[CVE-2026-89579][IMPORTANT] bpf: Harden bloom filter sizing and indexing on 32-bit kernels

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

CVE: CVE-2026-89579
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: bpf: Harden bloom filter sizing and indexing on 32-bit kernels
Commit: 3b7a13eccfcf97714ffc1ca6aa663d66d4a30e29
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3b7a13eccfcf97714ffc1ca6aa663d66d4a30e29
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89579
Analysis date: Fri, 11 Sep 2026 21:21:26 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89579 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89579

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
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CVE-2026-89579	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89579 CHECK WITH IMPACT FROM ORIG NN MODERATE	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-787;CWE-190;CWE-681;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'A memory corruption issue in the BPF bloom filter map exists on 32-bit kernels when the computed bitmap reaches the U32_MAX fallback case. The allocation size can wrap during bitmap byte calculation, leaving the map with bitset_mask equal to U32_MAX while allocating only a much smaller object. In addition, hashes above INT_MAX can become negative bit offsets when passed to test_bit() or set_bit() on 32-bit systems, allowing reads or writes before or after bloom bitset. For the CVSS the PR:L is used in the paranoid score because triggering may be possible from a local context with CAP_BPF or a delegated helper binary, which is less than full administrative control in practical exposure. Impact includes kernel memory corruption, denial of service, and plausible local privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	YES	BPF HARDWARE LINUS TEST  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5	YES	NO	checked

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

ActionableScore=7
ActionableScoreLower=6
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 6
* Conservative score: 6
* Raw paranoid score: 7
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate**::

No floor or cap changed the scores. The paranoid score is emitted as ActionableScore because it is higher than the conservative score.

2. Signal breakdown

Conservative signals:

* +2 Strong corruption primitive. The patch establishes attacker-influenced out-of-bounds write past the allocated BPF bloom filter object when 32-bit arithmetic wraps in the U32_MAX fallback case.
* +1 Independent signed-index OOB mechanism. Even with correct 512 MiB allocation, hashes above INT_MAX can become negative bit offsets on 32-bit x86 bitops, allowing access before bloom->bitset.
* +2 Strong LPE plausibility. The supplied commit text explicitly states that local privilege escalation is possible on a 32-bit x86 kernel using the under-allocation bug from a binary with CAP_BPF.
* +1 Integrity plausible. set_bit() can modify memory outside the intended bitmap, so unauthorized kernel memory modification is directly supported.
* +1 Practical exploit chain strongly implied. The exploitation note connects the concrete under-allocation OOB write to local privilege escalation.
* +1 Reliable kernel crash / strong DoS. A write past a much smaller allocated object in kernel memory realistically supports kernel crash or panic.
* -1 Reduced / delegated privilege. The described exploit path requires execution with CAP_BPF, which is below full host administrator control but still privilege-gated.
* -1 32-bit-specific deployment constraint. The bug is specifically limited to 32-bit kernels, with the strongest described exploitation case on 32-bit x86.

Paranoid additional signal:

* +1 Confidentiality plausible. bloom_map_peek_elem() uses test_bit() and returns a user-visible membership result, so the OOB read side can plausibly expose limited adjacent kernel-memory state through the boolean result.

Important non-awarded signals:

* RealUAF not awarded because this is not a lifetime or stale-reference bug.
* Controlled reclaim not required and not separately awarded because the patch already establishes direct OOB write.
* Arbitrary write not claimed because set_bit() gives a constrained bit-setting primitive, not a fully arbitrary write.
* Remote reachability not established. This is local BPF map usage, not a network-triggered bug.
* NamespaceScopedPrivilege=UNKNOWN because the complete BPF load/create/update path is not shown to be attacker-obtainable inside user namespaces.
* Extra availability not awarded beyond the single strong DoS signal.
* No repeated release/refcount signal applies.

3. Reachability analysis

The vulnerable path is reached through BPF bloom filter map creation and map operations. The supplied exploitation note identifies a local attacker path from a binary with CAP_BPF on a 32-bit x86 kernel.

This is below full host administrator in practical exposure when CAP_BPF is delegated to a binary or service, so HostAdminRequired=NO. It is not ordinary unprivileged reachability because CAP_BPF is still required by the described path.

The complete namespace-scoped chain is unresolved. BPF privilege, map creation, and update permissions are not shown to be fully obtainable by an ordinary attacker inside attacker-created namespaces, so NamespaceScopedPrivilege=UNKNOWN.

