Re: [PATCH 1/7] random: Simplify API for random address requests

From: Yann Droneaud <ydroneaud@opteya.com>
To: Jason Cooper <jason@lakedaemon.net>,
	william.c.roberts@intel.com, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org,
	kernel-hardening@lists.openwall.com
Cc: linux@arm.linux.org.uk, akpm@linux-foundation.org,
	keescook@chromium.org, tytso@mit.edu, arnd@arndb.de,
	gregkh@linuxfoundation.org, catalin.marinas@arm.com,
	will.deacon@arm.com, ralf@linux-mips.org,
	benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au,
	davem@davemloft.net, tglx@linutronix.de, mingo@redhat.com,
	hpa@zytor.com, x86@kernel.org, viro@zeniv.linux.org.uk,
	nnk@google.com, jeffv@google.com, dcashman@android.com
Subject: Re: [PATCH 1/7] random: Simplify API for random address requests
Date: Fri, 29 Jul 2016 10:59:14 +0200	[thread overview]
Message-ID: <1469782754.16837.20.camel@opteya.com> (raw)
In-Reply-To: <20160728204730.27453-2-jason@lakedaemon.net>

Hi,

Le jeudi 28 juillet 2016 à 20:47 +0000, Jason Cooper a écrit :
> To date, all callers of randomize_range() have set the length to 0,
> and check for a zero return value.  For the current callers, the only
> way to get zero returned is if end <= start.  Since they are all
> adding a constant to the start address, this is unnecessary.
> 
> We can remove a bunch of needless checks by simplifying the API to do
> just what everyone wants, return an address between [start, start +
> range).
> 
> While we're here, s/get_random_int/get_random_long/.  No current call
> site is adversely affected by get_random_int(), since all current
> range requests are < UINT_MAX.  However, we should match caller
> expectations to avoid coming up short (ha!) in the future.
> 
> Address generation within [start, start + range) behavior is
> preserved.
> 
> All current callers to randomize_range() chose to use the start
> address if randomize_range() failed.  Therefore, we simplify things
> by just returning the start address on error.
> 
> randomize_range() will be removed once all callers have been
> converted over to randomize_addr().
> 
> Signed-off-by: Jason Cooper <jason@lakedaemon.net>
> ---
>  drivers/char/random.c  | 26 ++++++++++++++++++++++++++
>  include/linux/random.h |  1 +
>  2 files changed, 27 insertions(+)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 0158d3bff7e5..3610774bcc53 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1840,6 +1840,32 @@ randomize_range(unsigned long start, unsigned
> long end, unsigned long len)
>  	return PAGE_ALIGN(get_random_int() % range + start);
>  }
>  
> +/**
> + * randomize_addr - 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.
> + *
> + * Before page alignment, the random address generated can be any value from
> + * @start, to @start + @range - 1 inclusive.
> + *
> + * If @start + @range would overflow, @range is capped.
> + *
> + * Return: A page aligned address within [start, start + range).

PAGE_ALIGN(start + range - 1) can be greater than start + range ..

In the worst case, when start = 0, range = ULONG_MAX, the result would
be 0.

In order to stay in the bounds, the start address must be rounded up,
and the random offset must be rounded down.

Something I haven't found the time to send was looking like this:

  unsigned long base = PAGE_ALIGN(start);

  range -= (base - start);
  range >>= PAGE_SHIFT;

  return base + ((get_random_int() % range) << PAGE_SHIFT);

>   On error,
> + * @start is returned.
> + */
> +unsigned long
> +randomize_addr(unsigned long start, unsigned long range)
> +{
> +	if (range == 0)
> +		return start;
> +
> +	if (start > ULONG_MAX - range)
> +		range = ULONG_MAX - start;
> +
> +	return PAGE_ALIGN(get_random_long() % range + start);
> +}
> +
>  /* 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.
> diff --git a/include/linux/random.h b/include/linux/random.h
> index e47e533742b5..f1ca2fa4c071 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -35,6 +35,7 @@ extern const struct file_operations random_fops, urandom_fops;
>  unsigned int get_random_int(void);
>  unsigned long get_random_long(void);
>  unsigned long randomize_range(unsigned long start, unsigned long end, unsigned long len);
> +unsigned long randomize_addr(unsigned long start, unsigned long range);
>  
>  u32 prandom_u32(void);
>  void prandom_bytes(void *buf, size_t nbytes);

Regards.

-- 
Yann Droneaud
OPTEYA