SMM based flash storage driver Version 2

This documents the API exposed by the x86 system management based storage driver.

SMMSTOREv2

SMMSTOREv2 is a SMM mediated driver to read from, write to and erase a predefined region in flash. It can be enabled by setting CONFIG_SMMSTORE=y and CONFIG_SMMSTORE_V2=y in menuconfig.

This can be used by the OS or the payload to implement persistent storage to hold for instance configuration data, without needing to implement a (platform specific) storage driver in the payload itself.

Storage size and alignment

SMMSTORE version 2 requires a minimum alignment of 64 KiB, which should be supported by all flash chips. Not having to perform read-modify-write operations is desired, as it reduces complexity and potential for bugs.

This can be used by a FTW (FaultTolerantWrite) implementation that uses at least two regions in an A/B update scheme. The FTW implementation in EDK2 uses three different regions in the store:

  • The variable store
  • The FTW spare block
  • The FTW working block

All regions must be block-aligned, and the FTW spare size must be larger than that of the variable store. FTW working block can be much smaller. With 64 KiB as block size, the minimum size of the FTW-enabled store is:

  • The variable store: 1 block = 64 KiB
  • The FTW spare block: 2 blocks = 2 * 64 KiB
  • The FTW working block: 1 block = 64 KiB

Therefore, the minimum size for EDK2 FTW is 4 blocks, or 256 KiB.

API

The API provides read and write access to an unformatted block storage.

Storage region

By default SMMSTOREv2 will operate on a separate FMAP region called SMMSTORE. The default generated FMAP will include such a region. On systems with a locked FMAP, e.g. in an existing vboot setup with a locked RO region, the option exists to add a cbfsfile called smm_store in the RW_LEGACY (if CHROMEOS) or in the COREBOOT FMAP regions. It is recommended for new builds using a handcrafted FMD that intend to make use of SMMSTORE to include a sufficiently large SMMSTORE FMAP region. It is mandatory to align the SMMSTORE region to 64KiB for compatibility with the largest flash erase operation.

When a default generated FMAP is used, the size of the FMAP region is equal to CONFIG_SMMSTORE_SIZE. UEFI payloads expect at least 64 KiB. To support a fault tolerant write mechanism, at least a multiple of this size is recommended.

Communication buffer

To prevent malicious ring0 code to access arbitrary memory locations, SMMSTOREv2 uses a communication buffer in CBMEM/HOB for all transfers. This buffer has to be at least 64 KiB in size and must be installed before calling any of the SMMSTORE read or write operations. Usually, coreboot will install this buffer to transfer data between ring0 and the SMM handler.

In order to get the communication buffer address, the payload or OS has to read the coreboot table with tag 0x0039, containing:

struct lb_smmstorev2 {
	uint32_t tag;
	uint32_t size;
	uint32_t num_blocks;	/* Number of writeable blocks in SMM */
	uint32_t block_size;	/* Size of a block in byte. Default: 64 KiB */
	uint32_t mmap_addr;	/* MMIO address of the store for read only access */
	uint32_t com_buffer;	/* Physical address of the communication buffer */
	uint32_t com_buffer_size;	/* Size of the communication buffer in byte */
	uint8_t apm_cmd;	/* The command byte to write to the APM I/O port */
	uint8_t unused[3];	/* Set to zero */
};

The absence of this coreboot table entry indicates that there’s no SMMSTOREv2 support.

Blocks

The SMMSTOREv2 splits the SMMSTORE FMAP partition into smaller chunks called blocks. Every block is at least the size of 64KiB to support arbitrary NOR flash erase ops. A payload or OS must make no further assumptions about the block or communication buffer size.

Generating the SMI

SMMSTOREv2 is called via an SMI, which is generated via a write to the IO port defined in the smi_cmd entry of the FADT ACPI table. %al contains APM_CNT_SMMSTORE=0xed and is written to the smi_cmd IO port. %ah contains the SMMSTOREv2 command. %ebx contains the parameter buffer to the SMMSTOREv2 command.

Return values

If a command succeeds, SMMSTOREv2 will return with SMMSTORE_RET_SUCCESS=0 in %eax. On failure SMMSTORE will return SMMSTORE_RET_FAILURE=1. For unsupported SMMSTORE commands SMMSTORE_REG_UNSUPPORTED=2 is returned.

NOTE 1: The caller must check the return value and should make no assumption on the returned data if %eax does not contain SMMSTORE_RET_SUCCESS.

NOTE 2: If the SMI returns without changing %ax, it can be assumed that the SMMSTOREv2 feature is not installed.

Calling arguments

SMMSTOREv2 supports 3 subcommands that are passed via %ah, the additional calling arguments are passed via %ebx.

NOTE: The size of the struct entries are in the native word size of smihandler. This means 32 bits in almost all cases.

- SMMSTORE_CMD_INIT = 4

This installs the communication buffer to use and thus enables the SMMSTORE handler. This command can only be executed once and is done by the firmware. Calling this function at runtime has no effect.

The additional parameter buffer %ebx contains a pointer to the following struct:

struct smmstore_params_init {
	uint32_t com_buffer;
	uint32_t com_buffer_size;
} __packed;

INPUT:

  • com_buffer: Physical address of the communication buffer (CBMEM)
  • com_buffer_size: Size in bytes of the communication buffer

- SMMSTORE_CMD_RAW_READ = 5

SMMSTOREv2 allows reading arbitrary data. It is up to the caller to initialize the store with meaningful data before using it.

The additional parameter buffer %ebx contains a pointer to the following struct:

struct smmstore_params_raw_read {
	uint32_t bufsize;
	uint32_t bufoffset;
	uint32_t block_id;
} __packed;

INPUT:

  • bufsize: Size of data to read within the communication buffer
  • bufoffset: Offset within the communication buffer
  • block_id: Block to read from

- SMMSTORE_CMD_RAW_WRITE = 6

SMMSTOREv2 allows writing arbitrary data. It is up to the caller to erase a block before writing it.

The additional parameter buffer %ebx contains a pointer to the following struct:

struct smmstore_params_raw_write {
        uint32_t bufsize;
        uint32_t bufoffset;
        uint32_t block_id;
} __packed;

INPUT:

  • bufsize: Size of data to write within the communication buffer
  • bufoffset: Offset within the communication buffer
  • block_id: Block to write to

- SMMSTORE_CMD_RAW_CLEAR = 7

SMMSTOREv2 allows clearing blocks. A cleared block will read as 0xff. By providing multiple blocks the caller can implement a fault tolerant write mechanism. It is up to the caller to clear blocks before writing to them.

struct smmstore_params_raw_clear {
	uint32_t block_id;
} __packed;

INPUT:

  • block_id: Block to erase

Security

Pointers provided by the payload or OS are checked to not overlap with SMM. This protects the SMM handler from being compromised.

As all information is exchanged using the communication buffer and coreboot tables, there’s no risk that a malicious application capable of issuing SMIs could extract arbitrary data or modify the currently running kernel.