From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98508][MODERATE REGULAR] drm/virtio: fix NULL pointer dereference on fence allocation failure Date: Fri, 23 Oct 2026 21:53:10 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98508 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: 2 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 9571ab7493561afb4624d69572a7ebb67d76d10f List-Id: CVE: CVE-2026-98508 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: drm/virtio: fix NULL pointer dereference on fence allocation failure Commit: 9571ab7493561afb4624d69572a7ebb67d76d10f Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9571ab7493561afb4624d69572a7ebb67d76d10f Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98508 Analysis date: Fri, 23 Oct 2026 21:53:10 -0400 ActionableScore: 2 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: An unchecked fence allocation failure in virtio_gpu_init_submit can pass a NULL fence to virtio_gpu_fence_event_create during a local DRM execbuffer ioctl, causing an Oops or kernel panic under memory pressure without evidence of memory corruption or privilege escalation. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98508 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98508 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: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-98508 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98508 CHECK WITH IMPACT FROM ORIG NN NONE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE RESULT=DIFF V39_AEGIS_ONE_STEP_UP=LOW->MODERATE TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-252;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'An unchecked allocation failure in virtio_gpu_init_submit can pass a NULL fence to virtio_gpu_fence_event_create during a DRM execbuffer ioctl. A local process with access to the virtio GPU DRM device can reach the path when a submission requires a fence and allocation fails under memory pressure. For the CVSS the PR is L because a local user context and device access are required. Triggering the exact failure has high complexity because the attacker must cause or coincide with the fence allocation failure. The resulting primitive is a NULL based invalid access at a fixed low address and the supported impact is denial of service only. There is no supported remote trigger memory corruption information disclosure privilege escalation or virtual machine escape. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to MODERATE because the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.6 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE KASAN OOB NULLPTR VIRT NOMEMCHK KERNEL_PANIC_PLUS_UAF HARDWARE LINUS TEST V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw conservative score: 2 * Conservative score: 2 * Raw paranoid score: 2 * Paranoid score: 2 * Final recommended bucket: **Ordinary Moderate**:: The conservative CVSS vector is CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L with score 2.5. The highest defensible paranoid CVSS vector is CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H with score 4.7. No ActionableScore floor or cap changes the result. The paranoid CVSS permits A:H when the Oops is fatal under the affected kernel configuration, but this is the same NULL dereference consequence and does not earn another ActionableScore point. 2. Signal breakdown * +1: A below-host-admin local process with access to the virtio GPU DRM render interface can invoke the execbuffer ioctl. * +1: The supplied Syzkaller trace demonstrates a kernel NULL pointer dereference and Oops. * +0: The access is a fixed NULL based invalid access near address 0x90. It is not a heap OOB write, UAF, arbitrary write, or controlled corruption primitive. * +0: System-wide panic is configuration dependent and is not counted separately from the demonstrated Oops. * +0: No confidentiality or integrity impact is supported. * +0: No remote reachability, guest-to-host escape, privilege escalation, controlled reclaim, callback dispatch, or authorization bypass is established. * +0: B5-R does not apply. There is no retry-accumulated heap OOB write or later consumption of corrupted heap state. * +0: B3-PENDING does not apply. No shared or file-backed storage write is involved. * +0: B4-PENDING does not apply. The patch restores allocation failure handling rather than an authorization decision. CWE-476 is primary because a NULL pointer is dereferenced. CWE-252 and CWE-703 also apply because the allocation failure return was not checked or propagated. 