• Linus Torvalds's avatar
    random: make get_random_int() more random · 8a0a9bd4
    Linus Torvalds authored
    It's a really simple patch that basically just open-codes the current
    "secure_ip_id()" call, but when open-coding it we now use a _static_
    hashing area, so that it gets updated every time.
    
    And to make sure somebody can't just start from the same original seed of
    all-zeroes, and then do the "half_md4_transform()" over and over until
    they get the same sequence as the kernel has, each iteration also mixes in
    the same old "current->pid + jiffies" we used - so we should now have a
    regular strong pseudo-number generator, but we also have one that doesn't
    have a single seed.
    
    Note: the "pid + jiffies" is just meant to be a tiny tiny bit of noise. It
    has no real meaning. It could be anything. I just picked the previous
    seed, it's just that now we keep the state in between calls and that will
    feed into the next result, and that should make all the difference.
    
    I made that hash be a per-cpu data just to avoid cache-line ping-pong:
    having multiple CPU's write to the same data would be fine for randomness,
    and add yet another layer of chaos to it, but since get_random_int() is
    supposed to be a fast interface I did it that way instead. I considered
    using "__raw_get_cpu_var()" to avoid any preemption overhead while still
    getting the hash be _mostly_ ping-pong free, but in the end good taste won
    out.
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    8a0a9bd4
random.c 49.3 KB