STALEUS: Breaking AMD SEV-SNP via Memory Incoherence

AMD SEV-SNP provides confidential computing capabilities on AMD platforms. To secure confidential virtual machines, AMD relies on a trusted co-processor known as the Platform Security Processor (PSP). The PSP executes on an Arm core, which is connected to the x86 interconnect via dedicated hardware bridges. We show that a malicious hypervisor can configure one of these bridges to render the PSP cache-incoherent with respect to the x86 caches. We present a novel attack called STALEUS that demonstrates how to leverage this vulnerability to break SEV-SNP. We show the impact of STALEUS by achieving arbitrary read and write primitives within a fully attested CVM