From: "Jason A. Donenfeld" Jason@zx2c4.com
commit 28f425e573e906a4c15f8392cc2b1561ef448595 upstream.
When /dev/random was directly connected with entropy extraction, without any expansion stage, extract_buf() was called for every 10 bytes of data read from /dev/random. For that reason, RDRAND was used rather than RDSEED. At the same time, crng_reseed() was still only called every 5 minutes, so there RDSEED made sense.
Those olden days were also a time when the entropy collector did not use a cryptographic hash function, which meant most bets were off in terms of real preimage resistance. For that reason too it didn't matter _that_ much whether RDSEED was mixed in before or after entropy extraction; both choices were sort of bad.
But now we have a cryptographic hash function at work, and with that we get real preimage resistance. We also now only call extract_entropy() every 5 minutes, rather than every 10 bytes. This allows us to do two important things.
First, we can switch to using RDSEED in extract_entropy(), as Dominik suggested. Second, we can ensure that RDSEED input always goes into the cryptographic hash function with other things before being used directly. This eliminates a category of attacks in which the CPU knows the current state of the crng and knows that we're going to xor RDSEED into it, and so it computes a malicious RDSEED. By going through our hash function, it would require the CPU to compute a preimage on the fly, which isn't going to happen.
Cc: Theodore Ts'o tytso@mit.edu Reviewed-by: Eric Biggers ebiggers@google.com Reviewed-by: Dominik Brodowski linux@dominikbrodowski.net Suggested-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 | 22 +++++++++------------- 1 file changed, 9 insertions(+), 13 deletions(-)
--- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -727,13 +727,8 @@ static void crng_reseed(struct crng_stat 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; - } + for (i = 0; i < 8; i++) + crng->state[i + 4] ^= buf.key[i]; memzero_explicit(&buf, sizeof(buf)); WRITE_ONCE(crng->init_time, jiffies); spin_unlock_irqrestore(&crng->lock, flags); @@ -1054,16 +1049,17 @@ static void extract_entropy(void *buf, s unsigned long flags; u8 seed[BLAKE2S_HASH_SIZE], next_key[BLAKE2S_HASH_SIZE]; struct { - unsigned long rdrand[32 / sizeof(long)]; + unsigned long rdseed[32 / sizeof(long)]; size_t counter; } block; size_t i;
trace_extract_entropy(nbytes, input_pool.entropy_count);
- for (i = 0; i < ARRAY_SIZE(block.rdrand); ++i) { - if (!arch_get_random_long(&block.rdrand[i])) - block.rdrand[i] = random_get_entropy(); + for (i = 0; i < ARRAY_SIZE(block.rdseed); ++i) { + if (!arch_get_random_seed_long(&block.rdseed[i]) && + !arch_get_random_long(&block.rdseed[i])) + block.rdseed[i] = random_get_entropy(); }
spin_lock_irqsave(&input_pool.lock, flags); @@ -1071,7 +1067,7 @@ static void extract_entropy(void *buf, s /* seed = HASHPRF(last_key, entropy_input) */ blake2s_final(&input_pool.hash, seed);
- /* next_key = HASHPRF(seed, RDRAND || 0) */ + /* next_key = HASHPRF(seed, RDSEED || 0) */ block.counter = 0; blake2s(next_key, (u8 *)&block, seed, sizeof(next_key), sizeof(block), sizeof(seed)); blake2s_init_key(&input_pool.hash, BLAKE2S_HASH_SIZE, next_key, sizeof(next_key)); @@ -1081,7 +1077,7 @@ static void extract_entropy(void *buf, s
while (nbytes) { i = min_t(size_t, nbytes, BLAKE2S_HASH_SIZE); - /* output = HASHPRF(seed, RDRAND || ++counter) */ + /* output = HASHPRF(seed, RDSEED || ++counter) */ ++block.counter; blake2s(buf, (u8 *)&block, seed, i, sizeof(block), sizeof(seed)); nbytes -= i;