[RFC PATCH 18/41] random: move arch_get_random_seed() calls in crng_reseed() into own loop

[Nicolai Stange]

x86's RDSEED/RDRAND insns have reportedly been slowed down significantly
due to the ucode update required to mitigate against the "Special Register
Buffer Data Sampling" vulnerability (CVE-2020-0543) and should not get
invoked from the interrupt path anymore.

In preparation of getting rid of that arch_get_random_long() call currently
found in add_interrupt_randomness(), move those arch_get_random_long()
calls in crng_reseed() into a separate loop and outside of the crng->lock.

There is no functional change.

Signed-off-by: Nicolai Stange <nstange@xxxxxxx>
---
 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 a49805d0d23c..1945249597e0 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1200,14 +1200,18 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
 		_crng_backtrack_protect(&primary_crng, buf.block,
 					CHACHA_KEY_SIZE);
 	}
-	spin_lock_irqsave(&crng->lock, flags);
+
 	for (i = 0; i < 8; i++) {
 		unsigned long	rv;
 		if (!arch_get_random_seed_long(&rv) &&
 		    !arch_get_random_long(&rv))
 			rv = random_get_entropy();
-		crng->state[i+4] ^= buf.key[i] ^ rv;
+		buf.key[i] ^= rv;
 	}
+
+	spin_lock_irqsave(&crng->lock, flags);
+	for (i = 0; i < 8; i++)
+		crng->state[i+4] ^= buf.key[i];
 	memzero_explicit(&buf, sizeof(buf));
 	crng->init_time = jiffies;
 	spin_unlock_irqrestore(&crng->lock, flags);
-- 
2.26.2