* [CVE-2026-99170][IMPORTANT] namespace: a mount that copy_tree() leaves out can't take over a lock
@ 2026-10-30 15:07 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-30 15:07 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99170
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: namespace: a mount that copy_tree() leaves out can't take over a lock
Commit: bd71d33923b9d6216bcee44331dd37142b0a8441
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bd71d33923b9d6216bcee44331dd37142b0a8441
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99170
Analysis date: Fri, 30 Oct 2026 11:07:12 -0400
ActionableScore: 7
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES
Summary:
A host-unprivileged user with attacker-created user and mount namespaces can bypass an `MNT_LOCKED` file cover and read the protected underlying file without a race.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99170 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99170
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-2026-99170 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-99170 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO 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:L/PR:L/UI:N/S:U/C:H/I:H/A:N';CWE-284;CWE-862;CWE-693;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N';BEST CVSS score: '7.1';DESCR 'A mount namespace protection bypass allows a local task that is root only in its own user namespace to transfer MNT_LOCKED to a bind mount of a mount namespace file and then remove that mount from a copied tree through unshare(CLONE_NEWNS). This can expose an underlying file that the locked cover was intended to hide. The reproducer reads SECRET instead of MASK after the namespace copy, providing a concrete confidentiality impact without a race. For the CVSS the PR:L value is selected because local code execution and creation of user and mount namespaces are required, but host root or a fully privileged administrative context is not required. Integrity impact depends on the permissions of the newly exposed object and availability impact is not demonstrated. The issue is local rather than network reachable and requires unprivileged user namespaces or equivalent namespace delegation. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7.1 and the production ActionableScore returned 7 with recommendation Strong Important candidate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore 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 a specific evidence based protection or manual review veto was applied and 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_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 and INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 and TEMP_KPANIC_MERGE_IMPORTANT_VETO and B4_CONFIRMED_AUTHORIZATION_PRESERVE and MANUAL_REVIEW_REQUIRED and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 5.3 and ActionableScore 4 with primitive class CLASS_A 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 Strong Important candidate (with actual score 7) MAYBE DISK RACE LEAK DEBUGFS LINUS INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 TEMP_KPANIC_MERGE_IMPORTANT_VETO B4_CONFIRMED_AUTHORIZATION_PRESERVE MANUAL_REVIEW_REQUIRED KPANIC_UNSUPPORTED_REMOVED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=7
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=YES
ProtectedResourceImpact=CONFIRMED
B4PendingReview=NO
B4PendingConfirmed=YES
1. ActionableScore
* Raw base score: 7.
* Conservative score: 7; no floor or cap applied.
* Raw paranoid score: 7.
* Paranoid score: 7; no additional speculative impact counted.
* Final recommended bucket: **Strong Important candidate**::
2. Signal breakdown
* +2 concrete authorization/protection bypass: `move_mount(..., MOVE_MOUNT_BENEATH)` transfers `MNT_LOCKED` to a mount-namespace-file mount which `copy_tree()` then omits, changing the intended denial/protection into an allowed namespace copy.
* +2 practical protected-resource access: the resulting namespace exposes the file beneath the locked cover.
* +1 local unprivileged trigger: the supplied reproducer drops to host UID 1000 and completes the attack using an attacker-created user and mount namespace.
* +1 practical exploit chain: move beneath locked cover → transfer lock → `unshare(CLONE_NEWNS)` omits lock-bearing mount → read underlying file.
* +1 confidentiality impact: the reproducer concretely reads `SECRET` rather than `MASK`.
* No memory corruption, UAF, integrity modification, availability impact, LPE, arbitrary file access, or host-root acquisition is shown. Sensitive-resource theft was not separately stacked with the demonstrated confidentiality effect.
3. Reachability analysis
A host administrator initially provisions the masked file and locked mount, but the exploiting actor is host UID 1000, not host UID 0. The actor creates a user namespace, becomes namespace root, creates the required mount namespace and child namespace-file mount, invokes `move_mount()`, and calls `unshare(CLONE_NEWNS)`. Thus the complete vulnerable operation is shown below host-admin authority and its privileged mount operations are namespace-scoped. Unprivileged-user-namespace availability is a deployment prerequisite; no race or special hardware is required.
4. Primitive and severity reading
This is a mount-namespace authorization and isolation-policy bypass, not memory corruption. The omitted `MNT_LOCKED` state lets active protection disappear during namespace copying. The clean fix rejects both creation of the unsafe beneath relationship and copying that would omit a locked namespace-file mount.
