[CVE-2026-45837][IMPORTANT] bpf: Fix use-after-free in arena_vm_close on fork

[CVE-2026-45837][IMPORTANT] bpf: Fix use-after-free in arena_vm_close on fork

[AL-KERNEL]

CVE: CVE-2026-45837
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: bpf: Fix use-after-free in arena_vm_close on fork
Commit: 723b9fa930cc277c15ce6b9ec9feec828cfac9d7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=723b9fa930cc277c15ce6b9ec9feec828cfac9d7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45837
Analysis date: Wed, 27 May 2026 06:02:16 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A BPF arena VMA lifetime bug can leave `vml->vma` pointing to a parent VMA after `fork()` and `munmap()`, allowing a child to trigger a use-after-free in `zap_pages()` via `bpf_arena_free_pages()`, with at least local DoS and plausible LPE impact.

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

The original Linux kernel CVE announcement for CVE-2026-45837 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45837

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.

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

CVE-2026-45837	IMPORTANT	CHECK	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:H';*CWE-416;*CWE-672;*CWE-664;BEST CVSS score: '7.8';DESCR 'A use-after-free can occur in the BPF arena VMA lifetime path because arena_vm_open only increments the vml mmap_count and does not register the child VMA in arena vma_list after fork. The vml vma pointer can still reference the parent VMA, so after the parent unmaps the arena the child can later call bpf_arena_free_pages and make zap_pages read a stale VMA pointer. For the CVSS the PR:L is used because triggering requires local access to BPF arena functionality, but this may be available through CAP_BPF, reduced capability service accounts, container root, or systems with less restrictive BPF policy rather than full host root. This is a real lifetime corruption bug in a high control BPF memory management path. Impact is at least local denial of service and may plausibly include privilege escalation due to use-after-free in kernel memory management state.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	READ BPF UAF LINUS  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	YES	NO	checked

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Memory corruption, strong corruption primitive: +2. The commit explicitly describes a stale `vml->vma` pointer and a use-after-free in `zap_pages()`.
* Real lifetime corruption: +1. The bug is a VMA lifetime mismatch across `fork()`, `munmap()`, and `mremap()` paths.
* Weak to strong LPE concern: +1 conservative, +2 paranoid. The UAF is in BPF arena VMA management, which is a high-control local kernel memory-management interface. A concrete exploit primitive is not shown, but UAF in this context is credible for privilege-escalation review.
* Reliable kernel crash / strong DoS: +1. The stale VMA access can trigger a kernel UAF and likely crash.
* Local high-control kernel data-plane API: +1. BPF arena gives local users with BPF access fine-grained control over mappings, lifetime, and memory operations.
* Availability impact realistic: +1. Kernel crash is a realistic outcome.
* Requires privileged BPF capability in typical deployments: -1 conservative. Access to BPF arena is commonly restricted by `CAP_BPF`, `CAP_SYS_ADMIN`, policy, or sysctls, but this is not equivalent to full host-root in delegated or reduced-capability environments.

Conservative total: 6.
Paranoid total: 7.

## 3. Reachability analysis

The bug is locally triggerable by an actor able to create and map a BPF arena, then manipulate VMA lifetime through `fork()`, `munmap()`, `bpf_arena_free_pages()`, and potentially `mremap()`. It is not network reachable.

Privileges depend heavily on BPF policy. On hardened systems, BPF arena access may require `CAP_BPF`, `CAP_SYS_ADMIN`, or equivalent privileged service context. In containerized or delegated environments, a reduced-capability root, service account, or container workload with BPF permissions may be enough, so treating this as full PR:H would understate practical risk.

The affected path is not a generic syscall for all unprivileged users, but BPF is a major high-risk kernel subsystem and the primitive is a real lifetime bug, not a simple warning or NULL dereference.

## 4. Severity interpretation

This behaves like an Important-class local kernel memory-corruption candidate. The realistic demonstrated impact is at least local DoS via UAF-triggered crash. The theoretical but credible risk is local privilege escalation because the stale pointer is in VMA/BPF arena memory-management state, and BPF interfaces often provide enough control to make lifetime bugs security-relevant.

This should not be auto-closed as ordinary Moderate. Even if a working LPE exploit is not demonstrated by the patch, the bug class, affected subsystem, and stale VMA pointer semantics justify manual Important-level review.

## 5. One-sentence report phrase

A BPF arena VMA lifetime bug can leave `vml->vma` pointing to a parent VMA after `fork()` and `munmap()`, allowing a child to trigger a use-after-free in `zap_pages()` via `bpf_arena_free_pages()`, with at least local DoS and plausible LPE impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: this is a real BPF arena use-after-free in VMA lifetime handling, with plausible local privilege-escalation implications even though the patch does not demonstrate a complete exploit chain.

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

Patch: bpf: Fix use-after-free in arena_vm_close on fork
Commit: 723b9fa930cc277c15ce6b9ec9feec828cfac9d7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=723b9fa930cc277c15ce6b9ec9feec828cfac9d7

Commit description:

arena_vm_open() only bumps vml->mmap_count but never registers the
child VMA in arena->vma_list. The vml->vma always points at the
parent VMA, so after parent munmap the pointer dangles. If the child
then calls bpf_arena_free_pages(), zap_pages() reads the stale
vml->vma triggering use-after-free.

Fix this by preventing the arena VMA from being inherited across
fork with VM_DONTCOPY, and preventing VMA splits via the may_split
callback.

Also reject mremap with a .mremap callback returning -EINVAL. A
same-size mremap(MREMAP_FIXED) on the full arena VMA reaches
copy_vma() through the following path:

  check_prep_vma()       - returns 0 early: new_len == old_len
                           skips VM_DONTEXPAND check
  prep_move_vma()        - vm_start == old_addr and
                           vm_end == old_addr + old_len
                           so may_split is never called
  move_vma()
    copy_vma_and_data()
      copy_vma()
        vm_area_dup()    - copies vm_private_data (vml pointer)
        vm_ops->open()   - bumps vml->mmap_count
      vm_ops->mremap()   - returns -EINVAL, rollback unmaps new VMA

The refcount ensures the rollback's arena_vm_close does not free
the vml shared with the original VMA.

Reported-by: Weiming Shi <bestswngs@gmail.com>
Reported-by: Xiang Mei <xmei5@asu.edu>
Fixes: 3174603 ("bpf: Introduce bpf_arena.")
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260413194245.21449-1-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/bpf/arena.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=723b9fa930cc277c15ce6b9ec9feec828cfac9d7

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

The original Linux kernel CVE announcement for CVE-2026-45837 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45837

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 6
  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: YES
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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).