Re: [kernel-hardening] [RFC patch 1/6] random: Simplify API for random address requests

From: Yann Droneaud <ydroneaud@opteya.com>
To: kernel-hardening@lists.openwall.com, william.c.roberts@intel.com,
	linux-mm@vger.kernel.org, linux-kernel@vger.kernel.org
Cc: Jason Cooper <jason@lakedaemon.net>,
	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, alyzyn@android.com,
	dcashman@android.com
Subject: Re: [kernel-hardening] [RFC patch 1/6] random: Simplify API for random address requests
Date: Wed, 27 Jul 2016 15:51:57 +0200	[thread overview]
Message-ID: <1469627517.16837.9.camel@opteya.com> (raw)
In-Reply-To: <20160726030201.6775-1-jason@lakedaemon.net>

Hi,

Le mardi 26 juillet 2016 à 03:01 +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.
> 

I agree.

> 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].
> 

I agree.

For the record:

http://lkml.kernel.org/r/cover.1390770607.git.ydroneaud@opteya.com

> 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 < MAX_UINT.  However, we should match caller
> expectations to avoid coming up short (ha!) in the future.
> 
> Signed-off-by: Jason Cooper <jason@lakedaemon.net>
> ---
>  drivers/char/random.c  | 17 ++++-------------
>  include/linux/random.h |  2 +-
>  2 files changed, 5 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 0158d3bff7e5..1251cb2cbab2 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1822,22 +1822,13 @@ unsigned long get_random_long(void)
>  EXPORT_SYMBOL(get_random_long);
>  
>  /*
> - * randomize_range() returns a start address such that
> - *
> - *    [...... <range> .....]
> - *  start                  end
> - *
> - * a <range> with size "len" starting at the return value is inside
> in the
> - * area defined by [start, end], but is otherwise randomized.
> + * randomize_addr() returns a page aligned address within [start,
> start +
> + * range]
>   */
>  unsigned long
> -randomize_range(unsigned long start, unsigned long end, unsigned
> long len)
> +randomize_addr(unsigned long start, unsigned long range)
>  {
> -	unsigned long range = end - len - start;
> -
> -	if (end <= start + len)
> -		return 0;
> -	return PAGE_ALIGN(get_random_int() % range + start);
> +	return PAGE_ALIGN(get_random_long() % range + start);
>  }
>  
>  /* Interface for in-kernel drivers of true hardware RNGs.
> diff --git a/include/linux/random.h b/include/linux/random.h
> index e47e533742b5..1ad877a98186 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -34,7 +34,7 @@ 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);