On Sat, Jul 30, 2016 at 8:42 AM, Jason Cooper <jason@xxxxxxxxxxxxxx> wrote:
> 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.
>
> 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@xxxxxxxxxxxxxx>
> ---
> Changes from v1:
> - Explicitly mention page_aligned start assumption (Yann Droneaud)
> - pick random pages vice random addresses (Yann Droneaud)
> - catch range=0 last
>
> drivers/char/random.c | 28 ++++++++++++++++++++++++++++
> include/linux/random.h | 1 +
> 2 files changed, 29 insertions(+)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 0158d3bff7e5..3bedf69546d6 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1840,6 +1840,34 @@ 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.
> + *
> + * If @start + @range would overflow, @range is capped.
> + *
> + * NOTE: Historical use of randomize_range, which this replaces, presumed that
> + * @start was already page aligned. This assumption still holds.
> + *
> + * Return: A page aligned address within [start, start + range). On error,
> + * @start is returned.
> + */
> +unsigned long
> +randomize_addr(unsigned long start, unsigned long range)
Since we're changing other things about this, let's try to document
its behavior in its name too and call this "randomize_page" instead.
If it requires a page-aligned value, we should probably also BUG_ON
it, or adjust the start too.
-Kees
> +{
> + 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.
> 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);
> --
> 2.9.2
>
--
Kees Cook
Chrome OS & Brillo Security
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