[PATCH 4.9 206/264] random: use static branch for crng_ready()

[Greg Kroah-Hartman]

From: "Jason A. Donenfeld" <Jason@xxxxxxxxx>

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@xxxxxxx>
Cc: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
Reviewed-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/char/random.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -78,8 +78,9 @@ 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;
@@ -109,6 +110,11 @@ 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);
 
@@ -268,7 +274,7 @@ 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));
@@ -783,6 +789,7 @@ static void extract_entropy(void *buf, s
 
 static void credit_init_bits(size_t nbits)
 {
+	static struct execute_work set_ready;
 	unsigned int new, orig, add;
 	unsigned long flags;
 
@@ -798,6 +805,7 @@ static void credit_init_bits(size_t nbit
 
 	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);
@@ -1307,7 +1315,7 @@ SYSCALL_DEFINE3(getrandom, char __user *
 	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)