linux-kernel - Re: [PATCH] random: use named fields for adjusting chacha state

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Date:   Thu, 20 Jan 2022 12:50:18 -0500
From:   "Theodore Ts'o" <tytso@....edu>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] random: use named fields for adjusting chacha state

On Thu, Jan 20, 2022 at 04:07:34PM +0100, Jason A. Donenfeld wrote:
> @@ -750,13 +756,13 @@ static bool crng_init_try_arch(struct crng_state *crng)
>  	bool arch_init = true;
>  	unsigned long rv;
>  
> -	for (i = 4; i < 16; i++) {
> +	for (i = 0; i < ARRAY_SIZE(crng->key); i++) {
>  		if (!arch_get_random_seed_long(&rv) &&
>  		    !arch_get_random_long(&rv)) {
>  			rv = random_get_entropy();
>  			arch_init = false;
>  		}
> -		crng->state[i] ^= rv;
> +		crng->key[i] ^= rv;
>  	}

This change means that we're only initializing the key, but we're not
initializing the counter/nonce (well, IV) value.  Could you fix this,
please?  

> @@ -768,13 +774,13 @@ static bool __init crng_init_try_arch_early(struct crng_state *crng)
>  	bool arch_init = true;
>  	unsigned long rv;
>  
> -	for (i = 4; i < 16; i++) {
> +	for (i = 0; i < ARRAY_SIZE(crng->key); i++) {
>  		if (!arch_get_random_seed_long_early(&rv) &&
>  		    !arch_get_random_long_early(&rv)) {
>  			rv = random_get_entropy();
>  			arch_init = false;
>  		}
> -		crng->state[i] ^= rv;
> +		crng->key[i] ^= rv;
>  	}

Same issue here.

> @@ -783,14 +789,14 @@ static bool __init crng_init_try_arch_early(struct crng_state *crng)
>  static void crng_initialize_secondary(struct crng_state *crng)
>  {
>  	chacha_init_consts(crng->state);
> -	_get_random_bytes(&crng->state[4], sizeof(u32) * 12);
> +	_get_random_bytes(&crng->key, sizeof(crng->key));
>  	crng_init_try_arch(crng);
>  	crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;
>  }

.... and here....


>  static void __init crng_initialize_primary(struct crng_state *crng)
>  {
> -	_extract_entropy(&crng->state[4], sizeof(u32) * 12);
> +	_extract_entropy(&crng->key, sizeof(crng->key));
>  	if (crng_init_try_arch_early(crng) && trust_cpu && crng_init < 2) {
>  		invalidate_batched_entropy();
>  		numa_crng_init();
> @@ -892,7 +898,7 @@ static size_t crng_fast_load(const u8 *cp, size_t len)

And here....

> @@ -994,9 +1000,9 @@ static void _extract_crng(struct crng_state *crng, u8 out[CHACHA_BLOCK_SIZE])
>  			crng_reseed(crng, crng == &primary_crng);
>  	}
>  	spin_lock_irqsave(&crng->lock, flags);
> -	chacha20_block(&crng->state[0], out);
> -	if (crng->state[12] == 0)
> -		crng->state[13]++;
> +	chacha20_block(crng->state, out);
> +	if (unlikely(!crng->counter[0] && !++crng->counter[1]))
> +		++crng->nonce;
>  	spin_unlock_irqrestore(&crng->lock, flags);
>  }

Minor nit, but I might do this as:

	if (unlikely(!crng->counter[0]) && !++crng->counter[1])
		++crng->nonce;


Cheers,

						- Ted