CVE-2026-46312: Linux Kernel vb2_dma_sg mmap Missing VM Flags — Medium Severity

Technical summary

The vulnerability resides in the vb2_dma_sg_mmap function within the videobuf2 subsystem. The function fails to set VM_DONTEXPAND and VM_DONTDUMP flags on virtual memory areas during mmap operations, which other similar DMA allocation paths (vb2_dma_contig) correctly implement. When imported DMA buffers from cameras using vb2_dma_sg_memops are subsequently mapped by DRM subsystem code, the missing flags trigger a WARN_ON assertion in drm_gem_mmap_obj, exposing a state inconsistency. The issue manifests during multimedia pipeline operations involving v4l2 capture and graphics rendering (e.g., gst-launch with v4l2src and GTK4 sinks).

Business impact

For most enterprise environments, impact is minimal since this affects niche multimedia and camera capture workflows, particularly on ARM-based systems like Apple Silicon Macs running specialized out-of-tree drivers. However, organizations deploying embedded vision systems, autonomous vehicle stacks, or media production workloads that combine v4l2 capture with GPU rendering should evaluate exposure. The kernel warning itself degrades system stability perception and can clutter logs, but does not enable privilege escalation, data theft, or service denial in isolation.

Affected systems

The Linux kernel is affected, specifically versions where vb2_dma_sg mmap functionality is present without the VM flags fix. ARM64 systems (particularly Apple Silicon) running newer kernels with concurrent DRM and videobuf2 integration are most visible in reported cases. Any Linux distribution shipping unpatched kernels used in camera capture, video processing, or multimedia applications could be vulnerable. The issue is architecture-neutral but most visible in systems integrating v4l2 camera drivers with GPU graphics stacks.

Exploitability

This is not exploitable for privilege escalation or code execution. The vulnerability requires local access and an active multimedia workflow combining v4l2 video capture with DRM-managed graphics buffers. The condition is triggered during normal mmap syscalls by unprivileged user processes, but the outcome is a kernel warning rather than memory corruption or system compromise. Exploitation difficulty is low (standard syscall sequence), but impact severity is low (log pollution and stability concerns rather than security breach).

Remediation

Apply Linux kernel patches that add VM_DONTEXPAND and VM_DONTDUMP flag initialization to the vb2_dma_sg_mmap function, bringing its behavior into alignment with vb2_dma_contig. Verify the patch against the upstream Linux repository. Systems not running multimedia video capture workflows combining v4l2 with GPU rendering have reduced urgency. Enterprises should prioritize kernels used in embedded vision, automotive, or production media systems.

Patch guidance

Obtain the fix from upstream Linux kernel releases or your distribution's kernel update channels. The patch modifies the vb2_dma_sg_mmap implementation to initialize vma->vm_flags with VM_DONTEXPAND and VM_DONTDUMP before the mmap operation completes, matching the pattern already used in vb2_dma_contig_mmap. Verify patch application against the vendor advisory and test in environments running v4l2 camera capture and GPU graphics workloads. Backport availability depends on your distribution's support lifecycle.

Detection guidance

Monitor kernel logs for WARN_ON messages originating from drm_gem_mmap_obj with the pattern 'WARNING: CPU.*at drivers/gpu/drm/drm_gem.c:1144'. These warnings indicate the vma->vm_flags assertion failure. Cross-reference with active multimedia processes (gst-launch, ffmpeg, camera applications) at the time of warning. Kernel taint flags will show 'W' (WARN) in dmesg output. Detection does not indicate exploitation but confirms exposure to the underlying condition.

Why prioritize this

Medium CVSS (5.5) reflects local-only access requirements and denial-of-availability impact via kernel instability rather than confidentiality or integrity breaches. Prioritization should account for whether your infrastructure runs v4l2 video capture pipelines; general Linux servers and desktops without multimedia workloads face minimal practical risk. Prioritize patching in embedded systems, automotive platforms, and media production environments where this code path is active.

Risk score, explained

CVSS 5.5 (Medium) reflects: AV:Local (requires local access), AC:Low (easy to trigger via standard mmap syscalls), PR:Low (unprivileged user can trigger), UI:None, S:Unchanged, C:None (no confidentiality impact), I:None (no integrity impact), A:High (kernel warning indicates memory protection failure risking system stability). The score appropriately captures that while exploitability is straightforward, actual harm is limited to systems with specific workload characteristics.

Frequently asked questions