From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-31787][MODERATE 7.0] xen/privcmd: fix double free via VMA splitting Date: Thu, 21 May 2026 16:23:40 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-31787 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: 8 X-AL-KERNEL-ActionableScore-Lower: 4 X-AL-KERNEL-Commit: dbf862ce9f009128ab86b234d91413a3e450beb4 List-Id: CVE: CVE-2026-31787 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: xen/privcmd: fix double free via VMA splitting Commit: dbf862ce9f009128ab86b234d91413a3e450beb4 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dbf862ce9f009128ab86b234d91413a3e450beb4 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31787 Analysis date: Thu, 21 May 2026 16:23:40 -0400 ActionableScore: 8 ActionableScore lower bound: 4 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A Xen privcmd VMA splitting bug can duplicate vm_private_data across VMAs, causing privcmd_close to free the same pages array twice and creating a local double-free memory corruption condition in a privileged virtualization control path. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-31787 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31787 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-2026-31787 MODERATE CHECK 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-415;*CWE-416;*CWE-664;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A double free can occur in Xen privcmd mappings because partial munmap can split a VMA and duplicate vm_private_data without an open callback or private reference fixup. Both VMAs can then point to the same pages array, and privcmd_close can free the same allocation twice when the split and surviving mappings are destroyed. For the CVSS the PR:L is used for the paranoid score because privcmd access may be delegated to Xen toolstack or device model processes that are not equivalent to full host root in practical risk terms. In typical host deployments PR:H is also reasonable because direct privcmd mapping normally requires administrative Xen control access. Impact is at least local denial of service via kernel memory corruption and in worst case may allow confidentiality or integrity impact because the primitive is a real double free.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6) YES DANGER LINUS KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=8 ActionableScoreLower=4 ## 1. ActionableScore * Conservative score: 4 * Paranoid score: 8 * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown Conservative signals: * Memory corruption, strong corruption primitive: +2. The patch describes a real double free of the same pages array after VMA splitting duplicates vm_private_data. * Real lifetime corruption: +1. The surviving VMA keeps a dangling pointer after the first privcmd_close frees the shared pages array. * Reliable kernel crash / strong DoS: +1. Double free in kernel memory management paths is a strong crash or corruption condition. * Privileged kernel/device-management memory corruption path: +1. This occurs in Xen privcmd, a highly trusted virtualization control interface. * Availability impact realistic: +1. A user able to create the mapping and partially unmap it can trigger kernel memory corruption or panic. * Requires admin/root/Xen control access in typical deployments: -2. Direct access to privcmd is normally restricted to dom0 root or Xen toolstack processes. Paranoid signals: * Local reduced-privilege trigger concern: +1. In real Xen deployments, privcmd access may be delegated to toolstack, qemu device model, or service processes that are not equivalent to full host root. * Strong LPE plausibility: +2. A double free is a real heap lifetime corruption primitive and may be exploitable beyond DoS if allocation reuse can be influenced. * Confidentiality impact plausible: +1. Kernel heap corruption may enable disclosure through corrupted objects, although no direct leak is shown. * Integrity impact plausible: +1. Double free can corrupt allocator state or reused kernel objects. * No remote/network signal. This is a local Xen control interface issue, not packet-triggered. ## 3. Reachability analysis The bug is triggered by userspace creating a Xen privcmd mapping and then performing a partial munmap that causes VMA splitting. In typical systems, /dev/xen/privcmd is restricted to dom0 root or trusted Xen management components, so the conservative interpretation applies a privilege penalty. However, Xen deployments often involve reduced-privilege toolstack or device-model processes with access to Xen control interfaces, and compromise of such a process is not equivalent to full host root. Namespaces do not meaningfully make this a normal unprivileged Linux interface. The path is only relevant on Xen systems with privcmd enabled and accessible. ## 4. Severity interpretation This is not an ordinary Moderate NULL dereference or resource leak. It is a concrete double free caused by duplicated VMA private data and missing split prevention. Realistic exploitation is likely at least local DoS from a privileged or semi-privileged Xen control context. Practical privilege escalation is not demonstrated by the patch alone, but the primitive is strong enough that Xen environments should treat it as an Important candidate requiring manual review. ## 5. One-sentence report phrase A Xen privcmd VMA splitting bug can duplicate vm_private_data across VMAs, causing privcmd_close to free the same pages array twice and creating a local double-free memory corruption condition in a privileged virtualization control path. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: the issue is a confirmed double free in a Xen kernel interface, with realistic DoS and plausible corruption-based privilege or isolation impact depending on how privcmd access is delegated. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: xen/privcmd: fix double free via VMA splitting Commit: dbf862ce9f009128ab86b234d91413a3e450beb4 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dbf862ce9f009128ab86b234d91413a3e450beb4 Commit description: privcmd_vm_ops defines .close (privcmd_close), but neither .may_split nor .open. When userspace does a partial munmap() on a privcmd mapping, the kernel splits the VMA via __split_vma(). Since may_split is NULL, the split is allowed. vm_area_dup() copies vm_private_data (a pages array allocated in alloc_empty_pages()) into the new VMA without any fixup, because there is no .open callback. Both VMAs now point to the same pages array. When the unmapped portion is closed, privcmd_close() calls: - xen_unmap_domain_gfn_range() - xen_free_unpopulated_pages() - kvfree(pages) The surviving VMA still holds the dangling pointer. When it is later destroyed, the same sequence runs again, which leads to a double free. Fix this issue by adding a .may_split callback denying the VMA split. This is XSA-487 / CVE-2026-31787 Fixes: d71f513 ("xen: privcmd: support autotranslated physmap guests.") Reported-by: Atharva Vartak Suggested-by: Atharva Vartak Signed-off-by: Juergen Gross Reviewed-by: Jan Beulich Signed-off-by: Greg Kroah-Hartman Changed files: drivers/xen/privcmd.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=dbf862ce9f009128ab86b234d91413a3e450beb4 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-31787 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31787 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:H/UI:N/S:U/C:L/I:L/A:H 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: 4 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: 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).