[CVE-2026-64098][MODERATE 7.0] drm/virtio: use uninterruptible resv lock for plane updates

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-64098
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: drm/virtio: use uninterruptible resv lock for plane updates
Commit: c86077d512ee980cc91322211d35dbcd3175f64c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c86077d512ee980cc91322211d35dbcd3175f64c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64098
Analysis date: Sun, 19 Jul 2026 14:37:11 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local DRM user can trigger a virtio_gpu plane update path where a failed dma_resv lock is ignored, allowing dma_resv_add_fence to run without the required lock and potentially corrupt the fence list, causing at least a local kernel crash and requiring manual review for memory-corruption impact.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64098 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64098

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-2026-64098	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-362;CWE-667;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race condition in the virtio GPU DRM plane update path can occur because the return value from virtio_gpu_array_lock_resv was ignored. If dma_resv_lock_interruptible returns EINTR or dma_resv_reserve_fences returns ENOMEM, the reservation lock may not be held while the later queue path still calls dma_resv_add_fence. This violates the dma_resv locking contract and can mutate the fence list concurrently with other readers or writers. With lockdep enabled this is reported as a warning, but without lockdep the same condition can corrupt the dma_resv fence list and may crash the kernel. For the CVSS the PR:L is used because a local user or local process with access to the DRM cursor ioctl can reach the affected plane update path, while full administrator privileges are not inherently required in typical graphics or virtual GPU deployments. The issue is not network reachable. Impact is at least local denial of service via kernel warning or crash, and in the paranoid score may include confidentiality and integrity impact because unlocked mutation of a shared kernel list is a plausible memory corruption candidate.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	LOCK DEADLOCK VIRT INIT RACE DMAorINTERRUPT HARDWARE LINUS SYZBOT KPANIC MEMORY 	YES	NO	checked

======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process with access to the DRM cursor or plane update ioctl path can reach the affected code.
* Memory corruption, weak or indirect corruption candidate: +1. The issue can call dma_resv_add_fence without holding the required dma_resv lock, which can corrupt the fence list, but no controlled overwrite or reclaim primitive is shown.
* Reliable kernel crash / strong DoS: +1. Lockdep reports a WARN, and the commit explicitly states that list corruption can lead to kernel instability or crash.
* Broad/common virtualization graphics exposure: +1. virtio_gpu is common in VM graphics deployments, although not universal.
* Hard or unreliable trigger condition: -1. The demonstrated syzbot path uses fault injection for ENOMEM, and the EINTR path requires signal interruption during lock wait.

Paranoid additional signals:

* Weak LPE concern: +1. Unlocked mutation of a shared kernel list is a plausible corruption sink, but exploitation beyond DoS is not demonstrated.
* Privileged kernel/device-management memory corruption path: +1. The corruption occurs in a trusted DRM/dma_resv synchronization object used for GPU buffer fencing.
* Integrity impact plausible: +1. In the paranoid interpretation, corrupted dma_resv fence-list state could affect synchronization semantics or object state.
* Confidentiality impact is not strongly supported. No read-back, info leak, or controlled disclosure primitive is shown.

## 3. Reachability analysis

The bug is locally reachable through the DRM_IOCTL_MODE_CURSOR / plane update path when virtio_gpu is present. Full root privileges are not inherently required because DRM device nodes are commonly accessible to logged-in local users through seat management, although exact access depends on distribution policy and device-node permissions.

Containers only matter if the virtio GPU DRM device is passed through or exposed into the container. The issue is not network reachable. It is also not a generic host-wide default path on systems without virtio_gpu, but virtio_gpu is common enough in virtualized desktop or graphical VM environments that this should not be treated as rare.

Call-site confidence: high. The provided patch includes both affected call sites and shows the ignored return value before dma_resv_add_fence can be reached.

## 4. Severity interpretation

This is stronger than an ordinary warning-only Moderate because the commit explicitly says that continuing without the reservation lock can corrupt the dma_resv fence list. Realistic exploitation evidence is mainly DoS or kernel warning/crash, with syzbot requiring fault injection for one path. The paranoid interpretation keeps it as Actionable Moderate because unlocked list mutation in kernel synchronization metadata is a plausible memory corruption candidate, but there is no demonstrated arbitrary write, UAF reclaim, callback control, or reliable LPE chain.