B4-PENDING is confirmed through the concrete path: UID 1000 namespace root → `MOVE_MOUNT_BENEATH` accepted → `copy_tree()` omits the lock-bearing mount → protected underlying file becomes readable. The resource impact is confirmed, though generalized host-file disclosure, integrity impact, and privilege escalation remain unproven. CLASS_A, mandatory review, and no automatic downgrade are warranted.
5. One-sentence report phrase
A host-unprivileged user with attacker-created user and mount namespaces can bypass an `MNT_LOCKED` file cover and read the protected underlying file without a race.
6. Manual review advice
MANUAL CHECK REQUIRED
The patch and reproducer establish a below-host-admin authorization boundary bypass with concrete protected-file disclosure.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: namespace: a mount that copy_tree() leaves out can't take over a lock
Commit: bd71d33923b9d6216bcee44331dd37142b0a8441
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bd71d33923b9d6216bcee44331dd37142b0a8441
Commit description:
Since commit c62a476 ("move_mount: transfer MNT_LOCKED") a mount
can be moved beneath a locked mount. The lock moves from the old cover
to the new mount which takes over the job of hiding what's under it and
the old cover can be unmounted. That works as long as the new mount
stays. But the caller picks the new mount and copy_tree() leaves a
mount of a mount namespace file out of every copy made without
CL_COPY_MNT_NS_FILE, together with everything mounted on it, and never
looks at MNT_LOCKED. unshare(CLONE_NEWNS) and mount propagation make
such copies.
So a task that is root in its own user namespace gets rid of a locked
cover on a file without a race and without touching the filesystem
below it. It makes a child with a mount namespace of its own, binds the
child's /proc/<pid>/ns/mnt beneath the cover with MOVE_MOUNT_BENEATH,
which hands the lock to that bind, and calls unshare(CLONE_NEWNS). The
new mount namespace has nothing on the file. That's the usual way to
mask a single file, a bind of /dev/null or of an empty file. With a
bind of an ordinary file the file stays covered.
copy_tree() already refuses to leave out a locked unbindable mount with
-EPERM since commit df7342b ("mount: Don't allow copying
MNT_UNBINDABLE|MNT_LOCKED mounts"). Do the same for a locked mount of a
mount namespace file. That also covers a tree copied with
CL_COPY_MNT_NS_FILE and attached into a mount namespace of another user
namespace where lock_mnt_tree() locks it. Note that unshare(CLONE_NEWNS)
and clone(CLONE_NEWNS) fail with EPERM from such a mount namespace from
then on. The mount can't be left out and it can't be unmounted.
And refuse to move a mount of a mount namespace file beneath a locked
mount in the first place. Otherwise the task ends up with a mount it
can't unmount and an unshare() that fails for good.
Chris also found a second mount that goes away by itself, a clone of an
automounted mount that shrink_submounts() took along with its parent.
Commit f78ddcc ("namespace: never expire a locked mount") took
care of that one.
Chris's reproducer is appended. Mode nsfile reads SECRET after the
unshare() on 7.3-rc7, with this patch the move_mount() fails with
EINVAL and the file stays covered. nsfile-control uses an ordinary file
and reads MASK both ways.
// SPDX-License-Identifier: GPL-2.0
/*
* Get rid of a locked cover with MOVE_MOUNT_BENEATH.
*
* The "admin" (root) makes a tmpfs W:
* W/D file, holds SECRET, covered by a bind of a file with MASK
* W/Dd/inner file, holds SECRET, Dd is covered by a bind of a directory
* whose "inner" holds MASK
* W/own directory of uid 1000 with the file "mine" and the
* directory "minedir", whose "inner" holds MINE
* W/dbg debugfs, mode 755. dbg/tracing is an automount point
* The "attacker" is uid 1000. It enters a new user namespace with a new mount
* namespace, so W and both covers are locked there.
*
* usage: prog MODE
* plain put a bind of own/mine beneath the cover of D, unmount the cover
* nsfile put a bind of the mount namespace file of a child beneath the
* cover of D, then unshare(CLONE_NEWNS)
* nsfile-control the same with a bind of own/mine
* shrink put a bind of the automounted tracefs beneath the cover of Dd,
* unmount the cover, put a tmpfs beneath W, call umount(W), which fails
* shrink-control the same with a bind of own/minedir
* The environment variable AUTOMOUNT_DIR names another automount point for "shrink".
*/
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <grp.h>
#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/mount.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/wait.h>
[Commit description truncated; see the upstream URL below]
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=bd71d33923b9d6216bcee44331dd37142b0a8441
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99170 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99170
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:H/I:L/A:N
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
The Best / paranoid CVSS score: 7.1
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: 7
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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