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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Herbert Xu <herbert@gondor.apana.org.au>,
        linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Dominik Brodowski <linux@dominikbrodowski.net>
Subject: [PATCH v2] hw_random: core: start hwrng kthread also for untrusted sources
Date:   Tue, 20 Sep 2022 16:21:59 +0200
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From: Dominik Brodowski <linux@dominikbrodowski.net>

Start the hwrng kthread even if the hwrng source has a quality setting
of zero. Then, every CRNG_RESEED_INTERVAL, one batch of data from this
zero-quality hwrng source will be mixed into the CRNG pool.

However, to avoid that an untrusted device assists in initializing the
CRNG, go to sleep in add_hwgenerator_randomness() in case the entropy
parameter passed to that function is zero.

This patch is based on the assumption that data from a hwrng source
will not actively harm the CRNG state, but that many hwrng sources
(such as TPM devices), even though they are assigend a quality level of
zero, actually provide some entropy, which is good to mix into the CRNG
pool every once in a while.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
---
This is Dominik's v1, verbatim, with the random.c changes split out, per
Herbert's request.

(It'd be nice if this would land soon, as there are other nice things
that could be layered on top of this change later.)

 drivers/char/hw_random/core.c | 36 ++++++++++-------------------------
 1 file changed, 10 insertions(+), 26 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 16f227b995e8..edb86c0cccda 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -52,7 +52,7 @@ MODULE_PARM_DESC(default_quality,
 
 static void drop_current_rng(void);
 static int hwrng_init(struct hwrng *rng);
-static void hwrng_manage_rngd(struct hwrng *rng);
+static int hwrng_fillfn(void *unused);
 
 static inline int rng_get_data(struct hwrng *rng, u8 *buffer, size_t size,
 			       int wait);
@@ -96,6 +96,15 @@ static int set_current_rng(struct hwrng *rng)
 	drop_current_rng();
 	current_rng = rng;
 
+	/* if necessary, start hwrng thread */
+	if (!hwrng_fill) {
+		hwrng_fill = kthread_run(hwrng_fillfn, NULL, "hwrng");
+		if (IS_ERR(hwrng_fill)) {
+			pr_err("hwrng_fill thread creation failed\n");
+			hwrng_fill = NULL;
+		}
+	}
+
 	return 0;
 }
 
@@ -167,8 +176,6 @@ static int hwrng_init(struct hwrng *rng)
 		rng->quality = 1024;
 	current_quality = rng->quality; /* obsolete */
 
-	hwrng_manage_rngd(rng);
-
 	return 0;
 }
 
@@ -454,10 +461,6 @@ static ssize_t rng_quality_store(struct device *dev,
 	/* the best available RNG may have changed */
 	ret = enable_best_rng();
 
-	/* start/stop rngd if necessary */
-	if (current_rng)
-		hwrng_manage_rngd(current_rng);
-
 out:
 	mutex_unlock(&rng_mutex);
 	return ret ? ret : len;
@@ -509,9 +512,6 @@ static int hwrng_fillfn(void *unused)
 		mutex_unlock(&reading_mutex);
 		put_rng(rng);
 
-		if (!quality)
-			break;
-
 		if (rc <= 0) {
 			pr_warn("hwrng: no data available\n");
 			msleep_interruptible(10000);
@@ -533,22 +533,6 @@ static int hwrng_fillfn(void *unused)
 	return 0;
 }
 
-static void hwrng_manage_rngd(struct hwrng *rng)
-{
-	if (WARN_ON(!mutex_is_locked(&rng_mutex)))
-		return;
-
-	if (rng->quality == 0 && hwrng_fill)
-		kthread_stop(hwrng_fill);
-	if (rng->quality > 0 && !hwrng_fill) {
-		hwrng_fill = kthread_run(hwrng_fillfn, NULL, "hwrng");
-		if (IS_ERR(hwrng_fill)) {
-			pr_err("hwrng_fill thread creation failed\n");
-			hwrng_fill = NULL;
-		}
-	}
-}
-
 int hwrng_register(struct hwrng *rng)
 {
 	int err = -EINVAL;
-- 
2.37.3