No network reachability is established. No special physical hardware is required. The issue is architecture/configuration constrained to 32-bit kernels, with x86 bitops specifically relevant for the negative-offset behavior. This is not a race and no timing penalty applies.

4. Primitive and severity interpretation

The exact primitive is kernel memory corruption through BPF bloom filter bitmap sizing and indexing bugs on 32-bit kernels.

There are two concrete mechanisms. First, 32-bit overflow in bitmap byte calculation can under-allocate the map while retaining bitset_mask == U32_MAX, allowing later set_bit() operations to write past the allocated object. Second, signed long bit indices on 32-bit kernels can reinterpret high u32 hash values as negative offsets, allowing test_bit() or set_bit() before bloom->bitset.

This is CLASS_A because an attacker-influenced OOB write is directly established and the supplied text explicitly states that local privilege escalation is possible with CAP_BPF. The demonstrated consequence includes kernel memory corruption and realistic DoS. The supplied exploitation note supports plausible LPE, so this should not be treated as ordinary crash-only BPF behavior.

ImportantCandidate=YES because this is a strong corruption primitive with explicit LPE relevance. ManualReview=REQUIRED because architecture-specific BPF kernel memory corruption with a stated LPE path needs human assessment. AutoDowngradeAllowed=NO because automatic downgrade would discard a concrete OOB write and supplied LPE signal.

Likely operational severity is Important, with practical exposure limited by 32-bit kernel use and CAP_BPF reachability.

5. One-sentence report phrase

A 32-bit BPF bloom filter sizing and indexing flaw can cause attacker-influenced out-of-bounds kernel reads and writes, and the supplied exploitation note states that local privilege escalation is possible on 32-bit x86 with CAP_BPF.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The bug establishes a concrete kernel OOB write primitive and supplied context explicitly connects it to possible local privilege escalation, even though reachability is constrained by CAP_BPF and 32-bit kernels.

7. Call-site confidence

Call-site confidence: high

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

Patch: bpf: Harden bloom filter sizing and indexing on 32-bit kernels
Commit: 3b7a13eccfcf97714ffc1ca6aa663d66d4a30e29
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3b7a13eccfcf97714ffc1ca6aa663d66d4a30e29

Commit description:

bloom_map_alloc() has two 32-bit-specific problems when the computed
bitmap reaches the U32_MAX fallback case.

First, BITS_TO_BYTES(U32_MAX) is evaluated with 32-bit arithmetic. The
addition performed by DIV_ROUND_UP wraps, so the map allocates only the
fixed-size bloom filter object while keeping bitset_mask == U32_MAX.
Subsequent updates can then write past the allocated object.

Second, fixing only the allocation size is not sufficient. The bloom hash
is a u32, but set_bit() takes a signed long bit number and x86 test_bit()
eventually feeds the index to variable_test_bit(long, ...). On 32-bit
kernels, hashes in [0x80000000, U32_MAX] therefore become negative bit
offsets. x86 bt/bts with a memory operand interpret those offsets relative
to the supplied base, so a map with bitset_mask == U32_MAX can read or
write before bloom->bitset even after allocating the full 512 MiB bitmap.

Keep the U32_MAX fallback, but split each hash into a word pointer and an
in-word bit number before calling test_bit() or set_bit(). The bitops
argument is then always in [0, BITS_PER_LONG - 1], while BIT_WORD(h) still
selects the intended word in the full bitmap.

Compute the bitset size from (u64)bitset_mask + 1 before passing the final
size to bpf_map_area_alloc(). This fixes the original under-allocation and
keeps the allocated storage consistent with the addressable bitset.

Exploitation note: local privilege escalation is possible on a 32-bit x86
kernel using the under-allocation bug from a binary with CAP_BPF.

Fixes: 9330986 ("bpf: Add bloom filter map implementation")
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/20260805060228.2703051-1-Jeremy.Jean@oss.cyber.gouv.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Assisted-by: Codex:gpt-5

Changed files:
  kernel/bpf/bloom_filter.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=3b7a13eccfcf97714ffc1ca6aa663d66d4a30e29

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

The original Linux kernel CVE announcement for CVE-2026-89579 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89579

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:H/UI:N/S:U/C:H/I:H/A:H
  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).