Insyde InsydeH2O (FvbServicesRuntimeDxe input buffer, DMA TOCTOU): Firmware Volume Block services are the abstraction

Impact

Firmware Volume Block services are the abstraction SMM uses to read and write the SPI flash. A DMA race on this handler's input buffer means an attacker influences what gets written to the boot flash - the most direct path in this whole batch to firmware that survives OS reinstall, disk replacement and reimaging between tenants.

Who can reach it

An attacker able to drive DMA at host memory while the SMI handler is mid-flight - a malicious PCIe device, a peripheral running attacker-flashed firmware (NIC, GPU, NVMe), or a tenant with a passed-through device that is not behind a correctly configured IOMMU. Notably does NOT require host root, which is what separates this family from the ordinary SMM callout bugs.

What to do

Firmware flash from the server OEM, not from Insyde - the fixed Insyde kernel has to be rebased by Dell/HPE/Lenovo/Supermicro and re-qualified before it reaches you, which for this batch ran months behind Insyde's own release. One reboot per node, so schedule it against a GPU drain. Fixed in kernel 5.2 / 05.27.21, 5.3 / 05.36.21, 5.4 / 05.44.21, 5.5 / 05.52.21. Verify SPI write protection (BIOS Lock Enable, protected range registers) is actually set - it blunts the primitive even before the flash lands. The compensating control that actually works here is the IOMMU, and Insyde says so in the advisory: enable VT-d/AMD-Vi with pre-boot DMA protection so the ACPI runtime buffer the handler reads is not reachable by an untrusted device. That is a BIOS setting, deployable fleet-wide without a flash, and it should be on already on any node that passes devices through to tenants. Patch the batch, not the CVE - Insyde filed one advisory per driver for the same defect, so fixing this one leaves every sibling handler reachable.

References