CVE-2021-47200

What it actually does

Use-after-free occurs in drm_gem_ttm_mmap when a GEM object's refcount reaches zero after the helper drops its reference on success, allowing a subsequent drm_gem_object_get to operate on freed memory.

Why it matters to the business

Kernel memory corruption or crashes in graphics subsystems can interrupt workloads on Linux systems handling display or GPU operations, with potential for privilege escalation in multi-user environments.

How an attack unfolds

An attacker with access to trigger DRM prime mmap operations on a GEM object whose reference count is exactly one causes the object to be freed during the call, then reuses the dangling pointer via the follow-on get operation to corrupt kernel memory.

How to fix it

Apply the upstream fix that acquires an extra reference before invoking the mmap helper; rebuild and deploy the patched Linux kernel on all affected systems.

How to confirm the fix worked

Confirm the presence of the reference-grab change before the mmap helper call in drm_gem_ttm_mmap; test by exercising prime mmap paths under memory-pressure conditions that produce refcount==1.

Typical attack path

1. Identify or create a DRM GEM object with refcount of 1
2. Invoke drm_gem_prime_mmap or equivalent path that reaches drm_gem_ttm_mmap
3. Trigger the reference drop that frees the object
4. Execute the subsequent drm_gem_object_get on the freed object to achieve UAF
5. Leverage the freed memory for corruption or control-flow hijacking

How to detect it

Review kernel logs for use-after-free warnings or slab corruption near drm_gem or ttm functions; inspect audit trails of DRM ioctl usage for unusual prime mmap sequences from untrusted processes.

What your security team should do

• Prioritize kernel updates on systems exposing DRM or TTM interfaces to untrusted processes
• Monitor for graphics-related kernel oops or memory-corruption alerts
• Restrict unprivileged access to /dev/dri and DRM ioctls where feasible