From: Theodore Ts'o tytso@mit.edu
commit dc12baacb95f205948f64dc936a47d89ee110117 upstream.
add_device_randomness() use of crng_fast_load() was highly problematic. Some callers of add_device_randomness() can pass in a large amount of static information. This would immediately promote the crng_init state from 0 to 1, without really doing much to initialize the primary_crng's internal state with something even vaguely unpredictable.
Since we don't have the speed constraints of add_interrupt_randomness(), we can do a better job mixing in the what unpredictability a device driver or architecture maintainer might see fit to give us, and do it in a way which does not bump the crng_init_cnt variable.
Also, since add_device_randomness() doesn't bump any entropy accounting in crng_init state 0, mix the device randomness into the input_pool entropy pool as well. This is related to CVE-2018-1108.
Reported-by: Jann Horn jannh@google.com Fixes: ee7998c50c26 ("random: do not ignore early device randomness") Cc: stable@kernel.org # 4.13+ Signed-off-by: Theodore Ts'o tytso@mit.edu Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/char/random.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 51 insertions(+), 4 deletions(-)
--- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -799,6 +799,10 @@ static void crng_initialize(struct crng_ crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1; }
+/* + * crng_fast_load() can be called by code in the interrupt service + * path. So we can't afford to dilly-dally. + */ static int crng_fast_load(const char *cp, size_t len) { unsigned long flags; @@ -876,6 +880,51 @@ static struct crng_state *select_crng(vo } #endif
+/* + * crng_slow_load() is called by add_device_randomness, which has two + * attributes. (1) We can't trust the buffer passed to it is + * guaranteed to be unpredictable (so it might not have any entropy at + * 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. + */ +static int crng_slow_load(const char *cp, size_t len) +{ + unsigned long flags; + static unsigned char lfsr = 1; + unsigned char tmp; + unsigned i, max = CHACHA20_KEY_SIZE; + const char * src_buf = cp; + char * dest_buf = (char *) &primary_crng.state[4]; + + if (!spin_trylock_irqsave(&primary_crng.lock, flags)) + return 0; + if (crng_init != 0) { + spin_unlock_irqrestore(&primary_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 % CHACHA20_KEY_SIZE]; + dest_buf[i % CHACHA20_KEY_SIZE] ^= src_buf[i % len] ^ lfsr; + lfsr += (tmp << 3) | (tmp >> 5); + } + spin_unlock_irqrestore(&primary_crng.lock, flags); + return 1; +} + static void crng_reseed(struct crng_state *crng, struct entropy_store *r) { unsigned long flags; @@ -1046,10 +1095,8 @@ void add_device_randomness(const void *b unsigned long time = random_get_entropy() ^ jiffies; unsigned long flags;
- if (!crng_ready()) { - crng_fast_load(buf, size); - return; - } + if (!crng_ready() && size) + crng_slow_load(buf, size);
trace_add_device_randomness(size, _RET_IP_); spin_lock_irqsave(&input_pool.lock, flags);