random: handle creditable entropy from atomic process context

The entropy accounting changes a static key when the RNG has
initialized, since it only ever initializes once. Static key changes,
however, cannot be made from atomic context, so depending on where the
last creditable entropy comes from, the static key change might need to
be deferred to a worker.

Previously the code used the execute_in_process_context() helper
function, which accounts for whether or not the caller is
in_interrupt(). However, that doesn't account for the case where the
caller is actually in process context but is holding a spinlock.

This turned out to be the case with input_handle_event() in
drivers/input/input.c contributing entropy:

  [<ffffffd613025ba0>] die+0xa8/0x2fc
  [<ffffffd613027428>] bug_handler+0x44/0xec
  [<ffffffd613016964>] brk_handler+0x90/0x144
  [<ffffffd613041e58>] do_debug_exception+0xa0/0x148
  [<ffffffd61400c208>] el1_dbg+0x60/0x7c
  [<ffffffd61400c000>] el1h_64_sync_handler+0x38/0x90
  [<ffffffd613011294>] el1h_64_sync+0x64/0x6c
  [<ffffffd613102d88>] __might_resched+0x1fc/0x2e8
  [<ffffffd613102b54>] __might_sleep+0x44/0x7c
  [<ffffffd6130b6eac>] cpus_read_lock+0x1c/0xec
  [<ffffffd6132c2820>] static_key_enable+0x14/0x38
  [<ffffffd61400ac08>] crng_set_ready+0x14/0x28
  [<ffffffd6130df4dc>] execute_in_process_context+0xb8/0xf8
  [<ffffffd61400ab30>] _credit_init_bits+0x118/0x1dc
  [<ffffffd6138580c8>] add_timer_randomness+0x264/0x270
  [<ffffffd613857e54>] add_input_randomness+0x38/0x48
  [<ffffffd613a80f94>] input_handle_event+0x2b8/0x490
  [<ffffffd613a81310>] input_event+0x6c/0x98

According to Guoyong, it's not really possible to refactor the various
drivers to never hold a spinlock there. And in_atomic() isn't reliable.

So, rather than trying to be too fancy, just punt the change in the
static key to a workqueue always. There's basically no drawback of doing
this, as the code already needed to account for the static key not
changing immediately, and given that it's just an optimization, there's
not exactly a hurry to change the static key right away, so deferal is
fine.
Reported-by: Guoyong Wang <guoyong.wang@mediatek.com>
Cc: stable@vger.kernel.org
Fixes: f5bda35f ("random: use static branch for crng_ready()")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -702,7 +702,7 @@ static void extract_entropy(void *buf, size_t len)
 
 static void __cold _credit_init_bits(size_t bits)
 {
-	static struct execute_work set_ready;
+	static DECLARE_WORK(set_ready, crng_set_ready);
 	unsigned int new, orig, add;
 	unsigned long flags;
 
@@ -718,8 +718,8 @@ static void __cold _credit_init_bits(size_t bits)
 
 	if (orig < POOL_READY_BITS && new >= POOL_READY_BITS) {
 		crng_reseed(NULL); /* Sets crng_init to CRNG_READY under base_crng.lock. */
-		if (static_key_initialized)
-			execute_in_process_context(crng_set_ready, &set_ready);
+		if (static_key_initialized && system_unbound_wq)
+			queue_work(system_unbound_wq, &set_ready);
 		atomic_notifier_call_chain(&random_ready_notifier, 0, NULL);
 		wake_up_interruptible(&crng_init_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);
@@ -890,8 +890,8 @@ void __init random_init(void)
 
 	/*
 	 * If we were initialized by the cpu or bootloader before jump labels
-	 * are initialized, then we should enable the static branch here, where
-	 * it's guaranteed that jump labels have been initialized.
+	 * or workqueues are initialized, then we should enable the static
+	 * branch here, where it's guaranteed that these have been initialized.
 	 */
 	if (!static_branch_likely(&crng_is_ready) && crng_init >= CRNG_READY)
 		crng_set_ready(NULL);