trezor-firmware/core/src/storage/sd_salt.py

from micropython import const

import storage.device
from trezor import io
from trezor.crypto import hmac
from trezor.crypto.hashlib import sha256
from trezor.utils import consteq

if False:
    from typing import Optional, TypeVar, Callable

    T = TypeVar("T", bound=Callable)

SD_CARD_HOT_SWAPPABLE = False
SD_SALT_LEN_BYTES = const(32)
SD_SALT_AUTH_TAG_LEN_BYTES = const(16)


class WrongSdCard(Exception):
    pass


def is_enabled() -> bool:
    return storage.device.get_sd_salt_auth_key() is not None


def compute_auth_tag(salt: bytes, auth_key: bytes) -> bytes:
    digest = hmac.new(auth_key, salt, sha256).digest()
    return digest[:SD_SALT_AUTH_TAG_LEN_BYTES]


def _get_device_dir() -> str:
    return "/trezor/device_{}".format(storage.device.get_device_id().lower())


def _get_salt_path(new: bool = False) -> str:
    return "{}/salt{}".format(_get_device_dir(), ".new" if new else "")


_ensure_filesystem_nesting_counter = 0


def ensure_filesystem(func: T) -> T:
    """Ensure the decorated function has access to SD card filesystem.

    Usage:
    >>> @ensure_filesystem
    >>> def do_something(arg):
    >>>     fs = io.FatFS()
    >>>     # the decorator guarantees that `fs` is mounted
    >>>     fs.unlink("/dir/" + arg)
    """
    # XXX
    # A slightly better design would be to make the decorated function take the `fs`
    # as an argument, but that is currently untypeable with mypy.
    # (see https://github.com/python/mypy/issues/3157)
    def wrapped_func(*args, **kwargs):  # type: ignore
        global _ensure_filesystem_nesting_counter

        sd = io.SDCard()
        if _ensure_filesystem_nesting_counter == 0:
            if not sd.power(True):
                raise OSError

        try:
            _ensure_filesystem_nesting_counter += 1
            fs = io.FatFS()
            fs.mount()
            # XXX do we need to differentiate failure types?
            # If yes, can the problem be derived from the type of OSError raised?
            return func(*args, **kwargs)
        finally:
            _ensure_filesystem_nesting_counter -= 1
            assert _ensure_filesystem_nesting_counter >= 0
            if _ensure_filesystem_nesting_counter == 0:
                fs.unmount()
                sd.power(False)

    return wrapped_func  # type: ignore


def _load_salt(fs: io.FatFS, auth_key: bytes, path: str) -> Optional[bytearray]:
    # Load the salt file if it exists.
    try:
        with fs.open(path, "r") as f:
            salt = bytearray(SD_SALT_LEN_BYTES)
            stored_tag = bytearray(SD_SALT_AUTH_TAG_LEN_BYTES)
            f.read(salt)
            f.read(stored_tag)
    except OSError:
        return None

    # Check the salt's authentication tag.
    computed_tag = compute_auth_tag(salt, auth_key)
    if not consteq(computed_tag, stored_tag):
        return None

    return salt


@ensure_filesystem
def load_sd_salt() -> Optional[bytearray]:
    salt_auth_key = storage.device.get_sd_salt_auth_key()
    if salt_auth_key is None:
        return None

    salt_path = _get_salt_path()
    new_salt_path = _get_salt_path(new=True)

    fs = io.FatFS()

    salt = _load_salt(fs, salt_auth_key, salt_path)
    if salt is not None:
        return salt

    # Check if there is a new salt.
    salt = _load_salt(fs, salt_auth_key, new_salt_path)
    if salt is None:
        # No valid salt file on this SD card.
        raise WrongSdCard

    # Normal salt file does not exist, but new salt file exists. That means that
    # SD salt regeneration was interrupted earlier. Bring into consistent state.
    # TODO Possibly overwrite salt file with random data.
    try:
        fs.unlink(salt_path)
    except OSError:
        pass

    # fs.rename can fail with a write error, which falls through as an OSError.
    # This should be handled in calling code, by allowing the user to retry.
    fs.rename(new_salt_path, salt_path)
    return salt


@ensure_filesystem
def set_sd_salt(salt: bytes, salt_tag: bytes, stage: bool = False) -> None:
    salt_path = _get_salt_path(stage)
    fs = io.FatFS()
    fs.mount()
    fs.mkdir("/trezor", True)
    fs.mkdir(_get_device_dir(), True)
    with fs.open(salt_path, "w") as f:
        f.write(salt)
        f.write(salt_tag)


@ensure_filesystem
def commit_sd_salt() -> None:
    salt_path = _get_salt_path(new=False)
    new_salt_path = _get_salt_path(new=True)

    fs = io.FatFS()
    try:
        fs.unlink(salt_path)
    except OSError:
        pass
    fs.rename(new_salt_path, salt_path)


@ensure_filesystem
def remove_sd_salt() -> None:
    salt_path = _get_salt_path()

    fs = io.FatFS()
    # TODO Possibly overwrite salt file with random data.
    fs.unlink(salt_path)