[kernel-hardening] Re: [PATCH v3 1/7] random: Simplify API for random address requests

[Yann Droneaud <ydroneaud@opteya.com>]

Hi,

Le mercredi 03 août 2016 à 23:39 +0000, Jason Cooper a écrit :
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 0158d3bff7e5..61cb434e3bea 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1840,6 +1840,39 @@ randomize_range(unsigned long start, unsigned
> long end, unsigned long len)
>  	return PAGE_ALIGN(get_random_int() % range + start);
>  }
>  
> +/**
> + * randomize_page - Generate a random, page aligned address
> + * @start:	The smallest acceptable address the caller will
> take.
> + * @range:	The size of the area, starting at @start, within
> which the
> + *		random address must fall.
> + *
> + * If @start + @range would overflow, @range is capped.
> + *
> + * NOTE: Historical use of randomize_range, which this replaces,
> presumed that
> + * @start was already page aligned.  We now align it regardless.
> + *
> + * Return: A page aligned address within [start, start + range).  On
> error,
> + * @start is returned.
> + */
> +unsigned long
> +randomize_page(unsigned long start, unsigned long range)
> +{

To prevent an underflow if start is not page aligned (but will one
would ever use a non aligned start address *and* range ? ...)

        if (range < PAGE_SIZE)
                return start;


> +	if (!PAGE_ALIGNED(start)) {
> +		range -= PAGE_ALIGN(start) - start;
> +		start = PAGE_ALIGN(start);
> +	}
> +
> +	if (start > ULONG_MAX - range)
> +		range = ULONG_MAX - start;
> +
> +	range >>= PAGE_SHIFT;
> +
> +	if (range == 0)
> +		return start;
> +
> +	return start + (get_random_long() % range << PAGE_SHIFT);
> +}
> +
>  /* Interface for in-kernel drivers of true hardware RNGs.
>   * Those devices may produce endless random bits and will be
> throttled
>   * when our pool is full.
> 

Regards.

-- 
Yann Droneaud
OPTEYA