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random: add wait_for_random_bytes() API

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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>
This commit is contained in:
Jason A. Donenfeld 2017-06-07 19:58:56 -04:00 committed by Theodore Ts'o
parent 4a072c71f4
commit e297a783e4
2 changed files with 32 additions and 10 deletions
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41
drivers/char/random.c
View File

@ -851,11 +851,6 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
}
}
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])
{
@ -1477,7 +1472,10 @@ static ssize_t extract_entropy_user(struct entropy_store *r, void __user *buf,
* 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)
{
@ -1506,6 +1504,24 @@ void get_random_bytes(void *buf, int nbytes)
}
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.
@ -1860,6 +1876,8 @@ const struct file_operations urandom_fops = {
SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count,
unsigned int, flags)
{
int ret;
if (flags & ~(GRND_NONBLOCK|GRND_RANDOM))
return -EINVAL;
@ -1872,9 +1890,9 @@ SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count,
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);
}
@ -2035,7 +2053,10 @@ static rwlock_t batched_entropy_reset_lock = __RW_LOCK_UNLOCKED(batched_entropy_
/*
* 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)

1
include/linux/random.h
View File

@ -34,6 +34,7 @@ extern void add_input_randomness(unsigned int type, unsigned int code,
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);