Xoshiro256++ will not work in the very unlikely case, that it is used with an all zeroes state. Add checks against this in all necessary places. Signed-off-by: Markus Theil --- lib/random32.c | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-) diff --git a/lib/random32.c b/lib/random32.c index 64fccfd64717..c01b763a7d40 100644 --- a/lib/random32.c +++ b/lib/random32.c @@ -26,6 +26,8 @@ #include #include +#define IS_ALL_ZERO_STATE(state) (!state->s[0] && !state->s[1] && !state->s[2] && !state->s[3]) + /** * prandom_u64_state - seeded pseudo-random number generator. * @state: pointer to state structure holding seeded state. @@ -40,6 +42,9 @@ u64 prandom_u64_state(struct rnd_state *state) const u64 result = rol64(state->s[0] + state->s[3], 23) + state->s[0]; const u64 t = state->s[1] << 17; + /* defensive check, as this fn returns always zero otherwise */ + BUG_ON(IS_ALL_ZERO_STATE(state)); + state->s[2] ^= state->s[0]; state->s[3] ^= state->s[1]; state->s[1] ^= state->s[2]; @@ -108,6 +113,12 @@ EXPORT_SYMBOL(prandom_bytes_state); * * splitmix64 init as suggested for xoshiro256++ * See: https://prng.di.unimi.it/splitmix64.c + * + * Seeding with this routine cannot result in an + * all zeroes state due to the addition operation + * with the fixed constant 0x9e3779b97f4a7c15! + * Nevertheless check early for such a state + * as a defensive mechanism. */ void prandom_seed_state(struct rnd_state *state, u64 seed) { @@ -120,6 +131,9 @@ void prandom_seed_state(struct rnd_state *state, u64 seed) z = (z ^ (z >> 27)) * 0x94d049bb133111eb; state->s[i] = z ^ (z >> 31); } + + /* shall never happen */ + BUG_ON(IS_ALL_ZERO_STATE(state)); } EXPORT_SYMBOL(prandom_seed_state); @@ -133,7 +147,16 @@ void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state) for_each_possible_cpu(i) { struct rnd_state *state = per_cpu_ptr(pcpu_state, i); - get_random_bytes(&state->s, sizeof(state->s)); + memset(state, 0, sizeof(struct rnd_state)); + /* + * Internal state MUST not be all zeroes. Check and repeat if necessary. + * + * Highly unlikely, that we ever need more than one round. Just defensive + * coding, as this could happen in theory. + */ + while (IS_ALL_ZERO_STATE(state)) { + get_random_bytes(&state->s, sizeof(state->s)); + } } } EXPORT_SYMBOL(prandom_seed_full_state); -- 2.49.0