[CVE-2026-98982][IMPORTANT] namespace: don't let a pseudo dentry become the root of a mount

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

CVE: CVE-2026-98982
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: namespace: don't let a pseudo dentry become the root of a mount
Commit: a256e31e0c54c4c37d2523aa52faabdcdd2ebc07
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a256e31e0c54c4c37d2523aa52faabdcdd2ebc07
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98982
Analysis date: Mon, 26 Oct 2026 16:17:53 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A reduced-privilege local actor can race mount teardown against lockless traversal, causing the kernel to consume a freed pseudo-dentry as an active mount root.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98982 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98982

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-98982	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98982 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=5 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: 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:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in mount namespace cloning can make a DCACHE_NORCU pseudo dentry the root of a mount even though this dentry is freed immediately at the final dput without an RCU grace period. A lockless walker in __follow_mount_rcu can find the mount while it is still hashed and then read d_seq, d_flags, or later dentry state after another thread unhooks the mount and frees its root. This provides a concrete use after free dereference with kernel crash potential and possible security impact if the freed slab object is reused during path traversal. For the CVSS the PR:L value reflects that reliable triggering requires a local process with access to a BPF token and usable mount namespace operations rather than network access. AC:H reflects the required race between mount teardown and lockless path walking. Impact is at least a local denial of service and the reuse of a freed kernel object makes limited or greater confidentiality and integrity impact plausible but not proven. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 6 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to IMPORTANT because the ActionableScore threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS and INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 and TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 5 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	MAYBE	DISK BPF UAF IMPROVEONLY LINUS INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 5. Active-structured RCU UAF calibration raises 3 → 5.
* Raw paranoid score: 4.
* Paranoid score: 6. The same floor plus one point for unknown nontrivial post-lifetime dentry consumption.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 — Real UAF: a mount remains discoverable by `__follow_mount_rcu()` while its `DCACHE_NORCU` root dentry can be immediately freed.
* +1 — Concrete RCU referenced-object lifetime mismatch: the visible mount references a dentry whose lifetime is not extended through the lockless reader interval.
* +1 — Reliable kernel crash/DoS is plausible from reads of freed `d_seq`, `d_flags`, or later dentry state.
* +1 — Attacker-influenced lifetime sequence: the actor can arrange cloning, mount teardown/final puts, and concurrent path walking.
* -1 — Reduced/delegated privilege: supplied evidence requires access to a BPF token and usable mount-namespace operations, but not effective host administration.
* -1 — Hard race: the supplied evidence expressly identifies a high-complexity teardown-versus-walker race.
* Paranoid +1 — the stale active dentry may undergo additional path-walk consumption after allocator reuse; this is unknown leverage, not controlled reclaim.

No arbitrary write, controlled reclaim, attacker-shaped replacement object, disclosure, LPE, or separate C/I impact is shown. The crash receives no duplicate Availability point.

3. Reachability analysis

The grounded actor is a local, reduced-privilege process with a BPF token and mount-cloning capability. Effective host-admin power is not required according to the supplied PR:L evidence, but numeric UID 0 and the exact capability namespace are unresolved. The complete chain is therefore not proven attacker-obtainable solely through a newly created user/mount namespace. No special hardware is required. Triggering requires a materially difficult but attacker-driven race.

4. Primitive and severity reading

This is an active structured-object UAF, not fixed stale storage or a cleanup-only dereference. A `DCACHE_NORCU` dentry is freed immediately while a lockless mount walker may still read and consume it. There is no repeated release on the same stale reference and no demonstrated controlled replacement, so the primitive remains CLASS_B rather than CLASS_A.

The write/corruption leverage after slab reuse is unresolved. Nevertheless, the attacker-influenced lifetime transition, privileged kernel path-walk consumer, and delegated mount/BPF-token boundary form a specific unknown cross-boundary active-state chain. That supports `ImportantCandidate=YES` and forbids automatic downgrade without claiming proven LPE or arbitrary kernel access. B4 authorization-state preservation does not apply.

5. One-sentence report phrase

A reduced-privilege local actor can race mount teardown against lockless traversal, causing the kernel to consume a freed pseudo-dentry as an active mount root.

6. Manual review advice

MANUAL CHECK REQUIRED

Review slab-reuse behavior, subsequent dentry consumers, and the exact user/mount-namespace capability chain.

7. Call-site confidence

Call-site confidence: high

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

Patch: namespace: don't let a pseudo dentry become the root of a mount
Commit: a256e31e0c54c4c37d2523aa52faabdcdd2ebc07
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a256e31e0c54c4c37d2523aa52faabdcdd2ebc07

Commit description:

d_alloc_pseudo() hands out dentries for pipes, sockets and other files
that are never anyone's child or parent. They carry DCACHE_NORCU and
dentry_free() frees them right away because no lockless path walk can
ever reach them. That holds as long as such a dentry isn't the root of
a mount. But bind mounting /proc/self/fd/<fd> of such a file does
exactly that.

Most callers of alloc_file_pseudo() put their files on kernel-internal
mounts and may_copy_tree() refuses those. bpf_token_create() doesn't. It
places the token file on the bpffs mount the caller handed it and that
mount is in the caller's mount namespace so the clone goes through:

  mount --bind /proc/self/fd/<token> <file>
  open_tree(tokfd, "", AT_EMPTY_PATH | OPEN_TREE_CLONE) + move_mount()

__follow_mount_rcu() then loads ->mnt_root of that mount, reads d_seq
and d_flags of the dentry and only then checks mount_lock. The dentry
can be gone by then. The final mntput() of an unmounted parent unhooks
a child that stayed attached to it under mount_lock alone. When the
child's holder does the final put right after that the root is dput()
and freed immediately while a walker that found the mount hashed still
looks at it.

Refuse to clone a mount with a DCACHE_NORCU dentry as its root. Nothing
sensible can be done with a bind mount of a bpf token anyway.

Fixes: 35f96de ("bpf: Introduce BPF token object")
Cc: stable@vger.kernel.org # v6.9+
Link: https://patch.msgid.link/20260930-work-mount-fixes-3-v1-17-be34c83956ae@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/namespace.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=a256e31e0c54c4c37d2523aa52faabdcdd2ebc07

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

The original Linux kernel CVE announcement for CVE-2026-98982 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98982

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: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: 5
  Paranoid ActionableScore: 6

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: NO
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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).