Re: [PATCH v2 01/10] random: introduce drain_entropy() helper to declutter crng_reseed()

[Eric Biggers]

On Sat, Feb 12, 2022 at 01:23:09PM +0100, Jason A. Donenfeld wrote:
> In preparation for separating responsibilities, break out the entropy
> count management part of crng_reseed() into its own function.
> 
> No functional changes.
> 
> Cc: Theodore Ts'o <tytso@xxxxxxx>
> Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> ---
>  drivers/char/random.c | 35 +++++++++++++++++++++++------------
>  1 file changed, 23 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 44a20a1a1b3a..436b146b33be 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -260,6 +260,7 @@ static struct {
>  };
>  
>  static void extract_entropy(void *buf, size_t nbytes);
> +static bool drain_entropy(void *buf, size_t nbytes);
>  
>  static void crng_reseed(void);
>  
> @@ -453,22 +454,13 @@ static void crng_slow_load(const void *cp, size_t len)
>  static void crng_reseed(void)
>  {
>  	unsigned long flags;
> -	int entropy_count;
>  	unsigned long next_gen;
>  	u8 key[CHACHA_KEY_SIZE];
>  	bool finalize_init = false;
>  
> -	/* First we make sure we have POOL_MIN_BITS of entropy in the pool,
> -	 * and then we drain all of it. Only then can we extract a new key.
> -	 */
> -	do {
> -		entropy_count = READ_ONCE(input_pool.entropy_count);
> -		if (entropy_count < POOL_MIN_BITS)
> -			return;
> -	} while (cmpxchg(&input_pool.entropy_count, entropy_count, 0) != entropy_count);
> -	extract_entropy(key, sizeof(key));
> -	wake_up_interruptible(&random_write_wait);
> -	kill_fasync(&fasync, SIGIO, POLL_OUT);
> +	/* Only reseed if we can, to prevent brute forcing a small amount of new bits. */
> +	if (!drain_entropy(key, sizeof(key)))
> +		return;
>  
>  	/* We copy the new key into the base_crng, overwriting the old one,
>  	 * and update the generation counter. We avoid hitting ULONG_MAX,
> @@ -893,6 +885,25 @@ static void extract_entropy(void *buf, size_t nbytes)
>  	memzero_explicit(&block, sizeof(block));
>  }
>  
> +/*
> + * First we make sure we have POOL_MIN_BITS of entropy in the pool,
> + * and then we drain all of it. Only then can we extract a new key
> + * with extract_entropy().
> + */
> +static bool drain_entropy(void *buf, size_t nbytes)
> +{
> +	unsigned int entropy_count;
> +	do {
> +		entropy_count = READ_ONCE(input_pool.entropy_count);
> +		if (entropy_count < POOL_MIN_BITS)
> +			return false;
> +	} while (cmpxchg(&input_pool.entropy_count, entropy_count, 0) != entropy_count);
> +	extract_entropy(buf, nbytes);
> +	wake_up_interruptible(&random_write_wait);
> +	kill_fasync(&fasync, SIGIO, POLL_OUT);
> +	return true;
> +}

Looks good, but perhaps the comment should clarify that drain_entropy() doesn't
destroy the entropy in the pool, but rather just extracts it and zeroes out the
entropy counter?

- Eric