linux-kernel - Re: [PATCH 2/3] random: Return nbytes filled from hw RNG

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Date:   Wed, 2 May 2018 09:44:25 +1000
From:   "Tobin C. Harding" <me@...in.cc>
To:     linux-kernel@...r.kernel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kees Cook <keescook@...omium.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Theodore Ts'o <tytso@....edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 2/3] random: Return nbytes filled from hw RNG

On Wed, May 02, 2018 at 09:33:39AM +1000, Tobin C. Harding wrote:
> Currently the function get_random_bytes_arch() has return value 'void'.
> If the hw RNG fails we currently fall back to using get_random_bytes().
> This defeats the purpose of requesting random material from the hw RNG
> in the first place.
> 
> There are currently no intree users of get_random_bytes_arch().
> 
> Only get random bytes from the hw RNG, make function return the number
> of bytes retrieved from the hw RNG.
> 
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Tobin C. Harding <me@...in.cc>
> ---
>  drivers/char/random.c  | 16 +++++++++-------
>  include/linux/random.h |  2 +-
>  2 files changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 031d18b31e0f..3a66507ea60b 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1725,26 +1725,28 @@ EXPORT_SYMBOL(del_random_ready_callback);
>   * key known by the NSA).  So it's useful if we need the speed, but
>   * only if we're willing to trust the hardware manufacturer not to
>   * have put in a back door.
> + *
> + * Return number of bytes filled in.
>   */
> -void get_random_bytes_arch(void *buf, int nbytes)
> +int __must_check get_random_bytes_arch(void *buf, int nbytes)
>  {
>  	char *p = buf;
> +	int left = nbytes;
>  
> -	trace_get_random_bytes_arch(nbytes, _RET_IP_);
> -	while (nbytes) {
> +	trace_get_random_bytes_arch(left, _RET_IP_);
> +	while (left) {
>  		unsigned long v;
> -		int chunk = min(nbytes, (int)sizeof(unsigned long));
> +		int chunk = min(left, (int)sizeof(unsigned long));
>  
>  		if (!arch_get_random_long(&v))
>  			break;
>  
>  		memcpy(p, &v, chunk);
>  		p += chunk;
> -		nbytes -= chunk;
> +		left -= chunk;
>  	}
>  
> -	if (nbytes)
> -		get_random_bytes(p, nbytes);
> +	return nbytes - left;
>  }
>  EXPORT_SYMBOL(get_random_bytes_arch);
>  
> diff --git a/include/linux/random.h b/include/linux/random.h
> index 2ddf13b4281e..308168c0a637 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -38,7 +38,7 @@ extern void get_random_bytes(void *buf, int nbytes);
>  extern int wait_for_random_bytes(void);
>  extern int add_random_ready_callback(struct random_ready_callback *rdy);
>  extern void del_random_ready_callback(struct random_ready_callback *rdy);
> -extern void get_random_bytes_arch(void *buf, int nbytes);
> +extern int get_random_bytes_arch(void *buf, int nbytes);
>  
>  #ifndef MODULE
>  extern const struct file_operations random_fops, urandom_fops;
> -- 
> 2.7.4
> 

Tested by loading a module with this init funciton

static int __init test_init(void)
{
	const int size = 256;
	char buf[size];
	int got;

	pr_info("test: module inserted\n");

	got = get_random_bytes_arch(buf, size);
	if (got != size)
		pr_warn("get_random_bytes_arch() failed, got %d bytes\n", got);

	return 0;
}
module_init(test_init);


Fails on machine without RDRAND and succeeds on machine with RDRAND

thanks,
Tobin.