random: fix crash on multiple early calls to add_bootloader_randomness()

Currently, if CONFIG_RANDOM_TRUST_BOOTLOADER is enabled, multiple calls
to add_bootloader_randomness() are broken and can cause a NULL pointer
dereference, as noted by Ivan T. Ivanov. This is not only a hypothetical
problem, as qemu on arm64 may provide bootloader entropy via EFI and via
devicetree.

On the first call to add_hwgenerator_randomness(), crng_fast_load() is
executed, and if the seed is long enough, crng_init will be set to 1.
On subsequent calls to add_bootloader_randomness() and then to
add_hwgenerator_randomness(), crng_fast_load() will be skipped. Instead,
wait_event_interruptible() and then credit_entropy_bits() will be called.
If the entropy count for that second seed is large enough, that proceeds
to crng_reseed().

However, both wait_event_interruptible() and crng_reseed() depends
(at least in numa_crng_init()) on workqueues. Therefore, test whether
system_wq is already initialized, which is a sufficient indicator that
workqueue_init_early() has progressed far enough.

If we wind up hitting the !system_wq case, we later want to do what
would have been done there when wqs are up, so set a flag, and do that
work later from the rand_initialize() call.

Reported-by: Ivan T. Ivanov <iivanov@suse.de>
Fixes: 18b915a ("efi/random: Treat EFI_RNG_PROTOCOL output as bootloader randomness")
Cc: stable@vger.kernel.org
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
[Jason: added crng_need_done state and related logic.]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Author: Dominik Brodowski

drivers/char/random.c

@@ -468,6 +468,7 @@ static struct crng_state primary_crng = {
  * its value (from 0->1->2).
  */
 static int crng_init = 0;
+static bool crng_need_final_init = false;
 #define crng_ready() (likely(crng_init > 1))
 static int crng_init_cnt = 0;
 static unsigned long crng_global_init_time = 0;
@@ -835,6 +836,36 @@ static void __init crng_initialize_primary(struct crng_state *crng)
 	crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;
 }
 
+static void crng_finalize_init(struct crng_state *crng)
+{
+	if (crng != &primary_crng || crng_init >= 2)
+		return;
+	if (!system_wq) {
+		/* We can't call numa_crng_init until we have workqueues,
+		 * so mark this for processing later. */
+		crng_need_final_init = true;
+		return;
+	}
+
+	invalidate_batched_entropy();
+	numa_crng_init();
+	crng_init = 2;
+	process_random_ready_list();
+	wake_up_interruptible(&crng_init_wait);
+	kill_fasync(&fasync, SIGIO, POLL_IN);
+	pr_notice("crng init done\n");
+	if (unseeded_warning.missed) {
+		pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
+			  unseeded_warning.missed);
+		unseeded_warning.missed = 0;
+	}
+	if (urandom_warning.missed) {
+		pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
+			  urandom_warning.missed);
+		urandom_warning.missed = 0;
+	}
+}
+
 #ifdef CONFIG_NUMA
 static void do_numa_crng_init(struct work_struct *work)
 {
@@ -989,25 +1020,7 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
 	memzero_explicit(&buf, sizeof(buf));
 	WRITE_ONCE(crng->init_time, jiffies);
 	spin_unlock_irqrestore(&crng->lock, flags);
-	if (crng == &primary_crng && crng_init < 2) {
-		invalidate_batched_entropy();
-		numa_crng_init();
-		crng_init = 2;
-		process_random_ready_list();
-		wake_up_interruptible(&crng_init_wait);
-		kill_fasync(&fasync, SIGIO, POLL_IN);
-		pr_notice("crng init done\n");
-		if (unseeded_warning.missed) {
-			pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
-				  unseeded_warning.missed);
-			unseeded_warning.missed = 0;
-		}
-		if (urandom_warning.missed) {
-			pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
-				  urandom_warning.missed);
-			urandom_warning.missed = 0;
-		}
-	}
+	crng_finalize_init(crng);
 }
 
 static void _extract_crng(struct crng_state *crng,
@@ -1780,6 +1793,8 @@ static void __init init_std_data(struct entropy_store *r)
 int __init rand_initialize(void)
 {
 	init_std_data(&input_pool);
+	if (crng_need_final_init)
+		crng_finalize_init(&primary_crng);
 	crng_initialize_primary(&primary_crng);
 	crng_global_init_time = jiffies;
 	if (ratelimit_disable) {
@@ -2288,7 +2303,8 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
 	 * We'll be woken up again once below random_write_wakeup_thresh,
 	 * or when the calling thread is about to terminate.
 	 */
-	wait_event_interruptible(random_write_wait, kthread_should_stop() ||
+	wait_event_interruptible(random_write_wait,
+			!system_wq || kthread_should_stop() ||
 			ENTROPY_BITS(&input_pool) <= random_write_wakeup_bits);
 	mix_pool_bytes(poolp, buffer, count);
 	credit_entropy_bits(poolp, entropy);