vulnerable local busy loop.

Host administrator privilege is not required for the runtime trigger once the filesystem is mounted, but prior privileged setup is normally required to mount and configure CephFS. This prior setup is not counted as host-admin requirement for the trigger itself.

No namespace-scoped capability issue is involved. There is no evidence that CAP_NET_ADMIN, user namespaces, or container-local capabilities are relevant to the complete trigger.

No special hardware, firmware, boot mode, or rare CPU feature is required. No narrow timing race is required.

4. Primitive and severity interpretation

The exact primitive is a progress-accounting and retry logic bug in ceph_trim_dentries() and ceph_cap_reclaim_work(). When no dentries are freed and no cap pressure exists, the code can still return -EAGAIN and immediately requeue reclaim work. The dir lease walk worsens this by touching valid leases, moving dentries, refreshing timestamps, and draining the scan budget without reclaim progress.

This is not an active structured UAF and not fixed stale storage. It is a DoS-only control-flow/resource-consumption issue.

There is no concrete repeated release/refcount operation. There is no evidence that an object lifetime ends and that the same stale reference later receives another put/free/refcount operation.

PrimitiveClass=CLASS_C because the demonstrated behavior is denial of service through CPU consumption and lock contention without an independently useful corruption primitive.

The demonstrated consequence is local performance degradation, lock contention, and latency impact for CephFS operations such as ceph_readdir(). A more severe operational DoS is possible under sustained production workload, but this remains resource exhaustion rather than kernel memory corruption.

ImportantCandidate=NO because no Important-preserving primitive is established. AutoDowngradeAllowed=YES because the issue is a pure resource/CPU busy-loop bug with no unresolved corruption, authorization, ownership, or privilege-escalation primitive.

Likely operational severity is ordinary Moderate or Low-like depending on deployment sensitivity and CephFS workload intensity.

5. One-sentence report phrase

CephFS cap reclaim can repeatedly requeue ceph_trim_dentries() without reclaim progress, causing CPU consumption and dentry_list_lock contention that may degrade CephFS client performance but does not show memory corruption or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the patch establishes a DoS-only resource consumption issue with no UAF, no repeated lifetime mutation, no memory corruption, no security-boundary bypass, and no remote unauthenticated trigger.

7. Call-site confidence

Call-site confidence: high

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

Patch: ceph: do not repeat ceph_trim_dentries() if no progress possible
Commit: 5a541eb401acb89d791da41189d7a79220164b93
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5a541eb401acb89d791da41189d7a79220164b93

Commit description:

ceph_cap_reclaim_work() re-queues itself for as long as
ceph_trim_dentries() returns -EAGAIN, which happens whenever a lease
walk exhausts its `nr_to_scan` budget.  This creates a busy loop that
consumes CPU without making any progress when there is nothing to
reclaim: with no cap pressure (`count==0`) and every scanned lease
still valid, each pass runs the full scan budget down to zero and
returns `-EAGAIN`, only to be queued again immediately.

The dir-lease walk made this worse.  When `expire_dir_lease` is
`false` (i.e. we have no intention of reclaiming dir leases),
__dir_lease_check() returned `TOUCH` for every valid lease.  `TOUCH`
moves the dentry to the tail of the list and resets `di->time` via
__dentry_dir_lease_touch(), so a walk over N valid leases pointlessly
rewrote the list, refreshed the timestamps (preventing them from ever
aging out) and always drained `nr_to_scan`, guaranteeing the `-EAGAIN`
requeue.

Fix this in three steps:

 - Return `KEEP` instead of `TOUCH` when `expire_dir_lease` is
   `false`.  If we are not going to reclaim the lease, leave it in
   place instead of churning the list and resetting its timestamp; the
   walk then terminates naturally (or via `STOP` at the first fresh
   lease).

 - Only return `-EAGAIN` from the first (dentry-lease) walk when something
   was actually freed.  A full batch that frees nothing means retrying
   the same list immediately is futile; fall through to the dir-lease
   walk instead.

