CVE-2025-37805: Fix for virtio sound driver uninitialized work_structs

The Linux kernel has received a targeted fix for a subtle but real correctness bug in the virtio sound driver that could trigger kernel workqueue warnings and disrupt system availability in virtualized environments: CVE-2025-37805 addresses uninitialized work_structs in the virtio_snd driver so that cancel_work_sync() is never invoked on an object that was never initialized.

Background / Overview​

Virtio is the standard paravirtualized device interface used by KVM/QEMU and many cloud hypervisors to present host-backed hardware (network, block, console, sound, etc.) to guests with minimal overhead. The virtio sound driver (virtio_snd) implements a virtual audio device that enables guests to capture/playback audio through the host audio stack. Because virtio spans host and guest code paths and is enabled in many kernel builds, small correctness bugs in the virtio drivers can surface as kernel warnings, crashes, or availability problems for both guests and hosts. This particular defect lives in the virtio PCM implementation (sound/virtio/virtio_pcm.c) and was cataloged and announced by the Linux kernel CVE team.
This CVE was publicly recorded on 8 May 2025, and maintainers supplied a series of stable-branch commits that close the window by ensuring work_struct objects are properly initialized before any cancel_work_sync() cleanup occurs. Multiple vulnerability databases and vendor trackers that repackage upstream advisories list the issue and the fix; Red Hat’s bug tracker captures the original problem report and the kernel trace that led to the diagnosis.

What went wrong: a practical, code-level explanation​

At a high level, the problem is a classic initialization-versus-cleanup race in an error path:

• virtsnd_probe() is the driver probe path that builds the virtio sound device state. As part of initialization it establishes an array of PCM substreams and sets snd->nsubstreams to the count of entries allocated.
• After allocation, probe continues with additional per-substream initialization steps (for example, preparing info structures and setting up a delayed-work item used to track elapsed periods).
• If a subsequent step in probe fails (for instance, an information allocation or a control-query fails), probe unwinds into the cleanup path by calling virtsnd_remove().
• virtsnd_remove() iterates over the substreams array and unconditionally calls cancel_work_sync() on each substream’s elapsed_period work_struct.
• If a failure occurred before the elapsed_period field was initialized for a substream, cancel_work_sync() is being called on memory that does not hold a valid work_struct; the kernel detects this and emits a workqueue warning (and in worst cases the cleanup can touch invalid memory, risking crashes or a hang).

The kernel CVE announcement lists the exact call trace sample observed by the reporter and identifies the affected file(s). The maintainers’ fix is simple and pragmatic: ensure the substreams structures (and, by extension, the work_struct members they contain) are initialized immediately after allocation so that any subsequent error path that triggers virtsnd_remove() will attempt to cancel a valid, initialized work item rather than garbage.

Evidence and verification​

The Linux kernel CVE announcement (the authoritative upstream announcement distributed to linux-cve-announce) describes the problem, identifies the affected file as sound/virtio/virtio_pcm.c, and lists the stable commits that carry the fixes. Those commit references are the definitive upstream corrections and are included in the kernel-stable trees.
Multiple independent vulnerability aggregators and vendor trackers re-published the same technical summary and CVSS scoring information. Rapid7 and other databases note the kernel backport commits and summarize the call trace observed when the bug is hit; Red Hat’s bugzilla also includes the same trace and the reporter’s reproduction notes, which corroborate the upstream narrative. Cross-checking these independent sources confirms the diagnosis (uninitialized work_struct cleanup on error) and the remedy (initialize substreams immediately after allocation).
A selection of stable commits are referenced in the upstream announcement; administrators who need to review the actual diffs can consult the kernel stable patch set named in the CVE announcement. Because the maintainers published stable-branch commits, the remediation can be applied either by upgrading to a kernel release that contains those commits or by cherry-picking the backport into a vendor kernel tree—recognizing, as the kernel team emphasizes, that cherry-picking individual commits carries risk and full kernel upgrades are the recommended route.

