From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63794][IMPORTANT] KVM: SVM: Fix page overflow in sev_dbg_crypt() for ENCRYPT path Date: Sun, 19 Jul 2026 09:43:56 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63794 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: 7 X-AL-KERNEL-ActionableScore-Lower: 5 X-AL-KERNEL-Commit: f701ae476cb92a3a3d8844bb39bb63b4512684c8 List-Id: CVE: CVE-2026-63794 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: KVM: SVM: Fix page overflow in sev_dbg_crypt() for ENCRYPT path Commit: f701ae476cb92a3a3d8844bb39bb63b4512684c8 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f701ae476cb92a3a3d8844bb39bb63b4512684c8 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63794 Analysis date: Sun, 19 Jul 2026 09:43:56 -0400 ActionableScore: 7 ActionableScore lower bound: 5 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A page-offset bounds bug in KVM AMD SEV `sev_dbg_crypt()` can let a local process with KVM/SEV ioctl access overflow a single-page kernel buffer in the ENCRYPT path, causing a host kernel crash and requiring manual review for possible local privilege escalation. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63794 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63794 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-2026-63794 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:U/C:H/I:H/A:H';*CWE-787;CWE-122;*CWE-20;BEST CVSS score: '7.8';DESCR 'A page overflow in KVM SVM SEV sev_dbg_crypt can occur in the ENCRYPT path because the per iteration length is bounded by the source page offset but not by the destination page offset. When d_off is greater than s_off, the single page dst_tpage buffer can be overrun by the PSP output and by the following copy into page_address(dst_tpage) plus the destination alignment offset. This is kernel memory corruption rather than a pure warning or resource leak, and KASAN reports a slab-use-after-free style bad access in sev_dbg_crypt. For the CVSS the PR:L is used because triggering requires local access to KVM/SEV ioctls such as a process that can open /dev/kvm and operate the affected VM. The issue is not network reachable. Impact is at least local denial of service via kernel crash and may allow privilege escalation due to an out of bounds kernel write. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) YES OOB DANGER VIRT UAF HARDWARE LINUS KPANIC MEMORY KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=7 ActionableScoreLower=5 ## 1. ActionableScore * Conservative score: 5 * Paranoid score: 7 * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown Conservative signals: * Local unprivileged or low-privileged trigger: +1. Triggering is through KVM VM ioctls, typically requiring access to `/dev/kvm`, not full host root. * Constrained kernel buffer overwrite: +1. The overflow is into/around a single-page temporary buffer and is limited by page-offset arithmetic. * Weak LPE concern: +1. The bug is a concrete kernel OOB write, but target selection and reliable object control are not demonstrated. * Reliable kernel crash / strong DoS: +1. KASAN shows a concrete bad write in `sev_dbg_crypt()` from the ioctl path. * Memory layout invariant restoration: +1. The patch restores consistency between transfer length and both source and destination page offsets. * Privileged kernel/device-management corruption path: +1. The corruption occurs in the KVM AMD SEV debug encryption path, a highly trusted virtualization context. * Corruption primitive highly constrained: -1. The overflow is small, up to roughly alignment-sized spillover, and the adjacent target is not shown to be attacker-selected. * Rare/configuration-dependent exposure: -1. The path requires AMD SEV/KVM support and use of SEV debug encrypt/decrypt functionality. Paranoid additional interpretation: * Memory corruption, strong corruption primitive: +2 instead of constrained-only treatment. The patch and KASAN trace show an OOB kernel write, including a `copy_from_user()` component in the ENCRYPT path. * Privilege escalation plausible: +2. Local kernel OOB writes in a KVM ioctl path are plausible LPE candidates even without a published reliable exploit. * Confidentiality and integrity impact plausible: +1/+1 in the paranoid view due to possible kernel memory corruption consequences. ## 3. Reachability analysis The bug is locally reachable through the KVM AMD SEV memory encryption ioctl path, specifically `sev_mem_enc_ioctl()` through `kvm_vm_ioctl()`. A realistic attacker needs the ability to create or control an affected KVM VM and issue SEV debug encrypt operations, which usually means access to `/dev/kvm` and suitable SEV hardware support. This is not network reachable. Namespaces do not automatically make this unprivileged, but container or service configurations that delegate `/dev/kvm` access can reduce the practical privilege barrier. The path is hardware and configuration dependent, so exposure is narrower than generic