linux-kernel - Re: [PATCH] Fix xoring of arch_get_random

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Message-ID: <20190529134200.GA31099@hmswarspite.think-freely.org>
Date:   Wed, 29 May 2019 09:42:00 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     linux-kernel@...r.kernel.org
Cc:     Steve Grubb <sgrubb@...hat.com>, Theodore Ts'o <tytso@....edu>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] Fix xoring of arch_get_random_long into crng->state array

On Tue, Apr 02, 2019 at 06:00:25PM -0400, Neil Horman wrote:
> When _crng_extract is called, any arch that has a registered
> arch_get_random_long method, attempts to mix an unsigned long value into
> the crng->state buffer, it only mixes in 32 of the 64 bits available,
> because the state buffer is an array of u32 values, even though 2 u32
> are expected to be filled (owing to the fact that it expects indexes 14
> and 15 to be filled).
> 
> Bring the expected behavior into alignment by casting index 14 to an
> unsignled long pointer, and xoring that in instead.
> 
> Tested successfully by myself
> 
> Signed-off-by: Neil Horman <nhorman@...driver.com>
> Reported-by: Steve Grubb <sgrubb@...hat.com>
> CC: "Theodore Ts'o" <tytso@....edu>
> CC: Arnd Bergmann <arnd@...db.de>
> CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  drivers/char/random.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 38c6d1af6d1c..8178618458ac 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -975,14 +975,16 @@ static void _extract_crng(struct crng_state *crng,
>  			  __u8 out[CHACHA_BLOCK_SIZE])
>  {
>  	unsigned long v, flags;
> -
> +	unsigned long *archrnd;
>  	if (crng_ready() &&
>  	    (time_after(crng_global_init_time, crng->init_time) ||
>  	     time_after(jiffies, crng->init_time + CRNG_RESEED_INTERVAL)))
>  		crng_reseed(crng, crng == &primary_crng ? &input_pool : NULL);
>  	spin_lock_irqsave(&crng->lock, flags);
> -	if (arch_get_random_long(&v))
> -		crng->state[14] ^= v;
> +	if (arch_get_random_long(&v)) {
> +		archrnd = (unsigned long *)&crng->state[14];
> +		*archrnd ^= v;
> +	}
>  	chacha20_block(&crng->state[0], out);
>  	if (crng->state[12] == 0)
>  		crng->state[13]++;
> -- 
> 2.20.1
> 
> 

Ping, Arnd, Ted, Greg, any comment here?
Neil