[PATCH 4.9 010/264] random: add wait_for_random_bytes() API

Greg Kroah-Hartman
2022-06-23T16:45:45Z

From: "Jason A. Donenfeld" <Jason@zx2c4.com>

commit e297a783e41560b44e3c14f38e420cba518113b8 upstream.

This enables users of get_random_{bytes,u32,u64,int,long} to wait until
the pool is ready before using this function, in case they actually want
to have reliable randomness.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/char/random.c | 41 +++++++++++++++++++++++++++++++----------
include/linux/random.h | 1 +
2 files changed, 32 insertions(+), 10 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -927,11 +927,6 @@ static void crng_reseed(struct crng_stat
}
}

-static inline void crng_wait_ready(void)
-{
- wait_event_interruptible(crng_init_wait, crng_ready());
-}
-
static void _extract_crng(struct crng_state *crng,
__u8 out[CHACHA20_BLOCK_SIZE])
{
@@ -1541,7 +1536,10 @@ static ssize_t extract_entropy_user(stru
* number of good random numbers, suitable for key generation, seeding
* TCP sequence numbers, etc. It does not rely on the hardware random
* number generator. For random bytes direct from the hardware RNG
- * (when available), use get_random_bytes_arch().
+ * (when available), use get_random_bytes_arch(). In order to ensure
+ * that the randomness provided by this function is okay, the function
+ * wait_for_random_bytes() should be called and return 0 at least once
+ * at any point prior.
*/
void get_random_bytes(void *buf, int nbytes)
{
@@ -1571,6 +1569,24 @@ void get_random_bytes(void *buf, int nby
EXPORT_SYMBOL(get_random_bytes);

/*
+ * Wait for the urandom pool to be seeded and thus guaranteed to supply
+ * cryptographically secure random numbers. This applies to: the /dev/urandom
+ * device, the get_random_bytes function, and the get_random_{u32,u64,int,long}
+ * family of functions. Using any of these functions without first calling
+ * this function forfeits the guarantee of security.
+ *
+ * Returns: 0 if the urandom pool has been seeded.
+ * -ERESTARTSYS if the function was interrupted by a signal.
+ */
+int wait_for_random_bytes(void)
+{
+ if (likely(crng_ready()))
+ return 0;
+ return wait_event_interruptible(crng_init_wait, crng_ready());
+}
+EXPORT_SYMBOL(wait_for_random_bytes);
+
+/*
* Add a callback function that will be invoked when the nonblocking
* pool is initialised.
*
@@ -1927,6 +1943,8 @@ const struct file_operations urandom_fop
SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count,
unsigned int, flags)
{
+ int ret;
+
if (flags & ~(GRND_NONBLOCK|GRND_RANDOM))
return -EINVAL;

@@ -1939,9 +1957,9 @@ SYSCALL_DEFINE3(getrandom, char __user *
if (!crng_ready()) {
if (flags & GRND_NONBLOCK)
return -EAGAIN;
- crng_wait_ready();
- if (signal_pending(current))
- return -ERESTARTSYS;
+ ret = wait_for_random_bytes();
+ if (unlikely(ret))
+ return ret;
}
return urandom_read(NULL, buf, count, NULL);
}
@@ -2102,7 +2120,10 @@ static rwlock_t batched_entropy_reset_lo
/*
* Get a random word for internal kernel use only. The quality of the random
* number is either as good as RDRAND or as good as /dev/urandom, with the
- * goal of being quite fast and not depleting entropy.
+ * goal of being quite fast and not depleting entropy. In order to ensure
+ * that the randomness provided by this function is okay, the function
+ * wait_for_random_bytes() should be called and return 0 at least once
+ * at any point prior.
*/
static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u64);
u64 get_random_u64(void)
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -34,6 +34,7 @@ extern void add_input_randomness(unsigne
extern void add_interrupt_randomness(int irq, int irq_flags) __latent_entropy;

extern void get_random_bytes(void *buf, int nbytes);
+extern int wait_for_random_bytes(void);
extern int add_random_ready_callback(struct random_ready_callback *rdy);
extern void del_random_ready_callback(struct random_ready_callback *rdy);
extern void get_random_bytes_arch(void *buf, int nbytes);