[CVE-2026-98996][LOW] fs: don't silently unmount busy mounts

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-98996
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fs: don't silently unmount busy mounts
Commit: bf2c8dec26fb792876b5e9bde7030c45d8be3908
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf2c8dec26fb792876b5e9bde7030c45d8be3908
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98996
Analysis date: Tue, 27 Oct 2026 02:39:45 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A namespace-privileged container can construct a propagated mount topology that causes a later host unmount to detach a still-referenced tree, producing cross-boundary mount-state and availability impact without UAF or memory corruption.

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

The original Linux kernel CVE announcement for CVE-2026-98996 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98996

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.

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

CVE-2026-98996	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98996 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=2 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:R/S:U/C:N/I:L/A:L';*CWE-664;CWE-754;CWE-670;Other CVSS 'AV:L/AC:H/PR:L/UI:R/S:U/C:N/I:N/A:L';BEST CVSS score: '3.3';DESCR 'propagate_mount_busy did not inspect every propagated mount copy that propagate_umount could detach. A container able to construct its own mount topology could keep an open file descriptor to a cloned tree and cause a later host synchronous unmount to succeed instead of returning EBUSY. For the CVSS the PR:L value reflects the need for mount administration rights inside an existing user or mount namespace rather than full host privileges. Exploitation also depends on a suitable shared or slave propagation relationship and a later host unmount operation. The demonstrated primitive is an incorrect mount detach and availability impact. The retained reference keeps the mount object alive and there is no supported UAF or other memory corruption primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 3.3 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 1.9 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	DISK IMPROVEONLY LINUS 	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 2.
* Conservative score: 2; no floor or cap applied.
* Raw paranoid score: 2.
* Paranoid score: 2; no technically supported stronger primitive.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Availability impact: synchronous host unmount can incorrectly detach a container mount tree that remains in use.
* +1 Cross-boundary interference: container-controlled mount topology affects the result and propagation consequences of a later host unmount.
* +1 Practical chain: the commit provides a concrete topology, retained tree FD, and subsequent host `umount` sequence.
* -1 Reduced/delegated privilege: constructing the topology requires mount-management rights within an existing namespace, below host-administrator authority.
* No UAF: the retained reference keeps the detached mount alive.
* No heap/OOB write, shared-backing write, authorization bypass, disclosure, arbitrary write, or privilege escalation is shown.
* No extra integrity point: the incorrect detach is already represented by the cross-boundary signal and no protected host data modification is established.

3. Reachability analysis

A container actor with mount-administration rights can construct the slave/shared topology and retain the cloned-tree FD. A separate host actor must later perform the synchronous unmount. Thus the attacker is below host admin, but the complete sequence is interaction-dependent. The attacker-controlled stages are namespace-local; because the final operation occurs in the host mount namespace, the complete chain is not proven namespace-scoped. No special hardware, boot option, race, or narrow timing is required.

4. Primitive and severity reading

The primitive is a faulty busy/reference decision: `propagate_mount_busy()` omits propagated copies that `propagate_umount()` will detach. This permits active mount topology to be detached despite a live reference. It is semantic invalid-state/availability behavior, CLASS_C, not a lifetime violation: the open FD preserves the mount object, so no stale post-free use occurs.

The shown effect is incorrect cross-namespace mount detachment. Kernel memory corruption, host panic, protected-resource access, and persistent host integrity compromise are unsupported. B5-R, B3-PENDING, and B4-PENDING do not apply. ImportantCandidate is therefore NO and automatic Moderate handling is allowed.

5. One-sentence report phrase

A namespace-privileged container can construct a propagated mount topology that causes a later host unmount to detach a still-referenced tree, producing cross-boundary mount-state and availability impact without UAF or memory corruption.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch and reproducer establish a constrained semantic detach bug, while the retained reference positively excludes the suspected UAF mechanism.

7. Call-site confidence

Call-site confidence: high

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

Patch: fs: don't silently unmount busy mounts
Commit: bf2c8dec26fb792876b5e9bde7030c45d8be3908
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf2c8dec26fb792876b5e9bde7030c45d8be3908

Commit description:

The propagate_mount_busy() helper exists to decide whether a synchronous
can suceed. It does that by looking at the copies of the victims in the
mounts its parent propagates to. One of those functions from propagation
hell.

It skips a copy that has child mounts except for the case of a single
child mount covering the copy's root. That algorithm used to be exact.
It isn't anymore.

Originally only childless copies were unmounted during propagation.
And 1064f87 ("mnt: Tuck mounts under others instead of creating
shadow/side mounts.") ended up adding covered mounts to both sides in
one go.

Along came 99b19d1 ("mnt: In propgate_umount handle visiting mounts
in any order") and that started to unmount more than before:

[1]: A copy of a mount is unmounted when each of its children is either
     its overmount or a copy of the victim itself. In the former case
     the overmount gets reparented. In the latter case the copy of the
     victim will end up being unmounted.

So propagate_umount() ended up being rewritten without adjusting
propagate_mount_busy(). And currently propagate_mount_busy() doesn't
look at copies of the victim that have two or more children nor a single
child that is not the overmount.

So any such copy in [1] is not checked for
references. The result is that a synchronous unmount succeeds, pulling
out a mount that is still in use.

A container can run into this with its own mounts. The host shares a
tree with the container and the host has a mount in that tree:

  mount -t tmpfs none /mnt
  mount --make-shared /mnt
  mount -t tmpfs none /mnt/a

The container's /mnt is a slave copy of the host mount. The container
now takes a detached copy of the tree and mounts that copy beneath its
propagated copy of /mnt/a and keeps the file descriptor to the tree
open:

  unshare -m --propagation unchanged
  mount --make-rslave /
  fd = open_tree(AT_FDCWD, "/mnt", OPEN_TREE_CLONE | AT_RECURSIVE)
  move_mount(fd, "", AT_FDCWD, "/mnt/a", MOVE_MOUNT_BENEATH)

  grep /mnt /proc/self/mountinfo
  775 355 ... /mnt ...     master:646
  777 775 ... /mnt/a ...   master:646
  776 777 ... /mnt/a ...   master:647
  778 777 ... /mnt/a/a ... master:647

The now attached tree (777) is now mounted beneath the container's copy
of /mnt/a. That's also where the host mount sits (as seen from the host
mount namespace). So the propagated copy (776) of the host's mount is on
top of its root and the copy of that (778) is inside it. Now the host
runs:

  umount /mnt/a

and it succeeds. The moved tree tree and the copy inside of it are gone
from the container. Now only the propagated copy is left and it got
reparented to where it was before:

  grep /mnt /proc/self/mountinfo
  775 355 ... /mnt ... master:646
  776 775 ... /mnt/a ...

But the file descriptor still refers to the moved tree. So a synchronous
umount detached a mount that is still in use.

Teach propagate_mount_busy() the same checks that propagate_umount()
does. It now checks for a single victim mount without children
for which it can assume that MNT_LOCKED is cleared on every copy like
propagate_umount() does. The copies form chains. IOW, when a copy itself
receives propagation from the victim's parent then the mount at the same
mountpoint inside that copy is a copy as well. Whether a mount receives
propagation from the victim's can simply be checked by walking its

[Commit description truncated; see the upstream URL below]

Changed files:
  fs/pnode.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=bf2c8dec26fb792876b5e9bde7030c45d8be3908

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

The original Linux kernel CVE announcement for CVE-2026-98996 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98996

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:R/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:R/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS score: 3.3

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).