ima: fix ima_alloc_atfm()

The patch 3bcced39ea7d: "ima: use ahash API for file hash
calculation" from Feb 26, 2014, leads to the following static checker
warning:

security/integrity/ima/ima_crypto.c:204 ima_alloc_atfm()
         error: buffer overflow 'hash_algo_name' 17 <= 17

Unlike shash tfm memory, which is allocated on initialization, the
ahash tfm memory allocation is deferred until needed.

This patch fixes the case where ima_ahash_tfm has not yet been
allocated and the file's signature/hash xattr contains an invalid hash
algorithm.  Although we can not verify the xattr, we still need to
measure the file.  Use the default IMA hash algorithm.

Changelog:
- set valid algo before testing tfm - based on Dmitry's comment

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>

security/integrity/ima/ima_crypto.c

@@ -200,7 +200,10 @@ static struct crypto_ahash *ima_alloc_atfm(enum hash_algo algo)
 	struct crypto_ahash *tfm = ima_ahash_tfm;
 	int rc;
 
-	if ((algo != ima_hash_algo && algo < HASH_ALGO__LAST) || !tfm) {
+	if (algo < 0 || algo >= HASH_ALGO__LAST)
+		algo = ima_hash_algo;
+
+	if (algo != ima_hash_algo || !tfm) {
 		tfm = crypto_alloc_ahash(hash_algo_name[algo], 0, 0);
 		if (!IS_ERR(tfm)) {
 			if (algo == ima_hash_algo)