From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-53200][MODERATE 7.0] KVM: arm64: nv: Fix handling of XN[0] when !FEAT_XNX Date: Thu, 25 Jun 2026 20:48:36 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-53200 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 5 X-AL-KERNEL-ActionableScore-Lower: 4 X-AL-KERNEL-Commit: b95976c2ea446044553a5f469c0bae13553d75ab List-Id: CVE: CVE-2026-53200 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: KVM: arm64: nv: Fix handling of XN[0] when !FEAT_XNX Commit: b95976c2ea446044553a5f469c0bae13553d75ab Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b95976c2ea446044553a5f469c0bae13553d75ab Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53200 Analysis date: Thu, 25 Jun 2026 20:48:36 -0400 ActionableScore: 5 ActionableScore lower bound: 4 Actionable bucket: Actionable Moderate at minimum Manual review required: YES Summary: KVM arm64 nested virtualization may incorrectly grant execute permission in shadow stage-2 mappings when FEAT_XNX is absent, allowing a local KVM-capable attacker to bypass intended execute-never restrictions without evidence of direct host memory corruption. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-53200 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53200 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 7.0 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-2026-53200 MODERATE CHECK Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:H/A:N';CWE-284;CWE-269;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:N/I:H/A:N';BEST CVSS score: '6.4';DESCR 'KVM arm64 nested virtualization can mishandle the XN execute-never permission when FEAT_XNX is not present because XN is already extracted from its page table bitfield but the code still applies FIELD_PREP style masking. This can turn the effective XN value into execute allowed and may grant execute permission in shadow stage-2 mappings even when the upper virtualization layer intended the mapping to be non-executable. For the CVSS the PR:L is used because triggering requires the ability to run or control a local KVM guest or nested virtualization workload, but does not normally require full host administrator privileges. The issue is not directly network reachable and is not triggered by packet traffic. Impact is primarily an integrity policy bypass in nested virtualization rather than a kernel crash. In the paranoid score, limited confidentiality impact is kept possible because execute permission bypasses can weaken guest or nested hypervisor isolation policies, but the patch does not show UAF, OOB access, arbitrary write, or a direct host privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) SKIP CVE-2026-53200 SKIP No affected files built, so skip this CVE NO - - unknown MAYBE VIRT SIMPLEFIX HARDWARE KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ActionableScoreLower=4 ## 1. ActionableScore * Conservative score: 4 * Paranoid score: 5 * Final recommended bucket: **Actionable Moderate at minimum** ## 2. Signal breakdown Conservative signals: * Local unprivileged trigger: +1 Triggering plausibly requires a local user or process able to run or control a KVM guest or nested virtualization workload through `/dev/kvm`. * Security-boundary bypass with practical permission impact: +2 The bug can incorrectly grant execute permission in shadow stage-2 mappings when execute-never should be preserved. This is a virtualization permission-model bypass. * Integrity impact plausible: +1 The main impact is weakening or bypassing intended executable-memory restrictions in nested virtualization. * Broad important subsystem exposure: +1 KVM is a security-sensitive and widely used virtualization subsystem, even though this specific arm64 nested-virtualization path is narrower. * Rare AND difficult-to-reach subsystem/configuration: -1 The affected path depends on arm64 nested virtualization and the absence of FEAT_XNX, so exposure is configuration and hardware dependent. Paranoid additional signal: * Confidentiality impact plausible: +1 In the worst defensible interpretation, execute-permission bypass in nested virtualization may weaken isolation or policy assumptions enough to justify limited confidentiality concern, but there is no demonstrated host memory disclosure primitive. Not counted: * Remote reachable: +0 No network-triggerable path is shown. * Generic memory corruption: +0 The patch does not indicate UAF, OOB write, arbitrary write, double free, or reclaim-based corruption. * Reliable kernel crash / strong DoS: +0 The described impact is permission bypass, not a crash path. * Strong LPE plausibility: +0 No direct guest-to-host privilege escalation primitive is demonstrated by the patch. ## 3. Reachability analysis The likely attacker is a local user, VM manager, or workload that can create or influence an arm64 KVM guest with nested virtualization enabled. In typical deployments this means access to `/dev/kvm`, often via membership in the `kvm` group or a virtualization service account, so this should be treated as PR:L rather than full host-root. Containers only matter if `/dev/kvm` is delegated into the container. If it is not delegated, the container cannot normally reach this path. The issue is not packet-triggered and is not remotely reachable by itself. The path is not universally exposed because it depends on arm64 KVM nested virtualization and systems where FEAT_XNX is absent. However, when this configuration exists, the bug affects a security-sensitive permission decision in the shadow stage-2 translation path. ## 4. Severity interpretation This behaves more like an actionable Moderate issue than an ordinary Moderate. It is not a memory corruption bug and does not by itself show a host code execution or kernel crash primitive. The practical impact is an execute-permission policy bypass in nested virtualization, which is security relevant and should not be auto-closed solely because the patch is small. The theoretical worst case is that weakened execute-never enforcement helps bypass guest or nested-hypervisor isolation policies. The realistic evidence from the patch supports integrity impact and limited paranoid confidentiality concern, but not arbitrary code execution in the host. ## 5. One-sentence report phrase KVM arm64 nested virtualization may incorrectly grant execute permission in shadow stage-2 mappings when FEAT_XNX is absent, allowing a local KVM-capable attacker to bypass intended execute-never restrictions without evidence of direct host memory corruption. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: this is a KVM permission-boundary bypass in nested virtualization. Even without a demonstrated memory corruption primitive, the affected code controls executable permissions across virtualization layers and should be reviewed manually for isolation impact. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: arm64: nv: Fix handling of XN[0] when !FEAT_XNX Commit: b95976c2ea446044553a5f469c0bae13553d75ab Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b95976c2ea446044553a5f469c0bae13553d75ab Commit description: XN has already been extracted from its bitfield position so using FIELD_PREP() on the mask that clears XN[0] is completely broken, having the effect of unconditionally granting execute permissions... Fix the obvious mistake by manipulating the right bit. Cc: stable@vger.kernel.org Fixes: d93febe ("KVM: arm64: nv: Forward FEAT_XNX permissions to the shadow stage-2") Reviewed-by: Wei-Lin Chang Signed-off-by: Oliver Upton Link: https://patch.msgid.link/20260602165901.52800-2-oupton@kernel.org Signed-off-by: Marc Zyngier Signed-off-by: Greg Kroah-Hartman Changed files: arch/arm64/include/asm/kvm_nested.h 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=b95976c2ea446044553a5f469c0bae13553d75ab ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-53200 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53200 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:C/C:N/I:H/A:N The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:H/A:N The Best / paranoid CVSS score: 6.4 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: 4 Paranoid ActionableScore: 5 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: YES Published priority for this report (same as in Subject): MODERATE 7.0 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).