Severity, exploitability, and practical impact​

• CVSS and severity: Multiple databases and vendors characterize this as a moderate severity bug; several trackers publish a CVSS v3.1 score around 5.5. The vector is local (attack requires local access to the guest or host that has the vulnerable driver loaded), and the primary impact is availability-related (kernel warnings, possible kernel instability).
• Attack surface: The flaw is local to code paths exercised when the device probes and when error-cleanup occurs; this implies an attacker who can manipulate device initialization or induce the probe failure (for example, by crafting a guest configuration or causing a certain device state) could trigger the condition. Remote unauthenticated exploitation is not indicated.
• Real-world consequences: The reported symptom is a kernel-level workqueue warning generated by cancel_work_sync(), which may escalate to a kernel OOPS or hang under some conditions. That translates into potential denial-of-service for the affected host or guest: noisy kernel logs, unexpected reboots, or service disruption—particularly problematic in multi-tenant virtualized infrastructures. The issue does not (based on the public record) disclose sensitive dvation-of-privilege path; availability and stability are the primary concerns.

Given the pattern of previous virtio-related fixes and the way virtio drivers interact with virtualization stacks, administrators should treat this as a stability/availability hazard that merits prompt remediation where virtio sound is used. Virtio subsystems have proven to be frequent targets for correctness bugs in the kernel; the reader should note the broader precedent of virtio fixes appearing across networking, block, and sound code over recent releases.

Who’s affected​

• Linux guests that enable the virtio sound driver (CONFIG_SND_VIRTIO) are directly in scope. Typical desktop-type guest images and some cloud images may ship with virtio sound enabled or as a module.
• Host kernels that implement virtio-snd for vhost-user or vhost-device-sound implementations can also be implicated if the same code paths are exercised in the host context.
• Virtualization platforms (KVM/QEMU, Proxmox, libvirt front-ends) that expose virtio-snd devices to guests are part of the deployment chain; a vulnerable guest kernel can crash independently of host kernel, but misbehavior that manifests as a host-guest interaction may surface on whichever side runs the vulnerable code.

Note: This is not a cross-network remote vulnerability; the attacker must be able to manipulate either the guest kernel environment or the guest configuration to exercise the probe/cleanup path that triggers the bug.

Mitigation and remediation guidance (practical steps for sysadmins)​

• Identify vulnerable systems
• Search kernel logs (dmesg or journalctl -k) for the characteristic workqueue warning and the call trace involving cancel_work_sync, __cancel_work_sync, or virtsnd_remove/virtsnd_probe. Example patterns to look for are the warning lines reported in public advisories. Use centralized logging to scan fleets for the signature trace.
• Apply vendor updates
• The recommended remediation is to install vendor-supplied kernel updates that incorporate the upstream stable commits listed in the CVE announcement. Distributions will backport these fixes into their kernel packaging; check your distribution’s security advisories or package feeds and apply kernel updates as soon as they are available.
• If immediate upgrade is impossible
• As a temporary mitigation, consider disabling or blacklisting the virtio_snd kernel module on affected systems that do not require virtual audio. This prevents the virtio sound probe paths from running and eliminates the immediate attack surface, at the cost of losing virtual audio support.
• For guests, remove virtual sound devices from VM configurations so the guest kernel never probes virtio-snd while you wait for a kernel update.
• Be careful: blacklisting or removing modules may interact with distribution packaging and life-cycle scripts; coordinate with your configuration management system.
• Avoid cherry-picking unless you know what you are doing
• The upstream kernel team explicitly discourages cherry-picking individual commits on production trees except when maintainers and packagers fully validate the backport. The safest path is to move to a kernel release or vendor patch that includes the endorsed stable commits.
• Detection and monitoring
• Add a detection rule to flag the kernel warning trace in logs and escalate to operations. Because this bug emits a clear kernel call trace in the log, it is straightforward to detect at scale.
• Test and validate
• After patching, validate by restarting affected guests or host services and monitoring kernel logs for recurrence. In environments with snapshotting and test VM pools, apply and validate in a staged rollout before broad deployment.

How to spot the problem in logs (indicators of compromise / evidence)​

Administrators should watch for kernel log entries resembling the sample trace published in the advisories: a workqueue warning referencing kernel/workqueue.c and a call trace that includes __flush_work, __cancel_work_sync, cancel_work_sync, virtsnd_remove, and virtsnd_probe. If you see those, immediate action is required—either patch or remove the virtio-snd device from the VM configuration to stop the probe path.

Why the fix is correct and low-risk​

The fix is defensive: it initializes the allocated substream structures immediately so the cleanup path always operates on a valid work_struct object. This is a standard kernel hygiene practice—initialize data before use—and the change is narrowly scoped to the driver’s probe/allocation path. The Linux kernel’s stable-branch commits address exactly this initialization gap; there is no broad refactor or API change involved, and the upstream kernel team has accepted and published the stable patches. Because the patch is small and local to sound/virtio, the risk of unintended side-effects is low compared to larger systemic changes.
That said, distribution packagers will continue the usual testing and backporting discipline, which is why operators should prefer vendor-supplied updates rather than ad-hoc cherry-picks.

