[CVE-2026-98980][MODERATE 7.0] namei: don't reveal overmounted entries in refwalk

[CVE-2026-98980][MODERATE 7.0] namei: don't reveal overmounted entries in refwalk

[AL-KERNEL]

CVE: CVE-2026-98980
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: namei: don't reveal overmounted entries in refwalk
Commit: e2b10181d913b4efa3a8dc67a532f859aa5f95e0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e2b10181d913b4efa3a8dc67a532f859aa5f95e0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98980
Analysis date: Mon, 26 Oct 2026 23:49:29 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A refwalk/unlink race can bypass an overmount’s pathname view and open the covered underlying file, while the required privilege boundary and cross-user impact remain unresolved.

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

The original Linux kernel CVE announcement for CVE-2026-98980 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98980

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-2026-98980	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98980 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both 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:N';CWE-362;*CWE-200;CWE-706;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N';BEST CVSS score: '6.3';DESCR 'A race in refwalk can let path lookup retain a dentry while unlink or rename marks it with DCACHE_CANT_MOUNT, detaches its overmounted mounts, and removes the name. If lookup has not crossed the mount before detachment, it can accept the underlying covered entry and bypass the view that the mount was intended to provide. The supplied reproducer reads SECRET_F instead of the mounted MARK_A, establishing a concrete confidentiality boundary bypass without evidence of UAF, OOB access, or kernel memory corruption. Exploitation is local and AC:H because it requires precise overlap between path walking and unlink or rename in a suitable mount topology. For the CVSS the PR:L value reflects that a local user context is required and that the necessary mount operations may be available through an unprivileged user namespace. Base integrity impact is limited because writes remain subject to inode permissions, while the paranoid case uses I:H because a write mode lookup could reach and modify the same underlying entry. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 6.3 and the production ActionableScore returned 5 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_KPANIC to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC 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 has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP. The independent weaker Aegis shadow evaluated CVSS unavailable and ActionableScore 0 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	DANGER DISK INIT RACE IMPROVEONLY LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

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

1. ActionableScore

* Raw base score: 5.
* Conservative score: 5; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; speculative write access or cross-user disclosure was not scored.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 strong attacker-influenced semantic reinterpret: refwalk obtains the underlying dentry while it is overmounted, then accepts that dentry after unlink detaches the mounts, resolving the same pathname to a materially different object.
* +1 confidentiality effect: the supplied reproducer concretely opens and reads `SECRET_F` where the mounted view should expose `MARK_A`.
* +1 practical chain: concurrent unlink plus refwalk leads directly to opening and reading the covered object.
* +1 important filesystem path: the defect affects VFS pathname resolution and mount-view integrity.
* No race penalty: the window is narrow, but repeated attempts are cheap and the unmodified reproducer recorded successful hits.
* No authorization-bypass points: the fix restores dentry/mount validation and returns `-ESTALE`; it does not restore an omitted credential, capability, permission, or LSM guard.
* No integrity point: write-mode access remains subject to the underlying inode permissions.
* No UAF, OOB write, arbitrary write, memory corruption, LPE, or protected cross-user victim is shown.

3. Reachability analysis

The reproducer controls the mount topology, unlink operations, and concurrent pathname walks. It uses `unshare(CLONE_NEWNS)`, tmpfs mounting, `open_tree()`, and `move_mount()`. The evidence does not establish whether the complete setup works for an ordinary user through an attacker-created user namespace, or instead requires effective host-administrator mount privileges. Thus both host-admin and namespace-scoped reachability remain unknown. No special hardware is required.

4. Primitive and severity reading

The primitive is an active pathname/mount-view consistency breach, not stale freed storage: a dentry found before mount detachment is consumed afterward as the underlying object without the validation used by RCU walk. The shown consequence is access to a mount-covered file in the reproducer. Cross-user confidentiality, permission bypass, persistent modification, and privilege escalation remain unproven.

This is CLASS_B because it is a concrete active-state semantic confusion with a later consumer. ImportantCandidate is YES because attacker-driven state transition, subsequent file access, and a credible mount-namespace trust boundary form a specific chain whose real authority boundary remains unresolved. Auto-downgrade is unsafe pending that review.

5. One-sentence report phrase

A refwalk/unlink race can bypass an overmount’s pathname view and open the covered underlying file, while the required privilege boundary and cross-user impact remain unresolved.

