[CVE-2026-98508][MODERATE REGULAR] drm/virtio: fix NULL pointer dereference on fence allocation failure

[CVE-2026-98508][MODERATE REGULAR] drm/virtio: fix NULL pointer dereference on fence allocation failure

[AL-KERNEL]

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:
	<TASK>
	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
	</TASK>
	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).