From: "Jason A. Donenfeld" Jason@zx2c4.com
commit 66e4c2b9541503d721e936cc3898c9f25f4591ff upstream.
Since we have a hash function that's really fast, and the goal of crng_slow_load() is reportedly to "touch all of the crng's state", we can just hash the old state together with the new state and call it a day. This way we dont need to reason about another LFSR or worry about various attacks there. This code is only ever used at early boot and then never again.
Cc: Theodore Ts'o tytso@mit.edu Reviewed-by: Dominik Brodowski linux@dominikbrodowski.net Reviewed-by: Eric Biggers ebiggers@google.com Signed-off-by: Jason A. Donenfeld Jason@zx2c4.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/char/random.c | 40 ++++++++++++++-------------------------- 1 file changed, 14 insertions(+), 26 deletions(-)
--- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -475,42 +475,30 @@ static size_t crng_fast_load(const u8 *c * all), and (2) it doesn't have the performance constraints of * crng_fast_load(). * - * So we do something more comprehensive which is guaranteed to touch - * all of the primary_crng's state, and which uses a LFSR with a - * period of 255 as part of the mixing algorithm. Finally, we do - * *not* advance crng_init_cnt since buffer we may get may be something - * like a fixed DMI table (for example), which might very well be - * unique to the machine, but is otherwise unvarying. + * So, we simply hash the contents in with the current key. Finally, + * we do *not* advance crng_init_cnt since buffer we may get may be + * something like a fixed DMI table (for example), which might very + * well be unique to the machine, but is otherwise unvarying. */ -static int crng_slow_load(const u8 *cp, size_t len) +static void crng_slow_load(const u8 *cp, size_t len) { unsigned long flags; - static u8 lfsr = 1; - u8 tmp; - unsigned int i, max = sizeof(base_crng.key); - const u8 *src_buf = cp; - u8 *dest_buf = base_crng.key; + struct blake2s_state hash; + + blake2s_init(&hash, sizeof(base_crng.key));
if (!spin_trylock_irqsave(&base_crng.lock, flags)) - return 0; + return; if (crng_init != 0) { spin_unlock_irqrestore(&base_crng.lock, flags); - return 0; + return; } - if (len > max) - max = len;
- for (i = 0; i < max; i++) { - tmp = lfsr; - lfsr >>= 1; - if (tmp & 1) - lfsr ^= 0xE1; - tmp = dest_buf[i % sizeof(base_crng.key)]; - dest_buf[i % sizeof(base_crng.key)] ^= src_buf[i % len] ^ lfsr; - lfsr += (tmp << 3) | (tmp >> 5); - } + blake2s_update(&hash, base_crng.key, sizeof(base_crng.key)); + blake2s_update(&hash, cp, len); + blake2s_final(&hash, base_crng.key); + spin_unlock_irqrestore(&base_crng.lock, flags); - return 1; }
static void crng_reseed(void)