Insyde InsydeH2O (FwBlockServiceSmm input buffer, DMA TOCTOU): The firmware block service is the SMM-side writer

Impact

The firmware block service is the SMM-side writer for the boot flash. Racing its input buffer with DMA corrupts SMRAM and puts the attacker on the flash-write path - a persistent implant that no reimage between tenant leases will remove, sitting underneath Secure Boot and underneath whatever the node later attests.

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 / 05.27.23, 5.3 / 05.36.23, 5.4 / 05.44.23, 5.5 / 05.52.23. Confirm SPI flash write protection is enforced as a stopgap. 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