evm: prevent racing during tfm allocation

There is a small chance of racing during tfm allocation.
This patch fixes it.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Acked-by: Mimi Zohar <zohar@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

security/integrity/evm/evm_crypto.c

@@ -27,26 +27,35 @@ static int evmkey_len = MAX_KEY_SIZE;
 
 struct crypto_shash *hmac_tfm;
 
+static DEFINE_MUTEX(mutex);
+
 static struct shash_desc *init_desc(void)
 {
 	int rc;
 	struct shash_desc *desc;
 
 	if (hmac_tfm == NULL) {
+		mutex_lock(&mutex);
+		if (hmac_tfm)
+			goto out;
 		hmac_tfm = crypto_alloc_shash(evm_hmac, 0, CRYPTO_ALG_ASYNC);
 		if (IS_ERR(hmac_tfm)) {
 			pr_err("Can not allocate %s (reason: %ld)\n",
 			       evm_hmac, PTR_ERR(hmac_tfm));
 			rc = PTR_ERR(hmac_tfm);
 			hmac_tfm = NULL;
+			mutex_unlock(&mutex);
 			return ERR_PTR(rc);
 		}
 		rc = crypto_shash_setkey(hmac_tfm, evmkey, evmkey_len);
 		if (rc) {
 			crypto_free_shash(hmac_tfm);
 			hmac_tfm = NULL;
+			mutex_unlock(&mutex);
 			return ERR_PTR(rc);
 		}
+out:
+		mutex_unlock(&mutex);
 	}
 
 	desc = kmalloc(sizeof(*desc) + crypto_shash_descsize(hmac_tfm),