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Commit d8b42964 authored by Jason A. Donenfeld's avatar Jason A. Donenfeld Committed by Greg Kroah-Hartman
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random: use static branch for crng_ready() 



commit f5bda35fba615ace70a656d4700423fa6c9bebee upstream.

Since crng_ready() is only false briefly during initialization and then
forever after becomes true, we don't need to evaluate it after, making
it a prime candidate for a static branch.

One complication, however, is that it changes state in a particular call
to credit_init_bits(), which might be made from atomic context, which
means we must kick off a workqueue to change the static key. Further
complicating things, credit_init_bits() may be called sufficiently early
on in system initialization such that system_wq is NULL.

Fortunately, there exists the nice function execute_in_process_context(),
which will immediately execute the function if !in_interrupt(), and
otherwise defer it to a workqueue. During early init, before workqueues
are available, in_interrupt() is always false, because interrupts
haven't even been enabled yet, which means the function in that case
executes immediately. Later on, after workqueues are available,
in_interrupt() might be true, but in that case, the work is queued in
system_wq and all goes well.

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Reviewed-by: default avatarDominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: default avatarJason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 087a14b9

drivers/char/random.c

+12 −4
Original line number Diff line number Diff line
@@ -77,8 +77,9 @@ static enum { 
	CRNG_EMPTY = 0, /* Little to no entropy collected */

	CRNG_EARLY = 1, /* At least POOL_EARLY_BITS collected */

	CRNG_READY = 2  /* Fully initialized with POOL_READY_BITS collected */

} crng_init = CRNG_EMPTY;

#define crng_ready() (likely(crng_init >= CRNG_READY))

} crng_init __read_mostly = CRNG_EMPTY;

static DEFINE_STATIC_KEY_FALSE(crng_is_ready);

#define crng_ready() (static_branch_likely(&crng_is_ready) || crng_init >= CRNG_READY)

/* Various types of waiters for crng_init->CRNG_READY transition. */

static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);

static struct fasync_struct *fasync;
@@ -108,6 +109,11 @@ bool rng_is_initialized(void) 
}

EXPORT_SYMBOL(rng_is_initialized);



static void crng_set_ready(struct work_struct *work)

{

	static_branch_enable(&crng_is_ready);

}



/* Used by wait_for_random_bytes(), and considered an entropy collector, below. */

static void try_to_generate_entropy(void);
@@ -267,7 +273,7 @@ static void crng_reseed(void) 
		++next_gen;

	WRITE_ONCE(base_crng.generation, next_gen);

	WRITE_ONCE(base_crng.birth, jiffies);

	if (!crng_ready())

	if (!static_branch_likely(&crng_is_ready))

		crng_init = CRNG_READY;

	spin_unlock_irqrestore(&base_crng.lock, flags);

	memzero_explicit(key, sizeof(key));
@@ -782,6 +788,7 @@ static void extract_entropy(void *buf, size_t nbytes) 


static void credit_init_bits(size_t nbits)

{

	static struct execute_work set_ready;

	unsigned int new, orig, add;

	unsigned long flags;
@@ -797,6 +804,7 @@ static void credit_init_bits(size_t nbits) 


	if (orig < POOL_READY_BITS && new >= POOL_READY_BITS) {

		crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */

		execute_in_process_context(crng_set_ready, &set_ready);

		process_random_ready_list();

		wake_up_interruptible(&crng_init_wait);

		kill_fasync(&fasync, SIGIO, POLL_IN);
@@ -1306,7 +1314,7 @@ SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count, unsigned int, 
	if (count > INT_MAX)

		count = INT_MAX;



	if (!(flags & GRND_INSECURE) && !crng_ready()) {

	if (!crng_ready() && !(flags & GRND_INSECURE)) {

		int ret;



		if (flags & GRND_NONBLOCK)