Commit 8a0a9bd4 authored May 05, 2009 by Linus Torvalds

random: make get_random_int() more random

It's a really simple patch that basically just open-codes the current
"secure_ip_id()" call, but when open-coding it we now use a _static_
hashing area, so that it gets updated every time.

And to make sure somebody can't just start from the same original seed of
all-zeroes, and then do the "half_md4_transform()" over and over until
they get the same sequence as the kernel has, each iteration also mixes in
the same old "current->pid + jiffies" we used - so we should now have a
regular strong pseudo-number generator, but we also have one that doesn't
have a single seed.

Note: the "pid + jiffies" is just meant to be a tiny tiny bit of noise. It
has no real meaning. It could be anything. I just picked the previous
seed, it's just that now we keep the state in between calls and that will
feed into the next result, and that should make all the difference.

I made that hash be a per-cpu data just to avoid cache-line ping-pong:
having multiple CPU's write to the same data would be fine for randomness,
and add yet another layer of chaos to it, but since get_random_int() is
supposed to be a fast interface I did it that way instead. I considered
using "__raw_get_cpu_var()" to avoid any preemption overhead while still
getting the hash be _mostly_ ping-pong free, but in the end good taste won
out.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 2c66fa7e

drivers/char/random.c

+12 −7

Original line number	Diff line number	Diff line
		@@ -1665,15 +1665,20 @@ EXPORT_SYMBOL(secure_dccp_sequence_number);
		* value is not cryptographically secure but for several uses the cost of
		* depleting entropy is too high
		*/
		DEFINE_PER_CPU(__u32 [4], get_random_int_hash);
		unsigned int get_random_int(void)
		{
		/*
		* Use IP's RNG. It suits our purpose perfectly: it re-keys itself
		* every second, from the entropy pool (and thus creates a limited
		* drain on it), and uses halfMD4Transform within the second. We
		* also mix it with jiffies and the PID:
		*/
		return secure_ip_id((__force __be32)(current->pid + jiffies));
		struct keydata *keyptr;
		__u32 *hash = get_cpu_var(get_random_int_hash);
		int ret;

		keyptr = get_keyptr();
		hash[0] += current->pid + jiffies + get_cycles() + (int)(long)&ret;

		ret = half_md4_transform(hash, keyptr->secret);
		put_cpu_var(get_random_int_hash);

		return ret;
		}

		/*

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