Insyde InsydeH2O (UsbCoreDxe USB working buffer, DMA TOCTOU): UsbCoreDxe builds its USB transaction working buffer

Impact

UsbCoreDxe builds its USB transaction working buffer outside SMRAM while the code consuming it runs inside SMM. The driver tries to sanitise pointers against a list of known-good buffer locations, but a pointer that misses the list is used anyway - so DMA tampering mid-transaction corrupts SMRAM and escalates to ring -2. A more subtle failure than the rest of the batch: the validation exists, it just does not fail closed.

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 the kernel releases named in the advisory (Insyde does not enumerate per-kernel versions for this one). Disabling USB legacy support on headless nodes reduces how often the vulnerable transaction path runs. 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