# SMM based flash storage driver

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

## SMMSTORE

SMMSTORE 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` 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.

The API provides append-only semantics for key/value pairs.

## API

### Storage region

By default SMMSTORE 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 recommended to align the `SMMSTORE` region to 64KiB for the largest flash erase op compatibility.

When a default generated FMAP is used the size of the FMAP region is equal to `CONFIG_SMMSTORE_SIZE`. UEFI payloads expect at least 64KiB. Given that the current implementation lacks a way to rewrite key-value pairs at least a multiple of this is recommended.

### generating the SMI

SMMSTORE 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 SMMSTORE command. `%ebx` contains the parameter buffer to the SMMSTORE command.

### Return values

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

NOTE1: The caller must check the return value and should make no assumption on the returned data if `%eax` does not contain `SMMSTORE_RET_SUCCESS`.

NOTE2: If the SMI returns without changing `%ax` assume that the SMMSTORE feature is not installed.

### Calling arguments

SMMSTORE 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_CLEAR = 1

This clears the `SMMSTORE` storage region. The argument in `%ebx` is unused.

#### - SMMSTORE_CMD_READ = 2

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

    struct smmstore_params_read {
    	void *buf;
    	ssize_t bufsize;
    };

INPUT:

- `buf`: is a pointer to where the data needs to be read

- `bufsize`: is the size of the buffer

OUTPUT:

- `buf`

- `bufsize`: returns the amount of data that has actually been read.

#### - SMMSTORE_CMD_APPEND = 3

SMMSTORE takes a key-value approach to appending data. key-value pairs are never updated, they are always appended. It is up to the caller to walk through the key-value pairs after reading SMMSTORE to find the latest one.

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

    struct smmstore_params_append {
    	void *key;
    	size_t keysize;
    	void *val;
    	size_t valsize;
    };

INPUT:

- `key`: pointer to the key data

- `keysize`: size of the key data

- `val`: pointer to the value data

- `valsize`: size of the value data

#### Security

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

However there’s no validation done on the source or destination pointing to DRAM. A malicious application that is able to issue SMIs could extract arbitrary data or modify the currently running kernel.

## External links

- A Tour Beyond BIOS Implementing UEFI Authenticated Variables in SMM with EDK II

Note, this differs significantly from coreboot’s implementation.