 - After both walks, bail out with success (0) when nothing was freed
   and there is no cap pressure (`count==0`).  There is no reason to
   keep retrying when we are not over the cap limit and made no
   progress.

Under real cap pressure (`count>0`) the reclaim path is unchanged and
still retries via `-EAGAIN`.

Without this patch, I saw 500 ceph_trim_dentries() calls per second on
our web servers.  This is very visible in `/proc/lock_stat` (5 minute
capture):

              class name    con-bounces    contentions   waittime-min   waittime-max waittime-total   waittime-avg    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total   holdtime-avg

 &mdsc->dentry_list_lock:        126180         128218           0.04        8063.44    15986965.20         124.69        1573354        5296812           0.04        8291.28    74164526.48          14.00
 -----------------------
 &mdsc->dentry_list_lock         111736          [<000000007b11e319>] __ceph_dentry_dir_lease_touch+0x7c/0xa8
 &mdsc->dentry_list_lock           2631          [<0000000050597999>] __dentry_leases_walk+0x64/0x2c8
 &mdsc->dentry_list_lock           3878          [<00000000c0022f62>] __ceph_dentry_lease_touch+0x5c/0xa8
 &mdsc->dentry_list_lock           9973          [<000000002f27cb6f>] __dentry_lease_unlist+0x50/0xa0
 -----------------------
 &mdsc->dentry_list_lock         123621          [<0000000050597999>] __dentry_leases_walk+0x64/0x2c8
 &mdsc->dentry_list_lock           1822          [<000000007b11e319>] __ceph_dentry_dir_lease_touch+0x7c/0xa8
 &mdsc->dentry_list_lock           2720          [<000000002f27cb6f>] __dentry_lease_unlist+0x50/0xa0
 &mdsc->dentry_list_lock             55          [<00000000c0022f62>] __ceph_dentry_lease_touch+0x5c/0xa8

With this patch:

              class name    con-bounces    contentions   waittime-min   waittime-max waittime-total   waittime-avg    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total   holdtime-avg

 &mdsc->dentry_list_lock:          1203           1215           0.16         408.88       33082.88          27.23        4320501        7357389           0.04         500.64     1961578.00           0.27
 -----------------------
 &mdsc->dentry_list_lock           1029          [<000000003c9aea8a>] __ceph_dentry_dir_lease_touch+0x7c/0xa8
 &mdsc->dentry_list_lock            169          [<000000002038c577>] __dentry_lease_unlist+0x50/0xa0
 &mdsc->dentry_list_lock             16          [<00000000c991106d>] __ceph_dentry_lease_touch+0x5c/0xa8
 &mdsc->dentry_list_lock              1          [<00000000612fe15f>] __dentry_leases_walk+0x64/0x2c8
 -----------------------
 &mdsc->dentry_list_lock            158          [<000000002038c577>] __dentry_lease_unlist+0x50/0xa0
 &mdsc->dentry_list_lock            858          [<000000003c9aea8a>] __ceph_dentry_dir_lease_touch+0x7c/0xa8
 &mdsc->dentry_list_lock            182          [<00000000612fe15f>] __dentry_leases_walk+0x64/0x2c8
 &mdsc->dentry_list_lock             17          [<00000000c991106d>] __ceph_dentry_lease_touch+0x5c/0xa8

__dentry_leases_walk() is almost gone.  The total wait time is reduced
by a factor of 483.  That will give some latency gains to
ceph_readdir().

Cc: stable@vger.kernel.org
Fixes: 37c4efc ("ceph: periodically trim stale dentries")
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Alex Markuze <amarkuze@redhat.com>

[Commit description truncated; see the upstream URL below]

Changed files:
  fs/ceph/dir.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=5a541eb401acb89d791da41189d7a79220164b93

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

The original Linux kernel CVE announcement for CVE-2026-89647 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89647

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:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).