Practical risk analysis for different environments​

• Desktop/laptop guests: If you or your users rely on virtualized audio, you should plan to update kernel packages when available. On single-user systems the risk is lower in practice because exploitation requires triggering a specific probe-failure path; nonetheless, noisy kernel warnings are disruptive.
• Multi-tenant cloud or virtualization hosts: The risk profile is higher. Kernel instability in guests can translate to support load and potential impact on orchestration services if many VMs are affected simultaneously. Large fleets should prioritize vendor kernel updates and use configuration management to temporarily remove virtual sound devices from images that don’t require sound.
• Embedded or appliance systems: Many appliances do not enable virtio-snd. Confirm whether CONFIG_SND_VIRTIO is enabled in the kernel builds you rely on; if not, you are not affected by this particular CVE.
• Security operations: The vulnerability is not a remote code execution or privilege escalation; it is an availability/stability issue that can be weaponized for denial-of-service in the right context. Treat it as an operational risk rather than a confidentiality or integrity breach.

What researchers and operators should watch for next​

• Backports and distro advisories: Watch vendor advisories for the fix to appear in distribution kernels. The linux-cve-announce message included commit references that maintainers can backport; vendors will announce when they release updates.
• Related virtio fixes: The virtio family of drivers has seen many small correctness fixes over recent releases. Operators should consider a broader policy of staying reasonably current with kernel maintenance releases when running virtualization stacks.
• Automated detection: Because the bug produces a consistent kernel trace, it is ideal for automated detection with log aggregation systems. Add a signature for the trace to your SIEM/ELK/Kibana rules to catch any fleet-wide occurrences quickly.

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Quick checklist for immediate action​

• Scan logs for the cancel_work_sync / virtsnd_remove trace and flag affected hosts/guests.
• Plan to apply vendor-provided kernel updates that include the stable commits listed in the CVE announcement.
• If you cannot update immediately, remove or disable virtio-snd devices or blacklist the virtio_snd module as a temporary mitigation.
• Validate fixes in a test pool and roll out updates in waves to production.
• Add log-based detection and alerting to catch regressions.

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Final analysis: strengths, limitations, and operational risk​

Strengths of the upstream response

• The fix is narrowly targeted, low risk, and implemented in the kernel stable branches so vendors can backport it. The maintainers documented the call trace and provided corrective commits, which helps packagers and operations teams respond quickly.
• The bug is readily detectable in kernel logs, so detection rules and fleet scans can identify affected systems reliably.

Limitations and caveats

• The vulnerability is local in nature, so while exploitation is not trivially remote, it remains meaningful in shared or multi-tenant virtualization environments. Attackers with guest-level access or the ability to manipulate VM configurations may be able to induce the condition at scale in cloud environments.
• Some public trackers note that reproduction has been limited in the wild; however, limited reproduction does not equal low risk—availability bugs can be triggered inadvertently by legitimate states, and the remediation is simple and low-cost.

Operational risk summary

• For most environments, the risk is manageable if operators apply vendor kernel updates in a standard maintenance window. For high-scale virtualization providers and multi-tenant clouds, prioritize the update and consider temporary device removal for vulnerable images to minimize customer impact during the patch cycle.

Conclusion​

CVE-2025-37805 is a classic kernel-initialization bug with a simple and safe upstream fix: initialize substream structures immediately after allocation so that cleanup code never calls cancel_work_sync() on uninitialized work_structs. The issue’s practical impact is availability (kernel warnings, possible OOPS or hangs) rather than data breach or privilege escalation, but in virtualized and multi-tenant contexts even a single poorly timed kernel OOPS can become a major incident. Operators should scan logs for the characteristic trace, apply vendor-supplied kernel updates that include the stable fixes, and—where immediate patching is not practical—disable virtio-snd devices or blacklist the module as a temporary mitigation. The kernel maintainers have published stable commits to correct the problem, and distribution maintainers are the right path to a safe, fully tested remediation.
By treating this as an operational stability issue, applying the upstream and vendor fixes, and adding simple log-detection rules, teams can eliminate the risk quickly and keep virtualized audio where it belongs—functional and safe, without noisy kernel surprises.

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Source: MSRC Security Update Guide - Microsoft Security Response Center