3. Reachability analysis The demonstrated path is virtio_gpu_execbuffer_ioctl to virtio_gpu_init_submit to virtio_gpu_fence_event_create. The path is entered through a local DRM execbuffer ioctl when the submission requires an output fence because of VIRTGPU_EXECBUF_FENCE_FD_OUT, output sync objects, buffer object handles, or a DRM fence event. The complete trigger also requires virtio_gpu_fence_alloc to return NULL. Memory pressure can produce this condition, but reliably forcing this specific small allocation to fail while retaining enough execution progress makes AC:H appropriate. The virtio GPU execbuffer ioctl is a render-capable DRM interface. An authorized render-node client can therefore be below host administrator privilege, although ordinary users without permission to open the device cannot reach it. For the CVSS the PR is L because a local user context and access to the DRM device file are required. Host UID 0 and host-administrator capabilities are not inherent requirements. An ordinary logged-in user is relevant only when granted access to the render node. A compositor or delegated graphics service can also reach the path if it holds the device file. DRM master status is not established as necessary for this render ioctl. Namespace capabilities do not independently grant access to the device, so the trigger is not classified as namespace-scoped. A virtio GPU device and the corresponding driver are prerequisites. In a virtual machine, this defect affects the kernel running the virtio GPU driver, normally the guest kernel. The supplied evidence does not establish corruption of the host or a virtual machine escape. No network trigger or special physical hardware is involved. 4. Primitive and severity interpretation The root cause is failure to check the return value of virtio_gpu_fence_alloc. On allocation failure, virtio_gpu_init_submit retains a NULL out_fence and passes it to virtio_gpu_fence_event_create. That function unconditionally accesses a fence member near offset 0x90, producing the reported NULL pointer dereference. The exact primitive is a fixed NULL based invalid access. It is not an active structured-state corruption primitive, stale object use, UAF, OOB access, repeated release, refcount mutation, or authorization-state error. The disassembly shows an intended store relative to a NULL base, but the low unmapped target produces a fault rather than a useful attacker-directed write. The demonstrated consequence is an Oops. Process termination or local graphics disruption is conservative. A kernel panic and system-wide denial of service are defensible only for configurations where this Oops is fatal. No useful corruption remains after excluding the crash, and no evidence supports information disclosure, protected-state modification, privilege escalation, or guest-to-host compromise. PrimitiveClass is CLASS_C because the behavior is a crash-capable invalid-state path without an independently useful corruption primitive. ImportantCandidate is NO because there is no unresolved Important-grade primitive. AutoDowngradeAllowed is YES because the issue is a constrained allocation-failure NULL dereference with DoS-only evidence. 5. One-sentence report phrase An unchecked fence allocation failure in virtio_gpu_init_submit can pass a NULL fence to virtio_gpu_fence_event_create during a local DRM execbuffer ioctl, causing an Oops or kernel panic under memory pressure without evidence of memory corruption or privilege escalation. 6. Manual review recommendation NO MANUAL CHECK NEEDED The patch and call path establish a local allocation-failure NULL dereference with no UAF, OOB write, authorization bypass, cross-boundary corruption, or useful post-fault primitive. