● Covered by 1 source · 1 reportMedium impact1 negative
System Management Mode Vulnerability Exploited with Long-Running Instruction
Key points
- SMM requires all cores to be in or out of SMM concurrently.
- A long-running instruction on one core can prevent it from joining SMM.
- This creates a state where cores are in different SMM states.
- The exploit requires an instruction lasting over 1 second (4 billion cycles).
SMM Security Model
System Management Mode (SMM) is a secure, ultra-privileged execution environment on x86 CPUs. Its security model relies on all CPU cores entering or exiting SMM at the same time. If this synchronization fails, the integrity of the SMM environment can be compromised.
Exploitation Method
The vulnerability is exploited by initiating an exceptionally long-running machine instruction on one CPU core. While this instruction is executing, another core attempts to enter SMM and invites the busy core to join. The SMM firmware waits for a limited time (approximately 1 second, or 4 billion CPU cycles) for all cores to synchronize. If the instruction on the first core is still running when this timeout expires, the second core proceeds into SMM without the first.
Resulting Vulnerability
This desynchronization leaves one core inside SMM and another outside, breaking the fundamental security assumption of SMM. The core outside SMM can then potentially attack the core that is still operating within the privileged SMM environment. The critical factor for this exploit is the existence of a machine instruction capable of running for an extended duration, exceeding the SMM synchronization timeout.
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Primary sources
GitHub cmc_internal/api GitHub github/collect GitHub xoreaxeaxeax/smiiiiiiiiiiiiiiii GitHub _private/browser GitHub get-started/accessibility GitHub open-source/sponsorsReporting from
A vulnerability in x86 System Management Mode (SMM) allows for exploitation by using a single, extremely long-running machine instruction. This method disrupts SMM's core security model, which requires all CPU cores to enter or exit SMM simultaneously. The exploit creates a state where one core is outside SMM while another is inside, enabling attacks on the in-SMM core.