[CVE-2026-74483][LOW] binfmt_misc: don't leak the user namespace when the mount fails

[CVE-2026-74483][LOW] binfmt_misc: don't leak the user namespace when the mount fails

[AL-KERNEL]

CVE: CVE-2026-74483
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: binfmt_misc: don't leak the user namespace when the mount fails
Commit: 87a4eb9bbb3497f749bbac612af22ddaa62d7b0d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=87a4eb9bbb3497f749bbac612af22ddaa62d7b0d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74483
Analysis date: Sat, 15 Aug 2026 13:47:26 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-74483 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74483

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74483	LOW	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-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A resource leak in binfmt_misc can occur when a user namespace mount fails before the superblock teardown path calls put_super. In that case the reference stored in sb s_fs_info can be lost, leaking one user namespace reference per failed mount attempt. For the CVSS the PR:L is used because a local user or process is required to create the user namespace and trigger the binfmt_misc mount path, even though full host root privileges are not required in common user namespace deployments. The issue is not network reachable and does not provide a demonstrated memory corruption primitive. Impact is denial of service through resource exhaustion after repeated failed mounts, especially under tight memory cgroup conditions where allocation failures can be induced more reliably.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-74483 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 21ca59b365c091d583f36ac753eaa8baf947be6f YES  NO NO unknown 	MAYBE	DISK INIT LEAK LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2

1. ActionableScore
* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like**::

2. Signal breakdown
* Local unprivileged trigger: +1. `binfmt_misc` supports `FS_USERNS_MOUNT`, and the commit states that an unprivileged caller in a user namespace can trigger failed mounts.
* Real lifetime corruption / refcount lifetime bug: +1. The bug leaks a `user_namespace` reference when mount setup fails before the normal release path runs.
* Availability impact realistic: +1 in the paranoid interpretation only. Repeated failed mounts under tight memcg conditions can accumulate leaked references and cause resource exhaustion.
* Hard or condition-dependent trigger: -1. Reliable triggering depends on inducing allocation failure during mount setup, typically using a tight memory cgroup.
* No generic memory corruption signal. The patch fixes a leaked reference, not UAF, double free, OOB write, type confusion, or arbitrary write.
* No confidentiality or integrity signal. There is no demonstrated information leak, file descriptor theft, credential exposure, or write primitive.

3. Reachability analysis
A local unprivileged user may trigger the path if unprivileged user namespaces and `binfmt_misc` mounts are available. Containers or delegated user namespaces can make this reachable without full host root privileges. The issue is not network reachable. Practical exploitation requires repeatedly causing `bm_fill_super()` failure, especially `simple_fill_super()` failure under tight memory cgroup pressure, so the attack is cumulative rather than immediate.

4. Severity interpretation
This behaves like a local resource exhaustion issue, not an Important-class memory corruption vulnerability. The theoretical impact is host or workload availability degradation by leaking one user namespace reference per failed mount attempt. The realistic evidence supports DoS through resource leakage, but not privilege escalation, confidentiality impact, or integrity impact. This is an ordinary Moderate / Low-like issue unless the affected product heavily relies on untrusted user namespaces with strict memcg isolation.

5. One-sentence report phrase
A failed `binfmt_misc` mount in a user namespace can leak a `user_namespace` reference, allowing a local unprivileged user under memory pressure to cause cumulative resource exhaustion.

6. Manual review recommendation
NO MANUAL CHECK NEEDED.
This is a local resource leak without memory corruption, network reachability, privilege escalation, or sensitive-object access.

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

Patch: binfmt_misc: don't leak the user namespace when the mount fails
Commit: 87a4eb9bbb3497f749bbac612af22ddaa62d7b0d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=87a4eb9bbb3497f749bbac612af22ddaa62d7b0d

Commit description:

bm_get_tree() takes a reference to the user namespace and hands it to
get_tree_keyed() as the sget key. sget_fc() moves that reference into
sb->s_fs_info and clears fc->s_fs_info, so from that point on the
superblock owns it and bm_free() doesn't see it anymore.

The superblock drops it in ->put_super(). But generic_shutdown_super()
only calls ->put_super() from inside the if (sb->s_root) branch, so
nothing releases it when bm_fill_super() fails:

- The kzalloc_obj() failure leaves s_root NULL and the whole branch is
  skipped.

- A simple_fill_super() failure in the file loop leaves s_root set, but
  s_op still points at simple_super_operations, which has no
  ->put_super(). bm_fill_super() installs s_ops only once
  simple_fill_super() returned success, and installing it earlier
  wouldn't help either because simple_fill_super() overwrites s_op.

Either way vfs_get_super() calls deactivate_locked_super() and the
reference is gone for good. binfmt_misc mounts are available in a user
namespace and both the inode and the dentry cache are SLAB_ACCOUNT, so
an unprivileged caller under a tight memory cgroup can fail
simple_fill_super() on demand and leak one user namespace per attempt.

Drop the reference in ->kill_sb() instead, which runs unconditionally,
the same way nfsd and rpc_pipefs release their keyed s_fs_info.

That also stops ->put_super() from clearing s_fs_info while the
superblock is still on @fs_supers. generic_shutdown_super() leaves it
there on purpose so that sget_fc() keeps finding it until kill_sb() has
run, but a NULL s_fs_info makes test_keyed_super() miss it, so a
concurrent mount for the same user namespace skips the grab_super()
wait and creates a second superblock for a namespace that is still
being torn down.

Link: https://patch.msgid.link/20260728-work-binfmt_misc-usernsleak-v1-1-dbd8d5e626e7@kernel.org
Fixes: 21ca59b ("binfmt_misc: enable sandboxed mounts")
Cc: stable@vger.kernel.org
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=87a4eb9bbb3497f749bbac612af22ddaa62d7b0d

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

The original Linux kernel CVE announcement for CVE-2026-74483 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74483

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: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: unknown
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).