[CVE-2026-46121][MODERATE 7.0] mm/damon/sysfs-schemes: protect memcg_path kfree() with damon_sysfs_lock

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

CVE: CVE-2026-46121
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mm/damon/sysfs-schemes: protect memcg_path kfree() with damon_sysfs_lock
Commit: b1e9f2d5870776347edef927f9bb3ea19b8e3abb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b1e9f2d5870776347edef927f9bb3ea19b8e3abb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46121
Analysis date: Thu, 28 May 2026 14:12:01 -0400
ActionableScore: 7
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
Concurrent DAMON sysfs `memcg_path` reads and writes can race with `kfree()` of the old path buffer, causing a use-after-free that may crash the kernel or expose freed heap data through sysfs.

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

The original Linux kernel CVE announcement for CVE-2026-46121 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46121

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-2026-46121	MODERATE	CHECK	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-667;*CWE-200;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '7';DESCR 'DAMON sysfs memcg_path reads and writes could race because the write path frees the old memcg_path buffer while direct reads were not protected by damon_sysfs_lock. A reader using a separate open sysfs file can therefore pass a freed string pointer to sysfs_emit(), resulting in a use-after-free. For the CVSS the PR:L is used in the paranoid delegated access model because DAMON sysfs access may be granted to a service account or container root even though normal setup is administrator controlled. AC:H is kept because reliable triggering requires a read versus write race using separate file instances. Impact is at least denial of service via kernel crash and may include confidentiality impact from freed heap data being emitted through sysfs. Integrity impact is only considered in the paranoid UAF model because no arbitrary write primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5) 	MAYBE	LOCK RACE UAF LINUS KPANIC MEMORY  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	-	-	checked

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

ActionableScore=7
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: **3**
* Paranoid score: **7**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged or delegated trigger, paranoid: **+1**. Normal DAMON sysfs access is usually admin controlled, but access may be delegated to service accounts, test workloads, or container root in some environments.
* Strong memory corruption primitive: **+2**. The commit explicitly describes a use-after-free of `memcg_path`.
* Real lifetime corruption: **+1**. The write path frees the old buffer while another reader can still pass the freed pointer to `sysfs_emit()`.
* Reliable kernel crash / strong DoS: **+1**. A read versus write race on a freed string pointer can cause kernel fault or crash.
* Confidentiality impact plausible: **+1**. Freed heap data may be emitted through sysfs if the memory is reused or stale contents are read as a string.
* Integrity impact plausible, paranoid: **+1**. UAF is a memory lifetime bug, but no arbitrary write or controlled reclaim primitive is demonstrated.
* Availability impact realistic: **+1**. Crash or fault is a realistic impact.
* Hard race / timing required: **-1**. Reliable triggering requires concurrent read and write through separate sysfs file instances.
* Requires admin in typical deployments, conservative: **-2**. DAMON sysfs is normally not available to arbitrary local users.

## 3. Reachability analysis

The issue is local and reachable through DAMON sysfs `memcg_path` read/write operations. It is not network reachable. In the default security model, creating and controlling DAMON sysfs schemes is typically privileged, which lowers the conservative score. However, if DAMON sysfs access is delegated to a service account, container root, or workload automation, the effective privilege can be closer to PR:L. The race is practical only when reads and writes happen via separate open files, because kernfs locking on the same open file does not cover that case.

## 4. Severity interpretation

This is more serious than an ordinary Moderate if delegated access is plausible, because the patch fixes an explicit UAF. The most realistic impact is local DoS and possible sysfs information disclosure from freed heap memory. Privilege escalation is not demonstrated, and there is no shown arbitrary write primitive, so it should not be treated as proven LPE. Still, UAF plus possible data emission through sysfs is enough for manual review.

## 5. One-sentence report phrase

Concurrent DAMON sysfs `memcg_path` reads and writes can race with `kfree()` of the old path buffer, causing a use-after-free that may crash the kernel or expose freed heap data through sysfs.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Manual review is required because this is an explicit sysfs-reachable UAF race with possible information disclosure, even though typical access is administrator controlled and exploitation requires timing.

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

Patch: mm/damon/sysfs-schemes: protect memcg_path kfree() with damon_sysfs_lock
Commit: b1e9f2d5870776347edef927f9bb3ea19b8e3abb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b1e9f2d5870776347edef927f9bb3ea19b8e3abb

Commit description:

Patch series "mm/damon/sysfs-schemes: fix use-after-free for [memcg_]path".

Reads of 'memcg_path' and 'path' files in DAMON sysfs interface could race
with their writes, results in use-after-free.  Fix those.

This patch (of 2):

damon_sysfs_scheme_filter->mmecg_path can be read and written by users,
via DAMON sysfs memcg_path file.  It can also be indirectly read, for the
parameters {on,off}line committing to DAMON.  The reads for parameters
committing are protected by damon_sysfs_lock to avoid the sysfs files
being destroyed while any of the parameters are being read.  But the
user-driven direct reads and writes are not protected by any lock, while
the write is deallocating the memcg_path-pointing buffer.  As a result,
the readers could read the already freed buffer (user-after-free).  Note
that the user-reads don't race when the same open file is used by the
writer, due to kernfs's open file locking.  Nonetheless, doing the reads
and writes with separate open files would be common.  Fix it by protecting
both the user-direct reads and writes with damon_sysfs_lock.

Link: https://lore.kernel.org/20260423150253.111520-1-sj@kernel.org
Link: https://lore.kernel.org/20260423150253.111520-2-sj@kernel.org
Fixes: 4f489fe ("mm/damon/sysfs-schemes: free old damon_sysfs_scheme_filter->memcg_path on write")
Co-developed-by: Junxi Qian <qjx1298677004@gmail.com>
Signed-off-by: Junxi Qian <qjx1298677004@gmail.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.16.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/damon/sysfs-schemes.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=b1e9f2d5870776347edef927f9bb3ea19b8e3abb

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

The original Linux kernel CVE announcement for CVE-2026-46121 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46121

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:H/UI:N/S:U/C:L/I:N/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: 3
  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: 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).