Exploiting System Management Mode with a very long interrupt

Researchers discovered a vulnerability in x86 CPU System Management Mode (SMM) that allows breaking its security model by exploiting an extremely long-running machine instruction. The attack works by having one CPU core execute an instruction lasting billions of cycles, preventing it from responding to SMM entry requests while another core exits SMM and performs unauthorized operations. This vulnerability exposes a fundamental flaw in SMM's synchronization mechanism, which assumes all cores can enter the secure environment nearly simultaneously.