[CVE-2026-74618][MODERATE REGULAR] binfmt_misc: don't warn when the mount is completed from another user namespace

[CVE-2026-74618][MODERATE REGULAR] binfmt_misc: don't warn when the mount is completed from another user namespace

[AL-KERNEL]

CVE: CVE-2026-74618
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: binfmt_misc: don't warn when the mount is completed from another user namespace
Commit: 37cf5cf1320a84a17225a1690547b8a0812ca94e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=37cf5cf1320a84a17225a1690547b8a0812ca94e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74618
Analysis date: Sat, 22 Aug 2026 12:18:28 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review recommended / Ordinary Moderate otherwise
Manual review required: YES

Summary:
A local unprivileged user can trigger a repeatable binfmt_misc WARN_ON by completing an fs_context from another user namespace, causing kernel log flooding and kernel panic only on systems configured with panic_on_warn.

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

The original Linux kernel CVE announcement for CVE-2026-74618 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74618

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 REGULAR
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-2026-74618	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-617;CWE-755;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'binfmt_misc can expose a reachable WARN_ON when an fscontext file descriptor created by fsopen is completed from another user namespace through fsconfig. A local unprivileged user can trigger this by using ordinary user and mount namespace operations and passing the fscontext fd across a process boundary. For the CVSS the PR:L is used because local code execution by a user process is required, even though no special capability is needed in the initial namespace. The issue is not network reachable and does not show a memory corruption primitive. Impact is mainly denial of service through repeated warnings, kernel taint, and log flooding. On systems booted with panic_on_warn, the same condition can cause a kernel panic.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Ordinary Moderate otherwise (with actual score 3)  SKIP CVE-2026-74618 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DISK WARNONLY HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended / Ordinary Moderate otherwise**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  The commit explicitly states that an unprivileged user can trigger the WARN_ON using ordinary user and mount namespace operations plus fsopen/fsconfig fd passing.

* Availability impact realistic: +1
  The WARN_ON is repeatable, not WARN_ON_ONCE, and can taint the kernel and flood logs. This is availability relevant, but not a strong default crash primitive.

Paranoid additional signal:

* Reliable kernel crash / strong DoS: +1
  On kernels booted with panic_on_warn, the same user-triggerable WARN_ON becomes a reliable local kernel panic.

Not awarded:

* No generic memory corruption.
  The patch only replaces WARN_ON(user_ns != current_user_ns()) with a normal conditional return.

* No privilege escalation plausible.
  The mount is still refused with EINVAL, and the commit states that nothing in bm_fill_super depends on the two namespaces matching.

* No confidentiality or integrity impact.
  There is no UAF, OOB access, stale pointer reuse, arbitrary write, fd theft, or permission bypass primitive.

* No remote reachability.
  The path is local VFS/fs_context control plane behavior.

## 3. Reachability analysis

A local unprivileged user can trigger the warning if unprivileged user namespaces and mount namespaces are available. The described path uses fsopen("binfmt_misc") in one namespace context, passes the fscontext fd to another process or namespace, and then completes the mount through fsconfig(FSCONFIG_CMD_CREATE). No initial namespace CAP_SYS_ADMIN is required in the demonstrated scenario.

If unprivileged user namespaces are disabled, reachability is reduced and may require administrative setup. The affected path is not network reachable. Realistic exploitation is limited to log flooding, kernel taint, and kernel panic only on panic_on_warn systems.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local DoS issue in default configurations. It becomes a borderline actionable Moderate only for environments where panic_on_warn is enabled or where repeated kernel warnings are treated as operationally severe.

Theoretical memory corruption potential is not supported by the patch. Realistic exploitation evidence supports a repeatable local warning and possible panic_on_warn crash, not LPE or data compromise.

## 5. One-sentence report phrase

A local unprivileged user can trigger a repeatable binfmt_misc WARN_ON by completing an fs_context from another user namespace, causing kernel log flooding and kernel panic only on systems configured with panic_on_warn.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended mainly to confirm product defaults for unprivileged user namespaces, binfmt_misc availability, and panic_on_warn policy. If panic_on_warn is disabled and warning log flooding is not considered urgent, this can be handled as ordinary Moderate or deferred maintenance.

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

Patch: binfmt_misc: don't warn when the mount is completed from another user namespace
Commit: 37cf5cf1320a84a17225a1690547b8a0812ca94e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=37cf5cf1320a84a17225a1690547b8a0812ca94e

Commit description:

fsopen() records the caller's user namespace in fc->user_ns and hands
back an ordinary file descriptor. Nothing ties the task that calls
fsconfig(FSCONFIG_CMD_CREATE) to the task that created the context. The
fd is inherited across fork() and exec() and it can be passed over a
unix socket.

Completing a context from another user namespace is allowed on purpose.
vfs_cmd_create() authorizes the create with mount_capable(), which for
FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN), and that
succeeds for a task holding CAP_SYS_ADMIN in an ancestor of fc->user_ns.
So an unprivileged task can reach the WARN_ON() in bm_fill_super():
create a user and a mount namespace in a child, call
fsopen("binfmt_misc") there, send the fscontext fd to the parent and let
the parent issue FSCONFIG_CMD_CREATE. Both namespaces come from a plain
unshare(1) and no capability is needed anywhere:

  WARNING: fs/binfmt_misc.c:938 at bm_fill_super+0xa2/0xc0 [binfmt_misc]
  CPU: 15 UID: 1000 PID: 3243382 Comm: fswarn
  Call Trace:
   get_tree_keyed+0x7d/0xb0
   bm_get_tree+0x34/0x90 [binfmt_misc]
   vfs_get_tree+0x2a/0x100
   vfs_cmd_create+0x60/0xf0
   __do_sys_fsconfig+0x4b2/0x500

The child needs the mount namespace because fsopen() itself gates on
may_mount(), which asks for CAP_SYS_ADMIN in the user namespace owning
the caller's mount namespace. fsconfig() doesn't repeat that check.

It is a WARN_ON() and not a WARN_ON_ONCE(), so the condition can be
raised in a loop to taint the kernel and flood the log, and it panics a
kernel booted with panic_on_warn.

Keep refusing the mount and stop warning about it. Nothing in
bm_fill_super() depends on the two namespaces matching, it derives
everything from sb->s_user_ns.

Fixes: 21ca59b ("binfmt_misc: enable sandboxed mounts")
Cc: stable@vger.kernel.org # v6.7+
Link: https://patch.msgid.link/20260802-work-fill_super-warn-v1-2-4e987911a39a@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/binfmt_misc.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=37cf5cf1320a84a17225a1690547b8a0812ca94e

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

The original Linux kernel CVE announcement for CVE-2026-74618 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74618

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2
  Paranoid ActionableScore: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).