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Message-ID: <1469782754.16837.20.camel@opteya.com>
Date: Fri, 29 Jul 2016 10:59:14 +0200
From: Yann Droneaud <ydroneaud@...eya.com>
To: Jason Cooper <jason@...edaemon.net>, william.c.roberts@...el.com,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Cc: linux@....linux.org.uk, akpm@...ux-foundation.org,
keescook@...omium.org, tytso@....edu, arnd@...db.de,
gregkh@...uxfoundation.org, catalin.marinas@....com, will.deacon@....com,
ralf@...ux-mips.org, benh@...nel.crashing.org, paulus@...ba.org,
mpe@...erman.id.au, davem@...emloft.net, tglx@...utronix.de,
mingo@...hat.com, hpa@...or.com, x86@...nel.org, viro@...iv.linux.org.uk,
nnk@...gle.com, jeffv@...gle.com, dcashman@...roid.com
Subject: Re: [PATCH 1/7] random: Simplify API for random address requests
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@...edaemon.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
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