[PATCHv4] random: Make /dev/random wait for input_pool initialized

[Bernd Edlinger]

Reading from /dev/random may return data while the getrandom
syscall is still blocking.

Those bytes are not yet cryptographically secure.

The first byte from /dev/random can have as little
as 8 bits entropy estimation.  Once a read blocks, it will
block until /proc/sys/kernel/random/read_wakeup_threshold
bits are available, which is usually 64 bits, but can be
configured as low as 8 bits.  A select will wake up when
at least read_wakeup_threshold bits are available.
Also when constantly reading bytes out of /dev/random
it will prevent the crng init done event forever.

Furthermore previously the input_pool got initialized when
more than 128 bits of raw entropy are accumulated, but that
is only credited 80 bits of pool entroy.  Therefore if
random_write_wakeup_bits is configured lower than 80 bits,
the code path that sends entropy to the blocking_pool can
get activated, before the CRNG is initialized and delays
the initialization of the CRNG until the blocking_pool is
filled to 75%.

Fixed by making read and select on /dev/random wait until
the input_pool is initialized which means that more than
128 bits of entropy have been fed into the input_pool.
This is guaranteed to happen after the CRNG is ready.
So after the first byte is readable from /dev/random
also /dev/urandom is guaranteed to be fully initialized.

Another minor tweak this patch makes, is when the primary
CRNG is periodically reseeded, we reserve twice the amount
of read_wakeup_threshold for fairness reasons, to keep
/dev/random readable when it is not accessed by user mode.

And finally, when the system runs for long times the jiffies
may roll over, but crng_global_init_time is not updated
when reseed every 5 minutes happens.  This could cause
CRNG reseeding all the time, making the input_pool run
low on entropy which would make /dev/random block
unexpectedly.

Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
---
v4 makes the /dev/random block until the input_pool has
reached 128 bits of entropy at least once.  Now make
everything depend on input_pool.initialized.
Additionally fixed a potential issue with jiffies roll
over in the CRNG reseeding logic, which would cause the
cgng_global_init_time to be in the future, causing
reseeding to happen all the time.
Finally added a fairness reserve to keep the /dev/random
in the readable state (select can check non-destructively),
when nothing but CRNG reseeding happens.
---
 drivers/char/random.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index bd449ad..97cde01 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -645,6 +645,7 @@ static void process_random_ready_list(void)
 static void credit_entropy_bits(struct entropy_store *r, int nbits)
 {
 	int entropy_count, orig;
+	int entropy_bits;
 	const int pool_size = r->poolinfo->poolfracbits;
 	int nfrac = nbits << ENTROPY_SHIFT;
 
@@ -702,8 +703,9 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
 	if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
 		goto retry;
 
+	entropy_bits = entropy_count >> ENTROPY_SHIFT;
 	r->entropy_total += nbits;
-	if (!r->initialized && r->entropy_total > 128) {
+	if (!r->initialized && entropy_bits >= 128) {
 		r->initialized = 1;
 		r->entropy_total = 0;
 	}
@@ -713,8 +715,6 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
 				  r->entropy_total, _RET_IP_);
 
 	if (r == &input_pool) {
-		int entropy_bits = entropy_count >> ENTROPY_SHIFT;
-
 		if (crng_init < 2 && entropy_bits >= 128) {
 			crng_reseed(&primary_crng, r);
 			entropy_bits = r->entropy_count >> ENTROPY_SHIFT;
@@ -722,10 +722,12 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
 
 		/* should we wake readers? */
 		if (entropy_bits >= random_read_wakeup_bits &&
+		    r->initialized &&
 		    wq_has_sleeper(&random_read_wait)) {
 			wake_up_interruptible(&random_read_wait);
 			kill_fasync(&fasync, SIGIO, POLL_IN);
 		}
+
 		/* If the input pool is getting full, send some
 		 * entropy to the blocking pool until it is 75% full.
 		 */
@@ -917,9 +919,11 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
 	} buf;
 
 	if (r) {
-		num = extract_entropy(r, &buf, 32, 16, 0);
+		num = extract_entropy(r, &buf, 32, 16, crng_ready()
+				      ? random_read_wakeup_bits / 4 : 0);
 		if (num == 0)
 			return;
+		crng_global_init_time = jiffies;
 	} else {
 		_extract_crng(&primary_crng, buf.block);
 		_crng_backtrack_protect(&primary_crng, buf.block,
@@ -1652,7 +1656,7 @@ EXPORT_SYMBOL(get_random_bytes);
  */
 int wait_for_random_bytes(void)
 {
-	if (likely(crng_ready()))
+	if (crng_ready())
 		return 0;
 	return wait_event_interruptible(crng_init_wait, crng_ready());
 }
@@ -1826,7 +1830,9 @@ _random_read(int nonblock, char __user *buf, size_t nbytes)
 
 	nbytes = min_t(size_t, nbytes, SEC_XFER_SIZE);
 	while (1) {
-		n = extract_entropy_user(&blocking_pool, buf, nbytes);
+		n = input_pool.initialized
+			? extract_entropy_user(&blocking_pool, buf, nbytes)
+			: 0;
 		if (n < 0)
 			return n;
 		trace_random_read(n*8, (nbytes-n)*8,
@@ -1840,6 +1846,7 @@ _random_read(int nonblock, char __user *buf, size_t nbytes)
 			return -EAGAIN;
 
 		wait_event_interruptible(random_read_wait,
+			input_pool.initialized &&
 			ENTROPY_BITS(&input_pool) >=
 			random_read_wakeup_bits);
 		if (signal_pending(current))
@@ -1884,7 +1891,8 @@ random_poll(struct file *file, poll_table * wait)
 	poll_wait(file, &random_read_wait, wait);
 	poll_wait(file, &random_write_wait, wait);
 	mask = 0;
-	if (ENTROPY_BITS(&input_pool) >= random_read_wakeup_bits)
+	if (input_pool.initialized &&
+		ENTROPY_BITS(&input_pool) >= random_read_wakeup_bits)
 		mask |= EPOLLIN | EPOLLRDNORM;
 	if (ENTROPY_BITS(&input_pool) < random_write_wakeup_bits)
 		mask |= EPOLLOUT | EPOLLWRNORM;
-- 
2.7.4