Insyde InsydeH2O (SmmResourceCheckDxe input buffer, DMA TOCTOU): The sharpest irony in the batch: SmmResourceCheckDxe

Impact

The sharpest irony in the batch: SmmResourceCheckDxe is the driver whose job is to validate SMM resource access, and its own input buffer is racy. An attacker who wins this race corrupts SMRAM through the guard rather than around it, which means the check other handlers rely on can be made to approve what it should reject.

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. 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