random: use hash function for crng_slow_load()

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>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 626c53c..866b855 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -477,42 +477,30 @@ static size_t crng_fast_load(const u8 *cp, size_t len)
  * 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;
-       }
-       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);
+               return;
        }
+
+       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)