[CVE-2026-63955][LOW] mm/vmalloc: do not trigger BUG() on BH disabled context

[CVE-2026-63955][LOW] mm/vmalloc: do not trigger BUG() on BH disabled context

[AL-KERNEL]

CVE: CVE-2026-63955
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mm/vmalloc: do not trigger BUG() on BH disabled context
Commit: ad7eff07b625f53c3fb513b30d7a8c5a79fbc7ce
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ad7eff07b625f53c3fb513b30d7a8c5a79fbc7ce
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63955
Analysis date: Sun, 19 Jul 2026 18:59:18 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline / Ordinary Moderate likely. Not an Important candidate
Manual review required: NO

Summary:
A reachable `BUG_ON` in `__get_vm_area_node()` can be triggered from local bridge/VLAN configuration when `vmalloc()` is called with bottom halves disabled, causing a local denial of service without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-63955 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63955

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-2026-63955	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-617;CWE-754;CWE-703;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A reachable BUG_ON in __get_vm_area_node can be triggered when vmalloc is called from process context with bottom halves disabled. The bridge VLAN add path can hold spin_lock_bh and then reach rhashtable allocation through kvmalloc and vmalloc, so in_interrupt returns true even though the caller is not in NMI or hard IRQ context. This can crash the kernel or trigger a severe availability failure during local network configuration. For the CVSS the PR:L is used in the higher paranoid score because CAP_NET_ADMIN can be delegated to container root or service accounts, so this is weaker than full host root in practical risk terms. The issue is not directly network reachable and is triggered through control plane configuration rather than packet traffic. Impact is denial of service only with C:N I:N A:H.';NO MANUAL CHECK; ,and ActionableScore result is Borderline / Ordinary Moderate likely. Not an Important candidate (with actual score 2)  SKIP CVE-2026-63955 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: c6307674ed82c0c57d6e1e3408e84ac449ab8e94 YES  NO NO unknown 	MAYBE	LOCK DANGER IMPROVEONLY SYZBOT MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline / Ordinary Moderate likely. Not an Important candidate**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The patch fixes a reachable `BUG_ON(in_interrupt())` that can be hit from the bridge VLAN path when `vmalloc()` is reached with BH disabled.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. The realistic trigger is local network configuration involving bridge/VLAN operations, normally requiring `CAP_NET_ADMIN`.
* Broad/common subsystem exposure: +1. `vmalloc()` is core MM code and Linux bridge is commonly deployed, although this is still a control-plane configuration path, not packet receive processing.

Conservative total: `+1 -2 +1 = 0`, rounded practically to 1 because the failure is a real kernel BUG path.

Paranoid signals:

* Local unprivileged or reduced-privilege trigger concern: +1. On systems with delegated network namespaces, container root or a service account may effectively obtain `CAP_NET_ADMIN` in a limited context, so the trigger can be weaker than full host root.
* Reliable kernel crash / strong DoS: +1. The failure mode is an intentional `BUG_ON`, not merely a warning.
* Broad/common subsystem exposure: +1. The affected path combines core MM allocation with bridge/rhashtable allocation behavior.

Paranoid total: `3`.

No memory corruption, UAF, OOB write, refcount misuse, page-cache corruption, information leak, or privilege-escalation primitive is indicated.

## 3. Reachability analysis

The bug is triggered locally through bridge/VLAN configuration paths that can call `rhashtable_lookup_insert_fast()` under `spin_lock_bh()`, then reach `kvmalloc()`, `vmalloc()`, and `__get_vm_area_node()`. In typical host deployments this requires administrative network privileges, usually `CAP_NET_ADMIN`, so conservative reachability is privileged local only.

Namespaces can matter. If an untrusted user can create or control a network namespace with bridge/VLAN configuration privileges, the practical privilege requirement can drop to a reduced-privilege or container-root scenario. The issue is not remotely network-triggerable by packet traffic and is not a data-plane parser bug.

Call-site confidence: high, because the commit message provides the relevant bridge-to-vmalloc call path.

## 4. Severity interpretation

This behaves like an ordinary Moderate / Low-like DoS issue, not an Important-class vulnerability. The theoretical impact is a local kernel crash or severe availability failure. Realistic exploitation evidence points only to availability impact from an overbroad assertion in BH-disabled process context.

There is no supported basis to score confidentiality or integrity impact. The patch only narrows the `BUG_ON()` condition from `in_interrupt()` to `in_nmi() || in_hardirq()`, and does not show stale object reuse, write-after-free, type confusion, arbitrary write, or sensitive-object access.

## 5. One-sentence report phrase

A reachable `BUG_ON` in `__get_vm_area_node()` can be triggered from local bridge/VLAN configuration when `vmalloc()` is called with bottom halves disabled, causing a local denial of service without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for urgent security triage because it is DoS-only, local/control-plane triggered, usually requires `CAP_NET_ADMIN`, and the patch does not indicate a memory corruption or security-boundary bypass primitive.

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

Patch: mm/vmalloc: do not trigger BUG() on BH disabled context
Commit: ad7eff07b625f53c3fb513b30d7a8c5a79fbc7ce
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ad7eff07b625f53c3fb513b30d7a8c5a79fbc7ce

Commit description:

__get_vm_area_node() currently triggers a BUG() if in_interrupt() returns
true.  However, in_interrupt() also reports true when BH are disabled.

The bridge code can call rhashtable_lookup_insert_fast() with bottom
halves disabled:

__vlan_add()
 -> br_fdb_add_local()
  spin_lock_bh(&br->hash_lock); <-- Disable BH
   -> fdb_add_local()
    -> fdb_create()
     -> rhashtable_lookup_insert_fast()
      -> kvmalloc()
       -> vmalloc()
        -> __get_vm_area_node()
         -> BUG_ON(in_interrupt())
  spin_unlock_bh(&br->hash_lock)

this triggers the BUG() despite the caller not being in NMI or
hard IRQ context.

Replace the in_interrupt() check with in_nmi() || in_hardirq().

Link: https://lore.kernel.org/20260515153009.2296191-1-urezki@gmail.com
Fixes: c630767 ("mm: kvmalloc: add non-blocking support for vmalloc")
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Ido Schimmel <idosch@nvidia.com>
Reported-by: syzbot+8b12fc6e0fb139765b58@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69ff8c7c.050a0220.1036b8.000b.GAE@google.com/
Reviewed-by: Baoquan He <baoquan.he@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/vmalloc.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=ad7eff07b625f53c3fb513b30d7a8c5a79fbc7ce

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

The original Linux kernel CVE announcement for CVE-2026-63955 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63955

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:N/I:N/A:H
  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: 1
  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: 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).