[PATCH 6.1 3/3] crypto: jitterentropy - replace long-held spinlock with mutex

[Eric Biggers <ebiggers@kernel.org>]

From: Haixin Xu <jerryxucs@gmail.com>

commit 01d798e9feb30212952d4e992801ba6bd6a82351 upstream.

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: bb5530e40824 ("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>
[ebiggers: backported without commit bb897c55042e ("crypto: jitter -
 replace LFSR with SHA3-256")]
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 crypto/jitterentropy-kcapi.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/crypto/jitterentropy-kcapi.c b/crypto/jitterentropy-kcapi.c
index b9edfaa51b273..29c830b47c5e2 100644
--- a/crypto/jitterentropy-kcapi.c
+++ b/crypto/jitterentropy-kcapi.c
@@ -40,6 +40,7 @@
 #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>
@@ -96,7 +97,7 @@ void jent_get_nstime(__u64 *out)
  ***************************************************************************/
 
 struct jitterentropy {
-	spinlock_t jent_lock;
+	struct mutex jent_lock;
 	struct rand_data *entropy_collector;
 };
 
@@ -109,7 +110,7 @@ static int jent_kcapi_init(struct crypto_tfm *tfm)
 	if (!rng->entropy_collector)
 		ret = -ENOMEM;
 
-	spin_lock_init(&rng->jent_lock);
+	mutex_init(&rng->jent_lock);
 	return ret;
 }
 
@@ -117,11 +118,11 @@ 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->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_random(struct crypto_rng *tfm,
@@ -131,7 +132,7 @@ 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);
 
@@ -157,7 +158,7 @@ static int jent_kcapi_random(struct crypto_rng *tfm,
 		ret = -EINVAL;
 	}
 
-	spin_unlock(&rng->jent_lock);
+	mutex_unlock(&rng->jent_lock);
 
 	return ret;
 }
-- 
2.56.0