From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63938][MODERATE 7.0] KVM: SEV: Check PSC request indices against the actual size of the buffer Date: Sun, 19 Jul 2026 20:01:46 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63938 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: 5198f70c09a5f6e9e5f5a0a2c6b388f24294b176 List-Id: CVE: CVE-2026-63938 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: KVM: SEV: Check PSC request indices against the actual size of the buffer Commit: 5198f70c09a5f6e9e5f5a0a2c6b388f24294b176 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5198f70c09a5f6e9e5f5a0a2c6b388f24294b176 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63938 Analysis date: Sun, 19 Jul 2026 20:01:46 -0400 ActionableScore: 5 ActionableScore lower bound: 4 Actionable bucket: Actionable Moderate at minimum Manual review required: YES Summary: KVM SEV-SNP PSC validation used the maximum GHCB entry count instead of the effective scratch area size, allowing a malicious guest to make host KVM parse PSC entries beyond the valid buffer with realistic DoS impact and possible limited integrity impact. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63938 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63938 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-2026-63938 MODERATE 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:L/I:L/A:H';CWE-125;CWE-129;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.9';DESCR 'KVM SEV-SNP PSC handling validates request indices against a maximum count derived from the full GHCB shared buffer, but the effective scratch area can be smaller when the guest supplied scratch pointer is not at the start of the GHCB buffer. A malicious guest can provide PSC metadata where end_entry is below VMGEXIT_PSC_MAX_COUNT but beyond the actual scratch area, causing KVM to read PSC entries outside the effective buffer during VMGEXIT handling. For the CVSS the PR:L is used because an attacker needs the ability to run or control a guest that can issue SEV-SNP PSC VMGEXIT requests, but no host administrator privileges are required. The issue is not directly network reachable. Impact is at least host denial of service, and the paranoid score treats the host-side out-of-bounds PSC parsing as possible limited confidentiality or integrity impact across the guest to host boundary.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) YES VIRT INIT SIMPLEFIX HARDWARE KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS YES NO guess ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ActionableScoreLower=4 ## 1. ActionableScore * Conservative score: 4 * Paranoid score: 5 * Final recommended bucket: **Actionable Moderate at minimum** ## 2. Signal breakdown Triggered signals: * +1 Local or guest controlled trigger: a malicious SEV-SNP guest can issue PSC VMGEXIT requests without host administrator privileges. * +1 Memory corruption, weak or indirect corruption candidate: KVM may parse PSC entries beyond the effective GHCB scratch area due to validation against VMGEXIT_PSC_MAX_COUNT instead of the actual buffer size. This is best treated as OOB read or invalid struct interpretation, not a strong overwrite primitive. * +1 Availability impact realistic: malformed PSC metadata can affect host-side KVM PSC handling and plausibly cause VM or host KVM failure. * +1 Cross-boundary isolation concern: the bug is in a guest-to-host KVM boundary path. * +1 Privileged kernel state-management path: PSC handling affects SEV-SNP page state management in trusted host kernel code. * -1 Rare or configuration-dependent exposure: requires KVM SEV-SNP and PSC/GHCB handling, not a broad default kernel path. Paranoid-only signal: * +1 Integrity impact plausible: if out-of-bounds parsed PSC entries are interpreted as valid page-state requests, unintended host-side handling of guest page state is possible. This is not enough to claim strong LPE, but it is enough to avoid auto-close. Not awarded: * No +2 remote reachable: this is not network-triggerable. * No +2 strong memory corruption: no OOB write, UAF reclaim, arbitrary write, type confusion, or callback control is shown. * No +2 strong privilege escalation: guest-to-host impact is concerning, but the patch does not demonstrate a practical host code execution or LPE primitive. * No confidentiality point in the conservative score: the likely OOB read is around GHCB scratch/shared guest-controlled data, not clearly host-secret disclosure. Call-site confidence: medium. The changed function and VMGEXIT context are visible, but the full downstream PSC processing path is not included in the provided patch. ## 3. Reachability analysis The likely attacker is a malicious SEV-SNP guest or guest-controlled kernel component capable of issuing PSC VMGEXIT requests with crafted GHCB scratch metadata. This does not require host root privileges, but it does require the attacker to control a guest running under KVM SEV-SNP. Namespaces and ordinary local containers on the host do not directly expose this path. The path is not default-enabled on all systems because it depends on AMD SEV-SNP virtualization support and a guest using PSC requests. Realistic exploitation is therefore configuration-dependent, but the guest-to-host boundary makes it security-relevant. ## 4. Severity interpretation This is stronger than an ordinary Moderate validation bug because the malformed input crosses the guest-to-host KVM boundary and can make host kernel code parse PSC entries outside the effective buffer. Realistically, the demonstrated impact is closer to host-side DoS or incorrect page-state handling than proven host privilege escalation. Theoretical memory corruption potential exists as OOB parsing or invalid object interpretation, but there is no evidence of attacker-controlled overwrite, UAF reclaim, arbitrary write, or reliable host code execution. ## 5. One-sentence report phrase KVM SEV-SNP PSC validation used the maximum GHCB entry count instead of the effective scratch area size, allowing a malicious guest to make host KVM parse PSC entries beyond the valid buffer with realistic DoS impact and possible limited integrity impact. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: this is a guest-to-host KVM boundary bug involving bounds validation and host-side parsing of guest-controlled PSC metadata. It should not be auto-closed even though the currently supported primitive is weaker than a demonstrated host memory corruption or LPE exploit. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: SEV: Check PSC request indices against the actual size of the buffer Commit: 5198f70c09a5f6e9e5f5a0a2c6b388f24294b176 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5198f70c09a5f6e9e5f5a0a2c6b388f24294b176 Commit description: When processing Page State Change (PSC) requests, validate the PSC buffer against the effective size of the scratch area, which could be less than the maximum size if the guest provided a pointer that isn't exactly at the start of the GHCB shared buffer. 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: <20260501202250.2115252-10-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=5198f70c09a5f6e9e5f5a0a2c6b388f24294b176 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63938 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63938 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:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H The Best / paranoid CVSS score: 7.9 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).