evm: Check also if *tfm is an error pointer in init_desc()

This patch avoids a kernel panic due to accessing an error pointer set by
crypto_alloc_shash(). It occurs especially when there are many files that
require an unsupported algorithm, as it would increase the likelihood of
the following race condition:

Task A: *tfm = crypto_alloc_shash() <= error pointer
Task B: if (*tfm == NULL) <= *tfm is not NULL, use it
Task B: rc = crypto_shash_init(desc) <= panic
Task A: *tfm = NULL

This patch uses the IS_ERR_OR_NULL macro to determine whether or not a new
crypto context must be created.

Cc: [email protected]
Fixes: d46eb36995 ("evm: crypto hash replaced by shash")
Co-developed-by: Krzysztof Struczynski <[email protected]>
Signed-off-by: Krzysztof Struczynski <[email protected]>
Signed-off-by: Roberto Sassu <[email protected]>
Signed-off-by: Mimi Zohar <[email protected]>

security/integrity/evm/evm_crypto.c

@@ -91,7 +91,7 @@ static struct shash_desc *init_desc(char type, uint8_t hash_algo)
 		algo = hash_algo_name[hash_algo];
 	}
 
-	if (*tfm == NULL) {
+	if (IS_ERR_OR_NULL(*tfm)) {
 		mutex_lock(&mutex);
 		if (*tfm)
 			goto out;