6. Manual review advice

MANUAL CHECK REQUIRED

Determine whether an unprivileged user namespace can perform the complete mount/unlink trigger and whether covered files belonging to another authority can be reached.

7. Call-site confidence

Call-site confidence: high

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

Patch: namei: don't reveal overmounted entries in refwalk
Commit: e2b10181d913b4efa3a8dc67a532f859aa5f95e0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e2b10181d913b4efa3a8dc67a532f859aa5f95e0

Commit description:

In rcuwalk the dentry is validated before it is accepted. step_into()
step_into() rechecks d_seq and __follow_mount_rcu() rechecks mount_lock.
An entry that gets unlinked in between causes the lookup to retry and
miss.

A refwalk doesn't do this. lookup_fast() takes a reference on the hashed
dentry and simply accepts it. So an unlink that happens after the
reference was taken isn't seen by refwalk. That's fine for a simple
file. We just happened to open it before it was unlinked, no problem.

For a mountpoint and specifically a locked mountpoint it very much
isn't. The unlink detaches all mounts and then removes the name. Any
refwalk that hasn't traversed the mounts yet simply reveals the
underlying entry. It's a very narrow window but it can be hit:

  reads of a covered file in 60 s, 3 walkers, ~15000 unlinks
    no widening            27
    that step + 200 us   3741

See the appended patch for a more reliable reproducer.

So check the entry after step_into(). unlink(), rmdir() and rename()
mark the dentry with dont_mount() before they detach the mounts and
remove the dentry.

So a dentry that is marked with DCACHE_CANT_MOUNT and is unhashed by the
time its mounts were looked at is a name that was unlinked under the
refwalk. DCACHE_CANT_MOUNT is read after the mounts were looked up and
it is set before they are detached by detach_mounts(). A refwalk that
missed the mounts will see DCACHE_CANT_MOUNT.

Plain d_unlinked() is fine. A rename takes the dentry off its hash chain
but ___d_drop() leaves d_hash.pprev set. So only __d_drop() and a rename
over the dentry unhash it. The only move that flips IS_ROOT splices in a
disconnected alias. That doesn't have DCACHE_CANT_MOUNT set.

Add the to step_into_slowpath(). ".." and LOOKUP_DOWN may legitimately
land on an unhashed directory. A refwalk that crossed onto a mount has
path.mnt different from nd->path.mnt. A dentry that a filesystem dropped
on its own (d_invalidate(), d_drop()) doesn't have the flag set and is
treated as before.

  // SPDX-License-Identifier: GPL-2.0
  /*
   * unlink_covered: unlink a file that is a mountpoint in a detached copy of
   * its mount, against walkers that open it through that copy.
   *
   * A is a tmpfs with the file f1 (content SECRET_F) and the plain file
   * MARK_A. A' is an open_tree(OPEN_TREE_CLONE) copy of A with MARK_A bound
   * on A'/f1, held through an O_PATH fd on its root once the tree fd is
   * closed. Walkers open f1 through that fd while the driver unlinks A/f1,
   * where nothing is mounted on it. A walker may read MARK_A or get ENOENT.
   * A read of SECRET_F is a hit: the name was found after its mount was
   * gone. The lockless walks use openat2(RESOLVE_CACHED).
   *
   * usage: unlink_covered [-t seconds] [-w walkers]
   */
  #ifndef _GNU_SOURCE
  #define _GNU_SOURCE
  #endif
  #include <errno.h>
  #include <fcntl.h>
  #include <pthread.h>
  #include <stdatomic.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <unistd.h>
  #include <sys/mount.h>
  #include <sys/stat.h>
  #include <sys/syscall.h>
  #include <linux/openat2.h>

  #ifndef __NR_open_tree
  #define __NR_open_tree 428
  #endif
  #ifndef __NR_move_mount
  #define __NR_move_mount 429
  #endif
  #ifndef __NR_openat2

[Commit description truncated; see the upstream URL below]

Changed files:
  fs/namei.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=e2b10181d913b4efa3a8dc67a532f859aa5f95e0

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

The original Linux kernel CVE announcement for CVE-2026-98980 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98980

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:H/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
  The Best / paranoid CVSS score: 6.3

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: 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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).