System Management Mode

System Management Mode (SMM) is an operating mode first released with the Intel 386SL and available in later microprocessors in the x86 architecture, in which all normal execution (including the operating system) is suspended, and special separate software (usually firmware or a hardware-assisted debugger) is executed in high-privilege mode.

Usage

Some uses of SMM are:

• primarily to handle system events like memory or chipset errors
• system safety functions, such as shutdown on high CPU temperature.
• power management operations, such as turning on fans.
• to emulate motherboard hardware that is unimplemented or buggy.
• to emulate a PS/2 mouse or keyboard from a USB one.
• system configuration, such as on Toshiba and IBM notebook computers
• to run high-privileged rootkits as shown at Black Hat 2008.
• to emulate or forward calls to a Trusted Platform Module (TPM).

Entering SMM

SMM is entered via the SMI (system management interrupt), which is caused by:

• motherboard hardware or chipset signaling via a designated pin of the processor chip. This signal can be an independent event
• SW SMI triggered by the system software via an I/O access to a location considered special by the motherboard logic (port 0B2h is common)
• an IO write to a location which the firmware has requested that the processor chip act on

Problems

• By design, the OS cannot override or disable SMIs.
• Since the SMM code (SMI handler) is installed by the system firmware (BIOS), the OS and the SMM code may have expectations about hardware settings that are incompatible, such as different ideas of how the APIC should be set up.
• Operations in SMM take CPU time away from the OS, since the CPU state must be stored to memory (SMRAM) and any write back caches must be flushed. This can destroy real-time behavior and cause clock ticks to get lost. Windows/Linux define an SMI Timeout within which SMM Handlers should complete their job and return control back to OS normal operations. Otherwise the OS will crash.
• A digital logic analyser may be required to determine if SMM is occurring.
• Recovering the SMI handler code to analyze it for bugs, vulnerabilities, and secrets requires a logic analyzer or dissassembly of the system firmware.
• SMI handling may cause unacceptable latencies in real-time systems.

See also

• MediaGX processor which implements nonexistent hardware via SMM
• Intel 80486SL
• Extensible Firmware Interface
• Coreboot implements an open source SMM/SMI handler for some chipsets

External links

• Badness of SMM, part 1 and part 2
• Intel 32/64 Architectures Software Developer’s Manual Volume 3B: System Programming Guide, Part 2
• AMD Hammer BIOS and Kernel Developer's guide, Chapter 6
• An exploit against SMM based on OpenBSD
• SMM based rootkit
• SMM Rootkit limitations. (and how to defeat it)