CVE-2021-47177

What it actually does

A sysfs leak occurs because iommu_device_sysfs_add() is invoked prior to error paths in alloc_iommu() without corresponding cleanup, leaving stale sysfs entries and associated kernel resources allocated.

Why it matters to the business

Over repeated IOMMU allocation failures the kernel may gradually exhaust limited sysfs and memory resources, potentially leading to degraded device management or stability issues in virtualized or IOMMU-enabled systems.

How an attack unfolds

An attacker with the ability to induce repeated IOMMU allocation errors (for example via crafted device configurations or driver interactions) can trigger the missing cleanup path, accumulating leaked sysfs objects without direct code execution or privilege escalation.

How to fix it

Update to a Linux kernel version containing the referenced commit that ensures iommu_device_sysfs_add() is cleaned on all subsequent error paths in alloc_iommu().

How to confirm the fix worked

Confirm the presence of the fix by inspecting the alloc_iommu() error handling for an explicit call to iommu_device_sysfs_del() (or equivalent) after the sysfs addition; test by inducing allocation failures and verifying no residual sysfs entries remain.

Typical attack path

1. Identify or create conditions that cause alloc_iommu() to fail after iommu_device_sysfs_add() succeeds
2. Repeat the error-triggering sequence to accumulate leaked sysfs entries
3. Monitor for resource pressure or observable side effects in IOMMU device enumeration

How to detect it

Inspect kernel logs and sysfs listings for unexpected IOMMU device entries that persist after allocation errors; correlate with dmesg entries referencing alloc_iommu failures.

What your security team should do

• Prioritize kernel updates on systems that rely on Intel VT-d IOMMU functionality
• Review IOMMU-related error logs for repeated allocation failures that could indicate leak accumulation
• Apply the fix during the next scheduled kernel maintenance window given low exploitability