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: drm/virtio: fix NULL pointer dereference on fence allocation failure Commit: 9571ab7493561afb4624d69572a7ebb67d76d10f Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9571ab7493561afb4624d69572a7ebb67d76d10f Commit description: virtio_gpu_fence_alloc() can fail due to memory pressure and return NULL, but its caller like virtio_gpu_init_submit() never checks it. Later, virtio_gpu_init_submit() passes the NULL fence to virtio_gpu_fence_event_create(), which unconditionally dereferences it. Found when fuzzing the virtio driver with Syzkaller: Oops: general protection fault, probably for non-canonical address 0xdffffc0000000012: 0000 [#1] SMP KASAN NOPTI KASAN: null-ptr-deref in range [0x0000000000000090-0x0000000000000097] CPU: 1 UID: 0 PID: 9991 Comm: syz.0.121 Not tainted 7.2.0 #4 PREEMPT(full) Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014 RIP: 0010:virtio_gpu_fence_event_create drivers/gpu/drm/virtio/virtgpu_submit.c:295 [inline] RIP: 0010:virtio_gpu_init_submit drivers/gpu/drm/virtio/virtgpu_submit.c:398 [inline] RIP: 0010:virtio_gpu_execbuffer_ioctl+0xc78/0x1aa0 drivers/gpu/drm/virtio/virtgpu_submit.c:505 Code: 85 ed 0f 85 21 09 00 00 e8 05 5a c9 fb 48 8b 44 24 10 48 8d b8 90 00 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 9a 0d 00 00 48 8b 44 24 10 4c 89 b0 90 00 00 00 RSP: 0018:ffffc900039dfad0 EFLAGS: 00010216 RAX: dffffc0000000000 RBX: ffffc900039dfdd8 RCX: ffffffff85f6fd3d RDX: 0000000000000012 RSI: ffffffff85f6fd4b RDI: 0000000000000090 RBP: 0000000000000000 R0virtio_gpu_virgl_process_cmd: ctrl 0x102, error 0x1203 R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880132c4000 R13: 0000000000000000 R14: ffff888073b6c700 R15: 000000000000003b FS: 00007fab480b96c0(0000) GS:ffff8880eb6e9000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007effbf5e55a8 CR3: 0000000048d19000 CR4: 0000000000350ef0 Call Trace: drm_ioctl_kernel+0x1f4/0x3e0 drivers/gpu/drm/drm_ioctl.c:817 drm_ioctl+0x5f4/0xc70 drivers/gpu/drm/drm_ioctl.c:914 vfs_ioctl fs/ioctl.c:51 [inline] __do_sys_ioctl fs/ioctl.c:597 [inline] __se_sys_ioctl fs/ioctl.c:583 [inline] __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0x116/0x800 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7fab471a82bd Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007fab480b9018 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 RAX: ffffffffffffffda RBX: 00007fab47435fa0 RCX: 00007fab471a82bd RDX: 00002000000000c0 RSI: 00000000c0406442 RDI: 0000000000000003 RBP: 00007fab480b9080 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001 R13: 00007fab47436038 R14: 00007fab47435fa0 R15: 00007ffe85ab0740 Modules linked in: ---[ end trace 0000000000000000 ]--- RIP: 0010:virtio_gpu_fence_event_create drivers/gpu/drm/virtio/virtgpu_submit.c:295 [inline] RIP: 0010:virtio_gpu_init_submit drivers/gpu/drm/virtio/virtgpu_submit.c:398 [inline] RIP: 0010:virtio_gpu_execbuffer_ioctl+0xc78/0x1aa0 drivers/gpu/drm/virtio/virtgpu_submit.c:505 Code: 85 ed 0f 85 21 09 00 00 e8 05 5a c9 fb 48 8b 44 24 10 48 8d b8 90 00 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 9a 0d 00 00 48 8b 44 24 10 4c 89 b0 90 00 00 00 RSP: 0018:ffffc900039dfad0 EFLAGS: 00010216 RAX: dffffc0000000000 RBX: ffffc900039dfdd8 RCX: ffffffff85f6fd3d RDX: 0000000000000012 RSI: ffffffff85f6fd4b RDI: 0000000000000090 RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880132c4000 R13: 0000000000000000 R14: ffff888073b6c700 R15: 000000000000003b FS: 00007fab480b96c0(0000) GS:ffff888098ae9000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f24c3759000 CR3: 0000000048d19000 CR4: 0000000000350ef0 ---------------- Code disassembly (best guess): 0: 85 ed test %ebp,%ebp 2: 0f 85 21 09 00 00 jne 0x929 8: e8 05 5a c9 fb call 0xfbc95a12 d: 48 8b 44 24 10 mov 0x10(%rsp),%rax 12: 48 8d b8 90 00 00 00 lea 0x90(%rax),%rdi 19: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax 20: fc ff df 23: 48 89 fa mov %rdi,%rdx 26: 48 c1 ea 03 shr $0x3,%rdx * 2a: 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1) <-- trapping instruction 2e: 0f 85 9a 0d 00 00 jne 0xdce 34: 48 8b 44 24 10 mov 0x10(%rsp),%rax 39: 4c 89 b0 90 00 00 00 mov %r14,0x90(%rax) Fix by checking virtio_gpu_fence_alloc() in virtio_gpu_init_submit() and returning -ENOMEM before any later code can dereference the NULL fence. Fixes: 70d1ace ("drm/virtio: Conditionally allocate virtio_gpu_fence") Cc: stable@vger.kernel.org [Commit description truncated; see the upstream URL below] Changed files: drivers/gpu/drm/virtio/virtgpu_submit.c drivers/gpu/drm/drm_ioctl.c fs/ioctl.c arch/x86/entry/syscall_64.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=9571ab7493561afb4624d69572a7ebb67d76d10f ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98508 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98508 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:H/PR:L/UI:N/S:U/C:N/I:N/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.7 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: unknown Paranoid ActionableScore: 2 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).