Insyde InsydeH2O (SdHostDriver SMI input buffer, DMA TOCTOU): DMA race on the SD host controller driver gives SMRAM

Impact

DMA race on the SD host controller driver gives SMRAM corruption and ring -2 escalation. On server hardware the SD/eMMC controller is often wired to platform or BMC-adjacent boot media rather than to anything a tenant uses, which makes this an easy driver to forget about - and an attractive one for an attacker, because nobody is watching it.

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 / 05.27.25, 5.3 / 05.36.25, 5.4 / 05.44.25, 5.5 / 05.52.25. Where the platform exposes it, disabling the unused SD/eMMC controller in BIOS removes the surface. 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