Linux kernel (drivers/iommu/amd): The AMD IOMMU busy-waits for command completion while holding its spinlock with

Impact

The AMD IOMMU busy-waits for command completion while holding its spinlock with interrupts disabled, so under heavy DMA map/unmap pressure the whole node soft-locks. One tenant generating aggressive IOMMU traffic stalls every other tenant's device on that IOMMU - a shared-node availability failure, and the kernel CNA rated it network-reachable because fabric traffic is what drives the mapping churn.

Who can reach it

Any workload that drives high-rate DMA mapping on an AMD-Vi host with iommu.strict=1: a tenant hammering unmap through a passed-through NIC or GPU, or high packet rates through an SR-IOV VF. Conditional on strict (non-deferred) IOMMU invalidation mode; no host privilege.

What to do

Update to a stable kernel carrying commits f2f65b28 / 715c2631. Interim: on AMD hosts, avoid iommu.strict=1 where your threat model tolerates deferred invalidation (note that lazy mode itself widens the stale-mapping window, so this is a genuine trade-off), and rate-limit tenant DMA mapping churn if the stack allows.

References