hwrng: core - let sleep be interrupted when unregistering hwrng

There are two deadlock scenarios that need addressing, which cause
problems when the computer goes to sleep, the interface is set down, and
hwrng_unregister() is called. When the deadlock is hit, sleep is delayed
for tens of seconds, causing it to fail. These scenarios are:

1) The hwrng kthread can't be stopped while it's sleeping, because it
   uses msleep_interruptible() which does not react to kthread_stop.

2) A normal user thread can't be interrupted by hwrng_unregister() while
   it's sleeping, because hwrng_unregister() is called from elsewhere.

We solve both issues by add a completion object called dying that
fulfils waiters once we have started the process in hwrng_unregister.

At the same time, we should cleanup a common and useless dmesg splat
in the same area.

Cc: <stable@vger.kernel.org>
Reported-by: Gregory Erwin <gregerwin256@gmail.com>
Fixes: fcd09c90c3c5 ("ath9k: use hw_random API instead of directly dumping into random.c")
Link: https://lore.kernel.org/all/CAO+Okf6ZJC5-nTE_EJUGQtd8JiCkiEHytGgDsFGTEjs0c00giw@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CAO+Okf5k+C+SE6pMVfPf-d8MfVPVq4PO7EY8Hys_DVXtent3HA@mail.gmail.com/
Link: https://bugs.archlinux.org/task/75138
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Acked-by: Kalle Valo <kvalo@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 15 insertions, 4 deletions

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 16f227b995e8..d7045dfaf16c 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -507,16 +507,17 @@ static int hwrng_fillfn(void *unused)
 			rng->quality = current_quality; /* obsolete */
 		quality = rng->quality;
 		mutex_unlock(&reading_mutex);
+
+		if (rc <= 0)
+			hwrng_msleep(rng, 10000);
+
 		put_rng(rng);
 
 		if (!quality)
 			break;
 
-		if (rc <= 0) {
-			pr_warn("hwrng: no data available\n");
-			msleep_interruptible(10000);
+		if (rc <= 0)
 			continue;
-		}
 
 		/* If we cannot credit at least one bit of entropy,
 		 * keep track of the remainder for the next iteration
@@ -570,6 +571,7 @@ int hwrng_register(struct hwrng *rng)
 
 	init_completion(&rng->cleanup_done);
 	complete(&rng->cleanup_done);
+	init_completion(&rng->dying);
 
 	if (!current_rng ||
 	    (!cur_rng_set_by_user && rng->quality > current_rng->quality)) {
@@ -617,6 +619,7 @@ void hwrng_unregister(struct hwrng *rng)
 
 	old_rng = current_rng;
 	list_del(&rng->list);
+	complete_all(&rng->dying);
 	if (current_rng == rng) {
 		err = enable_best_rng();
 		if (err) {
@@ -685,6 +688,14 @@ void devm_hwrng_unregister(struct device *dev, struct hwrng *rng)
 }
 EXPORT_SYMBOL_GPL(devm_hwrng_unregister);
 
+long hwrng_msleep(struct hwrng *rng, unsigned int msecs)
+{
+	unsigned long timeout = msecs_to_jiffies(msecs) + 1;
+
+	return wait_for_completion_interruptible_timeout(&rng->dying, timeout);
+}
+EXPORT_SYMBOL_GPL(hwrng_msleep);
+
 static int __init hwrng_modinit(void)
 {
 	int ret;