linux-kernel - [ 11/70] random: Use arch_get_random_int instead of cycle counter if avail

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Date:	Tue, 07 Aug 2012 04:28:06 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, "David S. Miller" <davem@...emloft.net>,
	"Theodore Tso" <tytso@....edu>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Matt Mackall <mpm@...enic.com>,
	Tony Luck <tony.luck@...el.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
Subject: [ 11/70] random: Use arch_get_random_int instead of cycle counter if avail

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds@...ux-foundation.org>

commit cf833d0b9937874b50ef2867c4e8badfd64948ce upstream.

We still don't use rdrand in /dev/random, which just seems stupid. We
accept the *cycle*counter* as a random input, but we don't accept
rdrand? That's just broken.

Sure, people can do things in user space (write to /dev/random, use
rdrand in addition to /dev/random themselves etc etc), but that
*still* seems to be a particularly stupid reason for saying "we
shouldn't bother to try to do better in /dev/random".

And even if somebody really doesn't trust rdrand as a source of random
bytes, it seems singularly stupid to trust the cycle counter *more*.

So I'd suggest the attached patch. I'm not going to even bother
arguing that we should add more bits to the entropy estimate, because
that's not the point - I don't care if /dev/random fills up slowly or
not, I think it's just stupid to not use the bits we can get from
rdrand and mix them into the strong randomness pool.

Link: http://lkml.kernel.org/r/CA%2B55aFwn59N1=m651QAyTy-1gO1noGbK18zwKDwvwqnravA84A@mail.gmail.com
Acked-by: "David S. Miller" <davem@...emloft.net>
Acked-by: "Theodore Ts'o" <tytso@....edu>
Acked-by: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Matt Mackall <mpm@...enic.com>
Cc: Tony Luck <tony.luck@...el.com>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Signed-off-by: H. Peter Anvin <hpa@...ux.intel.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/char/random.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 6035ab8..85da874 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -624,8 +624,8 @@ static struct timer_rand_state input_timer_state;
 static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 {
 	struct {
-		cycles_t cycles;
 		long jiffies;
+		unsigned cycles;
 		unsigned num;
 	} sample;
 	long delta, delta2, delta3;
@@ -637,7 +637,11 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 		goto out;
 
 	sample.jiffies = jiffies;
-	sample.cycles = get_cycles();
+
+	/* Use arch random value, fall back to cycles */
+	if (!arch_get_random_int(&sample.cycles))
+		sample.cycles = get_cycles();
+
 	sample.num = num;
 	mix_pool_bytes(&input_pool, &sample, sizeof(sample));
 


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