linux-kernel - [PATCHv2] random: Make /dev/random wait for crng

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Date:   Fri, 15 Feb 2019 13:58:20 +0000
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Theodore Ts'o <tytso@....edu>, Arnd Bergmann <arnd@...db.de>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [PATCHv2] random: Make /dev/random wait for crng_ready

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.

Fixed by making read and select on /dev/random wait until
the crng is fully initialized.

Signed-off-by: Bernd Edlinger <bernd.edlinger@...mail.de>
---
v2: Fixed one white space in the code.
    Also added some more details about the problem
    to the commit message.
---
 drivers/char/random.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index bd449ad..c8f16f0 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -721,7 +721,7 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
 		}
 
 		/* should we wake readers? */
-		if (entropy_bits >= random_read_wakeup_bits &&
+		if (crng_ready() && entropy_bits >= random_read_wakeup_bits &&
 		    wq_has_sleeper(&random_read_wait)) {
 			wake_up_interruptible(&random_read_wait);
 			kill_fasync(&fasync, SIGIO, POLL_IN);
@@ -1826,7 +1826,9 @@ _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 = crng_ready()
+			? extract_entropy_user(&blocking_pool, buf, nbytes)
+			: 0;
 		if (n < 0)
 			return n;
 		trace_random_read(n*8, (nbytes-n)*8,
@@ -1840,6 +1842,7 @@ _random_read(int nonblock, char __user *buf, size_t nbytes)
 			return -EAGAIN;
 
 		wait_event_interruptible(random_read_wait,
+			crng_ready() &&
 			ENTROPY_BITS(&input_pool) >=
 			random_read_wakeup_bits);
 		if (signal_pending(current))
@@ -1884,7 +1887,8 @@ 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 (crng_ready() &&
+		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