From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63939][IMPORTANT] KVM: SEV: Compute the correct max length of the in-GHCB scratch area Date: Sun, 19 Jul 2026 13:20:54 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63939 X-AL-KERNEL-Priority: IMPORTANT X-AL-KERNEL-Severity: IMPORTANT X-AL-KERNEL-Base-Severity: IMPORTANT X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 8 X-AL-KERNEL-ActionableScore-Lower: 7 X-AL-KERNEL-Commit: 6ca9400d36005ffdca25f80186bea781c7e1dc4c List-Id: CVE: CVE-2026-63939 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: KVM: SEV: Compute the correct max length of the in-GHCB scratch area Commit: 6ca9400d36005ffdca25f80186bea781c7e1dc4c Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6ca9400d36005ffdca25f80186bea781c7e1dc4c Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63939 Analysis date: Sun, 19 Jul 2026 13:20:54 -0400 ActionableScore: 8 ActionableScore lower bound: 7 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A KVM SEV VMGEXIT scratch area length bug can cause host kernel buffer overflow risk during PSC handling when a guest-provided in-GHCB scratch pointer is evaluated with only the minimum header length instead of the full available shared buffer. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63939 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63939 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. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-63939 IMPORTANT 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:C/C:H/I:H/A:H';CWE-131;*CWE-787;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H';BEST CVSS score: '8.8';DESCR 'A host-side buffer overflow risk exists in the KVM SEV VMGEXIT path because the GHCB scratch area length can be computed from the minimum PSC header length instead of the maximum usable region ending at the GHCB shared buffer boundary. A malicious or controlled SEV guest can provide the scratch pointer and reach the PSC handling path, causing KVM to use incorrect bounds for guest controlled scratch data. For the CVSS the PR:L is used because triggering requires control of a guest or a process that can run a KVM guest, but does not require host administrator privileges. The issue is not directly reachable from the network and is a local virtualization boundary attack surface. Impact is at least host denial of service and in the paranoid case may include confidentiality and integrity impact because the bug class is buffer overflow in host kernel context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8) YES OOB VIRT INIT SIMPLEFIX HARDWARE KPANIC MEMORY KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=8 ActionableScoreLower=7 ## 1. ActionableScore * Conservative score: **7** * Paranoid score: **8** * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown Conservative signals: * **Local unprivileged or reduced-privilege trigger: +1** Triggering requires control of a guest or a userspace process able to run a KVM SEV guest, not full host root in typical virtualization threat models. * **Privilege escalation plausible: +2** The affected path is guest-to-host KVM SEV VMGEXIT handling. A host-side buffer overflow in this context is a plausible virtualization boundary escape or host privilege impact candidate. * **Generic memory corruption, strong corruption primitive: +2** The commit explicitly says KVM needs the full scratch area length to avoid buffer overflows. The scratch pointer is guest-provided and the affected buffer is used by host kernel KVM code. * **Reliable kernel crash / strong DoS: +1** Even without proving code execution, malformed PSC scratch handling can plausibly corrupt host kernel memory or crash the host. * **Cross-boundary isolation impact: +1** The bug is in the guest-to-host SEV interface, crossing a VM isolation boundary. * **Confidentiality impact plausible: +1** In the paranoid interpretation, host kernel memory corruption can plausibly expose host or other guest state. * **Integrity impact plausible: +1** In the paranoid interpretation, host kernel memory corruption can plausibly modify host kernel state. * **Rare/configuration-dependent exposure: -1** Requires AMD SEV/SEV-ES/SNP style KVM deployment and PSC VMGEXIT handling, not a universal KVM path. Paranoid score uses the same basis but treats the host-side buffer overflow as high-value memory corruption across a virtualization boundary, raising the practical triage score to **8**. Call-site confidence: **medium**. The patch shows the vulnerable length computation helper and the commit explains PSC consumers need the full scratch area to avoid buffer overflows, but the downstream PSC copy/write call sites are not included in the provided diff. ## 3. Reachability analysis The likely attacker is a malicious or controlled SEV guest, or a local user with enough access to create/run such a guest through KVM. This is not directly network-reachable. Namespace/container exposure depends on whether `/dev/kvm` is delegated into containers or available to non-root users via the `kvm` group. The path is not default-exposed on all systems, but it is security-sensitive on hosts offering SEV guests because guest-provided VMGEXIT state is processed by host kernel KVM code. ## 4. Severity interpretation This behaves more like an **Important-class candidate** than an ordinary Moderate issue. The realistic demonstrated impact from the patch is host-side buffer overflow prevention in KVM SEV PSC handling. The exact exploitability is not proven by the diff alone, but the primitive is stronger than a simple NULL dereference or validation-only fix because it involves guest-controlled scratch area bounds in host kernel virtualization code. Conservative handling should not auto-close it. ## 5. One-sentence report phrase A KVM SEV VMGEXIT scratch area length bug can cause host kernel buffer overflow risk during PSC handling when a guest-provided in-GHCB scratch pointer is evaluated with only the minimum header length instead of the full available shared buffer. ## 6. Manual review recommendation **MANUAL CHECK REQUIRED** This is required because the patch explicitly addresses a buffer overflow risk in host-side KVM SEV code reachable from a guest-controlled VMGEXIT path, which is a virtualization boundary and potential guest-to-host impact surface. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: SEV: Compute the correct max length of the in-GHCB scratch area Commit: 6ca9400d36005ffdca25f80186bea781c7e1dc4c Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6ca9400d36005ffdca25f80186bea781c7e1dc4c Commit description: When setting the length of the GHCB scratch area, and the area is in the GHCB shared buffer, set the effective length of the scratch area to the max possible size given the start of the guest-provided pointer, and the end of the shared buffer. The code was "fine" when first introduced, as KVM doesn't consult the length of the buffer when emulating MMIO, because the passed in @len always specifies the *max* size required. But for PSC requests, the incoming @len is just the minimum length (to process the header), and KVM needs to know the full size of the scratch area to avoid buffer overflows (spoiler alert). Opportunistically rename @len => @min_len to better reflect its role. Fixes: 9b54e24 ("KVM: SEV: Add support to handle Page State Change VMGEXIT") Cc: stable@vger.kernel.org Reviewed-by: Tom Lendacky Reviewed-by: Michael Roth Signed-off-by: Sean Christopherson Message-ID: <20260501202250.2115252-7-seanjc@google.com> Signed-off-by: Paolo Bonzini Signed-off-by: Greg Kroah-Hartman Changed files: arch/x86/kvm/svm/sev.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=6ca9400d36005ffdca25f80186bea781c7e1dc4c ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63939 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63939 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:L/UI:N/S:C/C:L/I:L/A:H The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H The Best / paranoid CVSS score: 8.8 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: 7 Paranoid ActionableScore: 8 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).