## 5. One-sentence report phrase

A local DRM user can trigger a virtio_gpu plane update path where a failed dma_resv lock is ignored, allowing dma_resv_add_fence to run without the required lock and potentially corrupt the fence list, causing at least a local kernel crash and requiring manual review for memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch explicitly identifies unlocked mutation of the dma_resv fence list and possible list corruption, which is enough to avoid auto-closing as a simple DoS, but the current evidence does not justify treating it as a strong Important-class LPE without further manual analysis.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: drm/virtio: use uninterruptible resv lock for plane updates
Commit: c86077d512ee980cc91322211d35dbcd3175f64c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c86077d512ee980cc91322211d35dbcd3175f64c

Commit description:

virtio_gpu_cursor_plane_update() and virtio_gpu_resource_flush() lock
the framebuffer BO's dma_resv via virtio_gpu_array_lock_resv() and
ignore its return value. The function can fail with -EINTR from
dma_resv_lock_interruptible() (signal during lock wait) or with
-ENOMEM from dma_resv_reserve_fences() (fence slot allocation),
leaving the resv lock not held. The queue path then walks the object
array and calls dma_resv_add_fence(), which requires the lock held;
with lockdep enabled this trips dma_resv_assert_held():

  WARNING: drivers/dma-buf/dma-resv.c:296 at dma_resv_add_fence+0x71e/0x840
  Call Trace:
   virtio_gpu_array_add_fence
   virtio_gpu_queue_ctrl_sgs
   virtio_gpu_queue_fenced_ctrl_buffer
   virtio_gpu_cursor_plane_update
   drm_atomic_helper_commit_planes
   drm_atomic_helper_commit_tail
   commit_tail
   drm_atomic_helper_commit
   drm_atomic_commit
   drm_atomic_helper_update_plane
   __setplane_atomic
   drm_mode_cursor_universal
   drm_mode_cursor_common
   drm_mode_cursor_ioctl
   drm_ioctl
   __x64_sys_ioctl

Beyond the WARN, mutating the dma_resv fence list without the lock
races with concurrent readers/writers and can corrupt the list.

Both call sites run inside the .atomic_update plane callback, which
DRM atomic helpers do not allow to fail (by the time it runs, the
commit has been signed off to userspace and there is no clean
rollback path). Moving the lock acquisition to .prepare_fb was
rejected because the broader lock scope deadlocks against other BO
locking paths in the same atomic commit.

Introduce virtio_gpu_lock_one_resv_uninterruptible() that uses
dma_resv_lock() instead of dma_resv_lock_interruptible(). This
eliminates the -EINTR failure mode -- the realistic syzbot trigger
-- without extending the lock hold across the commit. The helper
locks a single BO and rejects nents > 1 with -EINVAL; both fix
sites lock exactly one BO.

Use it from virtio_gpu_cursor_plane_update() and
virtio_gpu_resource_flush(); check the return value to handle the
remaining -ENOMEM case from dma_resv_reserve_fences() by freeing
the objs and skipping the plane update for that frame. The
framebuffer BOs touched here are not shared with other contexts
and lock contention is expected to be brief, so the loss of
signal-interruptibility is acceptable.

Other callers of virtio_gpu_array_lock_resv() (the ioctl paths)
continue to use the interruptible variant.

The bug was reported by syzbot, triggered via fault injection
(fail_nth) on the DRM_IOCTL_MODE_CURSOR path, which forces the
-ENOMEM branch in dma_resv_reserve_fences().

Reported-by: syzbot+72bd3dd3a5d5f39a0271@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=72bd3dd3a5d5f39a0271
Fixes: 5cfd31c ("drm/virtio: fix virtio_gpu_cursor_plane_update().")
Cc: stable@vger.kernel.org
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patch.msgid.link/20260519082247.34470-1-kartikey406@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/dma-buf/dma-resv.c
  drivers/gpu/drm/virtio/virtgpu_drv.h
  drivers/gpu/drm/virtio/virtgpu_gem.c
  drivers/gpu/drm/virtio/virtgpu_plane.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=c86077d512ee980cc91322211d35dbcd3175f64c

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64098 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64098

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 4
  Paranoid ActionableScore: 6

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).