KVM, but it is still security-relevant in virtualization deployments. Call-site confidence: high. The provided trace and patch show the relevant ioctl call path and the exact overflow location. ## 4. Severity interpretation This is not an ordinary Moderate cleanup bug. The patch fixes a concrete page-boundary overflow in kernel memory, and KASAN demonstrates an actual bad write in the KVM AMD SEV path. Conservative handling is Actionable Moderate because the primitive is constrained and configuration-dependent. Paranoid handling is Strong Important candidate because the issue is a local kernel OOB write in a virtualization subsystem and may plausibly support privilege escalation with further exploit development. ## 5. One-sentence report phrase A page-offset bounds bug in KVM AMD SEV `sev_dbg_crypt()` can let a local process with KVM/SEV ioctl access overflow a single-page kernel buffer in the ENCRYPT path, causing a host kernel crash and requiring manual review for possible local privilege escalation. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: this is concrete kernel memory corruption in a KVM ioctl path, with KASAN evidence and plausible LPE relevance despite constrained overwrite size and SEV-specific reachability. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: SVM: Fix page overflow in sev_dbg_crypt() for ENCRYPT path Commit: f701ae476cb92a3a3d8844bb39bb63b4512684c8 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f701ae476cb92a3a3d8844bb39bb63b4512684c8 Commit description: In sev_dbg_crypt(), the per-iteration transfer length is bounded by the source page offset (PAGE_SIZE - s_off) but not by the destination page offset (PAGE_SIZE - d_off). When d_off > s_off, the encrypt path (__sev_dbg_encrypt_user) performs a read-modify-write using a single-page intermediate buffer (dst_tpage): 1. __sev_dbg_decrypt() expands the size to round_up(len + (d_off & 15), 16) before issuing the PSP command. If len + (d_off & 15) > PAGE_SIZE, the PSP writes beyond the end of the 4096-byte dst_tpage allocation. 2. The subsequent memcpy()/copy_from_user() into page_address(dst_tpage) + (d_off & 15) of 'len' bytes overflows by up to 15 bytes under the same condition. Trigger example: s_off = 0, d_off = 1, debug.len = PAGE_SIZE - the PSP is instructed to write round_up(4097, 16) = 4112 bytes to a 4096-byte buffer. Fix by also bounding len by (PAGE_SIZE - d_off), the same check that sev_send_update_data() already performs for its single-page guest region. ================================================================== BUG: KASAN: slab-use-after-free in sev_dbg_crypt+0x993/0xd10 [kvm_amd] Write of size 4095 at addr ff110062293bb009 by task sev_dbg_test/228214 CPU: 96 UID: 0 PID: 228214 Comm: sev_dbg_test Tainted: G U W 7.0.0-smp--5ce9b0c48211-dbg #156 PREEMPTLAZY Tainted: [U]=USER, [W]=WARN Hardware name: Google Astoria/astoria, BIOS 0.20250817.1-0 08/25/2025 Call Trace: dump_stack_lvl+0x54/0x70 print_report+0xbc/0x260 kasan_report+0xa2/0xd0 kasan_check_range+0x25f/0x2c0 __asan_memcpy+0x40/0x70 sev_dbg_crypt+0x993/0xd10 [kvm_amd] sev_mem_enc_ioctl+0x33c/0x450 [kvm_amd] kvm_vm_ioctl+0x65d/0x6d0 [kvm] __se_sys_ioctl+0xb2/0x100 do_syscall_64+0xe8/0x870 entry_SYSCALL_64_after_hwframe+0x4b/0x53 The buggy address belongs to the physical page: page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x7fe72b6a0 pfn:0x62293bb memcg:ff11000112827d82 flags: 0x1400000000000000(node=1|zone=1) raw: 1400000000000000 0000000000000000 dead000000000122 0000000000000000 raw: 00000007fe72b6a0 0000000000000000 00000001ffffffff ff11000112827d82 page dumped because: kasan: bad access detected Memory state around the buggy address: ff110062293bbf00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ff110062293bbf80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >ff110062293bc000: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc ^ ff110062293bc080: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc ff110062293bc100: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc ================================================================== Disabling lock debugging due to kernel taint Fixes: 24f41fb ("KVM: SVM: Add support for SEV DEBUG_DECRYPT command") Fixes: 7d1594f ("KVM: SVM: Add support for SEV DEBUG_ENCRYPT command") Cc: stable@vger.kernel.org Signed-off-by: Ashutosh Desai [sean: add sample KASAN splat, Fixes, and stable@] Link: https://patch.msgid.link/20260501203537.2120074-2-seanjc@google.com Signed-off-by: Sean Christopherson 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=f701ae476cb92a3a3d8844bb39bb63b4512684c8 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63794 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63794 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: Not available The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 7.8 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: 5 Paranoid ActionableScore: 7 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).