crypto: jitterentropy - replace long-held spinlock with mutex

jent_kcapi_random() serializes the shared jitterentropy state, but it
currently holds a spinlock across the jent_read_entropy() call. That
path performs expensive jitter collection and SHA3 conditioning, so
parallel readers can trigger stalls as contending waiters spin for
the same lock.

To prevent non-preemptible lock hold, replace rng->jent_lock with a
mutex so contended readers sleep instead of spinning on a shared lock
held across expensive entropy generation.

Fixes: bb5530e ("crypto: jitterentropy - add jitterentropy RNG")
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Yuan Tan <yuantan098@gmail.com>
Suggested-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Haixin Xu <jerryxucs@gmail.com>
Reviewed-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Author: hxjerry

crypto/jitterentropy-kcapi.c

@@ -42,6 +42,7 @@
 #include <linux/fips.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/slab.h>
 #include <linux/time.h>
 #include <crypto/internal/rng.h>
@@ -193,7 +194,7 @@ int jent_read_random_block(void *hash_state, char *dst, unsigned int dst_len)
  ***************************************************************************/
 
 struct jitterentropy {
-	spinlock_t jent_lock;
+	struct mutex jent_lock;
 	struct rand_data *entropy_collector;
 	struct crypto_shash *tfm;
 	struct shash_desc *sdesc;
@@ -203,7 +204,7 @@ static void jent_kcapi_cleanup(struct crypto_tfm *tfm)
 {
 	struct jitterentropy *rng = crypto_tfm_ctx(tfm);
 
-	spin_lock(&rng->jent_lock);
+	mutex_lock(&rng->jent_lock);
 
 	if (rng->sdesc) {
 		shash_desc_zero(rng->sdesc);
@@ -218,7 +219,7 @@ static void jent_kcapi_cleanup(struct crypto_tfm *tfm)
 	if (rng->entropy_collector)
 		jent_entropy_collector_free(rng->entropy_collector);
 	rng->entropy_collector = NULL;
-	spin_unlock(&rng->jent_lock);
+	mutex_unlock(&rng->jent_lock);
 }
 
 static int jent_kcapi_init(struct crypto_tfm *tfm)
@@ -228,7 +229,7 @@ static int jent_kcapi_init(struct crypto_tfm *tfm)
 	struct shash_desc *sdesc;
 	int size, ret = 0;
 
-	spin_lock_init(&rng->jent_lock);
+	mutex_init(&rng->jent_lock);
 
 	/* Use SHA3-256 as conditioner */
 	hash = crypto_alloc_shash(JENT_CONDITIONING_HASH, 0, 0);
@@ -257,7 +258,6 @@ static int jent_kcapi_init(struct crypto_tfm *tfm)
 		goto err;
 	}
 
-	spin_lock_init(&rng->jent_lock);
 	return 0;
 
 err:
@@ -272,7 +272,7 @@ static int jent_kcapi_random(struct crypto_rng *tfm,
 	struct jitterentropy *rng = crypto_rng_ctx(tfm);
 	int ret = 0;
 
-	spin_lock(&rng->jent_lock);
+	mutex_lock(&rng->jent_lock);
 
 	ret = jent_read_entropy(rng->entropy_collector, rdata, dlen);
 
@@ -298,7 +298,7 @@ static int jent_kcapi_random(struct crypto_rng *tfm,
 		ret = -EINVAL;
 	}
 
-	spin_unlock(&rng->jent_lock);
+	mutex_unlock(&rng->jent_lock);
 
 	return ret;
 }