Re: [PATCH v4] random: absorb fast pool into input pool after fast load

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Feb 21, 2022 at 03:58:16PM +0100, Jason A. Donenfeld wrote:
> During crng_init == 0, we never credit entropy in add_interrupt_
> randomness(), but instead dump it directly into the primary_crng. That's
> fine, except for the fact that we then wind up throwing away that
> entropy later when we switch to extracting from the input pool and
> xoring into (and later in this series overwriting) the primary_crng key.
> The two other early init sites -- add_hwgenerator_randomness()'s use
> crng_fast_load() and add_device_ randomness()'s use of crng_slow_load()
> -- always additionally give their inputs to the input pool. But not
> add_interrupt_randomness().
> 
> This commit fixes that shortcoming by calling mix_pool_bytes() after
> crng_fast_load() in add_interrupt_randomness(). That's partially
> verboten on PREEMPT_RT, where it implies taking spinlock_t from an IRQ
> handler. But this also only happens during early boot and then never
> again after that. Plus it's a trylock so it has the same considerations
> as calling crng_fast_load(), which we're already using.
> 
> Cc: Theodore Ts'o <tytso@mit.edu>
> Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
> Suggested-by: Eric Biggers <ebiggers@google.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> v4 has no changes except for a commit message nit, per Eric's request.
> 
>  drivers/char/random.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index d31b0b3afe2e..f3179c67010b 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -850,6 +850,10 @@ void add_interrupt_randomness(int irq)
>  		    crng_fast_load((u8 *)fast_pool->pool, sizeof(fast_pool->pool)) > 0) {
>  			fast_pool->count = 0;
>  			fast_pool->last = now;
> +			if (spin_trylock(&input_pool.lock)) {
> +				_mix_pool_bytes(&fast_pool->pool, sizeof(fast_pool->pool));
> +				spin_unlock(&input_pool.lock);
> +			}
>  		}
>  		return;
>  	}
> -- 

Reviewed-by: Eric Biggers <ebiggers@google.com>

- Eric