• Emese Revfy's avatar
    gcc-plugins: Add latent_entropy plugin · 38addce8
    Emese Revfy authored
    This adds a new gcc plugin named "latent_entropy". It is designed to
    extract as much possible uncertainty from a running system at boot time as
    possible, hoping to capitalize on any possible variation in CPU operation
    (due to runtime data differences, hardware differences, SMP ordering,
    thermal timing variation, cache behavior, etc).
    At the very least, this plugin is a much more comprehensive example for
    how to manipulate kernel code using the gcc plugin internals.
    The need for very-early boot entropy tends to be very architecture or
    system design specific, so this plugin is more suited for those sorts
    of special cases. The existing kernel RNG already attempts to extract
    entropy from reliable runtime variation, but this plugin takes the idea to
    a logical extreme by permuting a global variable based on any variation
    in code execution (e.g. a different value (and permutation function)
    is used to permute the global based on loop count, case statement,
    if/then/else branching, etc).
    To do this, the plugin starts by inserting a local variable in every
    marked function. The plugin then adds logic so that the value of this
    variable is modified by randomly chosen operations (add, xor and rol) and
    random values (gcc generates separate static values for each location at
    compile time and also injects the stack pointer at runtime). The resulting
    value depends on the control flow path (e.g., loops and branches taken).
    Before the function returns, the plugin mixes this local variable into
    the latent_entropy global variable. The value of this global variable
    is added to the kernel entropy pool in do_one_initcall() and _do_fork(),
    though it does not credit any bytes of entropy to the pool; the contents
    of the global are just used to mix the pool.
    Additionally, the plugin can pre-initialize arrays with build-time
    random contents, so that two different kernel builds running on identical
    hardware will not have the same starting values.
    Signed-off-by: default avatarEmese Revfy <re.emese@gmail.com>
    [kees: expanded commit message and code comments]
    Signed-off-by: default avatarKees Cook <keescook@chromium.org>
latent_entropy_plugin.c 17 KB