From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63852][MODERATE REGULAR] drm/amdgpu/vcn: set no_user_fence for VCN v4.0.3 enc ring [ Upstream Date: Sun, 19 Jul 2026 11:42:39 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63852 X-AL-KERNEL-Priority: MODERATE REGULAR X-AL-KERNEL-Severity: MODERATE REGULAR X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 4 X-AL-KERNEL-ActionableScore-Lower: 2 X-AL-KERNEL-Commit: c12a5d35033c0640c57c10d7111c010c7b9c2c8e List-Id: CVE: CVE-2026-63852 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: drm/amdgpu/vcn: set no_user_fence for VCN v4.0.3 enc ring [ Upstream Commit: c12a5d35033c0640c57c10d7111c010c7b9c2c8e Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c12a5d35033c0640c57c10d7111c010c7b9c2c8e Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63852 Analysis date: Sun, 19 Jul 2026 11:42:39 -0400 ActionableScore: 4 ActionableScore lower bound: 2 Actionable bucket: Borderline, manual review recommended Manual review required: YES Summary: The amdgpu VCN v4.0.3 encoder ring allowed user fence submissions despite lacking 64-bit user fence write support, allowing a local render-node user to trigger an unsupported GPU path that may cause failed submissions, GPU reset, or denial of service. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63852 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63852 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 REGULAR 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-2026-63852 MODERATE 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:L/I:L/A:H';*CWE-20;CWE-754;CWE-693;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.6';DESCR 'The amdgpu VCN v4.0.3 encoder ring accepted command submissions with user fences even though this ring does not support 64-bit user fence writes. A local process with access to the DRM render node could submit a CS job using a user fence and reach an unsupported hardware path, which may cause a failed submission, GPU engine hang, GPU reset, or denial of service for graphics or media workloads. For the CVSS the PR:L is used because reliable triggering requires local code execution with access to amdgpu command submission, not full administrator privileges. The issue is not network reachable and it is triggered through the local DRM control path. Impact is at least local denial of service. In the paranoid score, limited confidentiality or integrity impact is kept because user fence handling involves GPU writes and the unsupported 64-bit write path should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3) SKIP CVE-2026-63852 UNKNOWN SKIP No affected files built, so skip this CVE NO - - unknown MAYBE SIMPLEFIX HARDWARE INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=4 ActionableScoreLower=2 ## 1. ActionableScore * Conservative score: 2 * Paranoid score: 4 * Final recommended bucket: **Borderline, manual review recommended**:: ## 2. Signal breakdown Conservative signals: * Local unprivileged trigger: +1 A local process with access to the amdgpu DRM render node can submit command submissions. This usually does not require full root privileges. * Availability impact realistic: +1 Submitting a user fence to a VCN ring that does not support 64-bit user fence writes can plausibly cause failed submissions, VCN engine hang, GPU reset, or media workload denial of service. Paranoid additional signals: * Weak or indirect corruption candidate: +1 User fence handling involves GPU-side writes to a submitted fence location, and the ring lacks support for the expected 64-bit write behavior. The patch does not show a concrete kernel memory corruption primitive, but the unsupported write path is worth review. * Integrity impact plausible: +1 Limited integrity impact is plausible in the paranoid interpretation because an unsupported GPU fence write path may affect GPU-visible memory or command completion state. This is not enough to claim arbitrary kernel memory corruption. Not applied: * No remote reachable signal. This is a local DRM command submission path. * No strong memory corruption signal. The patch does not show UAF, OOB write, double free, refcount abuse, type confusion, or arbitrary write. * No LPE signal. There is no demonstrated reclaim, callback control, controlled overwrite, or privilege boundary bypass. * No high-control API bonus. GPU command submission is a rich device API, but the patch only rejects an unsupported user fence feature and does not show control over kernel object lifetime or allocator layout. * No full rare-hardware penalty. The issue is hardware specific to VCN v4.0.3, but amdgpu DRM render-node access is an ordinary local interface on affected systems. Call-site confidence: medium. The patch only shows the ring flag change, but the `no_user_fence` flag is conventionally consumed by amdgpu CS validation to reject unsupported user fence submissions. ## 3. Reachability analysis The realistic trigger is a local user or local process that can open the amdgpu DRM render node and submit CS jobs using user fences. This is often available to desktop or GPU workload users through `/dev/dri/renderD*`, depending on device permissions and group membership. Namespaces or containers may matter if GPU render nodes are passed through into a container. In that case, a containerized workload with render-node access may trigger the affected path without host root. The path is not network reachable. A remote attacker would need an additional service or workload that accepts remote input and translates it into local amdgpu CS submissions. The vulnerable condition is hardware and driver specific. It requires VCN v4.0.3 encoder ring support and user fence submission to that ring. ## 4. Severity interpretation This behaves more like a Borderline or low Actionable Moderate candidate than a strong Important issue. The conservative interpretation is local device denial of service. The patch is primarily a validation hardening change that prevents unsupported user fence submissions. The paranoid interpretation justifies manual review because user fences imply GPU write behavior, and the unsupported 64-bit fence write path could have limited integrity consequences in GPU-visible memory. However, the patch does not demonstrate kernel memory corruption, arbitrary write, information disclosure, or a practical privilege escalation path. ## 5. One-sentence report phrase The amdgpu VCN v4.0.3 encoder ring allowed user fence submissions despite lacking 64-bit user fence write support, allowing a local render-node user to trigger an unsupported GPU path that may cause failed submissions, GPU reset, or denial of service. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Reason: the likely impact is local GPU DoS, but the user fence write semantics make this worth manual confirmation to exclude GPU-visible memory corruption or cross-process effects. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: drm/amdgpu/vcn: set no_user_fence for VCN v4.0.3 enc ring [ Upstream Commit: c12a5d35033c0640c57c10d7111c010c7b9c2c8e Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c12a5d35033c0640c57c10d7111c010c7b9c2c8e Changed files: drivers/gpu/drm/amd/amdgpu/vcn_v4_0_3.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=c12a5d35033c0640c57c10d7111c010c7b9c2c8e ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63852 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63852 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:L The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS score: 6.6 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: 2 Paranoid ActionableScore: 4 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: NO Published priority for this report (same as in Subject): MODERATE REGULAR 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).