[PATCH v2 4/9] random: ensure early RDSEED goes through mixer on init

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Continuing the reasoning of "random: use RDSEED instead of RDRAND in
entropy extraction" from this series, at init time we also don't want to
be xoring RDSEED directly into the crng. Instead it's safer to put it
into our entropy collector and then re-extract it, so that it goes
through a hash function with preimage resistance.

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 81786bef0a8e..75dc370d83b5 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1218,24 +1218,18 @@ int __init rand_initialize(void)
 	bool arch_init = true;
 	unsigned long rv;
 
+	mix_pool_bytes(utsname(), sizeof(*(utsname())));
 	mix_pool_bytes(&now, sizeof(now));
 	for (i = BLAKE2S_BLOCK_SIZE; i > 0; i -= sizeof(rv)) {
-		if (!arch_get_random_seed_long(&rv) &&
-		    !arch_get_random_long(&rv))
-			rv = random_get_entropy();
-		mix_pool_bytes(&rv, sizeof(rv));
-	}
-	mix_pool_bytes(utsname(), sizeof(*(utsname())));
-
-	extract_entropy(&primary_crng.state[4], sizeof(u32) * 12);
-	for (i = 4; i < 16; i++) {
 		if (!arch_get_random_seed_long_early(&rv) &&
 		    !arch_get_random_long_early(&rv)) {
 			rv = random_get_entropy();
 			arch_init = false;
 		}
-		primary_crng.state[i] ^= rv;
+		mix_pool_bytes(&rv, sizeof(rv));
 	}
+
+	extract_entropy(&primary_crng.state[4], sizeof(u32) * 12);
 	if (arch_init && trust_cpu && crng_init < 2) {
 		invalidate_batched_entropy();
 		crng_init = 2;
-- 
2.35.0