random: credit architectural init the exact amount

commit 12e45a2 upstream.

RDRAND and RDSEED can fail sometimes, which is fine. We currently
initialize the RNG with 512 bits of RDRAND/RDSEED. We only need 256 bits
of those to succeed in order to initialize the RNG. Instead of the
current "all or nothing" approach, actually credit these contributions
the amount that is actually contributed.

Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/random.c

@@ -896,21 +896,21 @@ early_param("random.trust_bootloader", parse_trust_bootloader);
  */
 int __init random_init(const char *command_line)
 {
-	size_t i;
 	ktime_t now = ktime_get_real();
-	bool arch_init = true;
+	unsigned int i, arch_bytes;
 	unsigned long rv;
 
 #if defined(LATENT_ENTROPY_PLUGIN)
 	static const u8 compiletime_seed[BLAKE2S_BLOCK_SIZE] __initconst __latent_entropy;
 	_mix_pool_bytes(compiletime_seed, sizeof(compiletime_seed));
 #endif
 
-	for (i = 0; i < BLAKE2S_BLOCK_SIZE; i += sizeof(rv)) {
+	for (i = 0, arch_bytes = BLAKE2S_BLOCK_SIZE;
+	     i < BLAKE2S_BLOCK_SIZE; i += sizeof(rv)) {
 		if (!arch_get_random_seed_long_early(&rv) &&
 		    !arch_get_random_long_early(&rv)) {
 			rv = random_get_entropy();
-			arch_init = false;
+			arch_bytes -= sizeof(rv);
 		}
 		_mix_pool_bytes(&rv, sizeof(rv));
 	}
@@ -921,8 +921,8 @@ int __init random_init(const char *command_line)
 
 	if (crng_ready())
 		crng_reseed();
-	else if (arch_init && trust_cpu)
-		credit_init_bits(BLAKE2S_BLOCK_SIZE * 8);
+	else if (trust_cpu)
+		credit_init_bits(arch_bytes * 8);
 
 	return 0;
 }