On Mon, Sep 26, 2022 at 6:22 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Mon, 26 Sep 2022 18:03:32 +0200 "Jason A. Donenfeld" <Jason@xxxxxxxxx> wrote: > > > The full RNG initialization relies on some timestamps, made possible > > with general functions like time_init() and timekeeping_init(). However, > > these are only available rather late in initialization. Meanwhile, other > > things, such as memory allocator functions, make use of the RNG much > > earlier. > > > > So split RNG initialization into two phases. We can give arch randomness > > very early on, and then later, after timekeeping and such are available, > > initialize the rest. > > > > This ensures that, for example, slabs are properly randomized if RDRAND > > is available. Another positive consequence is that on systems with > > RDRAND, running with CONFIG_WARN_ALL_UNSEEDED_RANDOM=y results in no > > warnings at all. > > Please give a full description of the user-visible runtime effects of > this shortcoming. Sure. I'll expand that paragraph to read: This ensures that, for example, slabs are properly randomized if RDRAND is available. Without this, CONFIG_SLAB_FREELIST_RANDOM=y loses a degree of its security, because its random seed is potentially deterministic, since it hasn't yet incorporated RDRAND. It also makes it possible to use a better seed in kfence, which currently relies on only the cycle counter. Another positive consequence is that on systems with RDRAND, running with CONFIG_WARN_ALL_UNSEEDED_RANDOM=y results in no warnings at all.
