dm ima: prefix dm table hashes in ima log with hash algorithm

The active/inactive table hashes measured in the ima log do not contain
the information about hash algorithm.  This information is useful for the
attestation servers to recreate the hashes and compare them with the ones
present in the ima log to verify the table contents.

Prefix the table hashes in various DM events in ima log with the hash
algorithm used to compute those hashes.
Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-ima.c

@@ -186,6 +186,11 @@ void dm_ima_measure_on_table_load(struct dm_table *table, unsigned int status_fl
 	struct crypto_shash *tfm = NULL;
 	u8 *digest = NULL;
 	bool noio = false;
+	/*
+	 * In below hash_alg_prefix_len assignment +1 is for the additional char (':'),
+	 * when prefixing the hash value with the hash algorithm name. e.g. sha256:<hash_value>.
+	 */
+	const size_t hash_alg_prefix_len = strlen(DM_IMA_TABLE_HASH_ALG) + 1;
 
 	ima_buf = dm_ima_alloc(DM_IMA_MEASUREMENT_BUF_LEN, GFP_KERNEL, noio);
 	if (!ima_buf)
@@ -204,7 +209,7 @@ void dm_ima_measure_on_table_load(struct dm_table *table, unsigned int status_fl
 	if (dm_ima_alloc_and_copy_device_data(table->md, &device_data_buf, num_targets, noio))
 		goto error;
 
-	tfm = crypto_alloc_shash("sha256", 0, 0);
+	tfm = crypto_alloc_shash(DM_IMA_TABLE_HASH_ALG, 0, 0);
 	if (IS_ERR(tfm))
 		goto error;
 
@@ -315,12 +320,15 @@ void dm_ima_measure_on_table_load(struct dm_table *table, unsigned int status_fl
 	if (r < 0)
 		goto error;
 
-	digest_buf = dm_ima_alloc((digest_size*2)+1, GFP_KERNEL, noio);
+	digest_buf = dm_ima_alloc((digest_size*2) + hash_alg_prefix_len + 1, GFP_KERNEL, noio);
+
 	if (!digest_buf)
 		goto error;
 
+	snprintf(digest_buf, hash_alg_prefix_len + 1, "%s:", DM_IMA_TABLE_HASH_ALG);
+
 	for (i = 0; i < digest_size; i++)
-		snprintf((digest_buf+(i*2)), 3, "%02x", digest[i]);
+		snprintf((digest_buf + hash_alg_prefix_len + (i*2)), 3, "%02x", digest[i]);
 
 	if (table->md->ima.active_table.hash != table->md->ima.inactive_table.hash)
 		kfree(table->md->ima.inactive_table.hash);

drivers/md/dm-ima.h

@@ -16,6 +16,7 @@
 #define DM_IMA_TARGET_METADATA_BUF_LEN	128
 #define DM_IMA_TARGET_DATA_BUF_LEN	2048
 #define DM_IMA_DEVICE_CAPACITY_BUF_LEN	128
+#define DM_IMA_TABLE_HASH_ALG		"sha256"
 
 #ifdef CONFIG_IMA