eCryptfs: ensure copy to crypt_stat->cipher does not overrun

The patch 237fead61998: "[PATCH] ecryptfs: fs/Makefile and
fs/Kconfig" from Oct 4, 2006, leads to the following static checker
warning:

  fs/ecryptfs/crypto.c:846 ecryptfs_new_file_context()
  error: off-by-one overflow 'crypt_stat->cipher' size 32.  rl = '0-32'

There is a mismatch between the size of ecryptfs_crypt_stat.cipher
and ecryptfs_mount_crypt_stat.global_default_cipher_name causing the
copy of the cipher name to cause a off-by-one string copy error. This
fix ensures the space reserved for this string is the same size including
the trailing zero at the end throughout ecryptfs.

This fix avoids increasing the size of ecryptfs_crypt_stat.cipher
and also ecryptfs_parse_tag_70_packet_silly_stack.cipher_string and instead
reduces the of ECRYPTFS_MAX_CIPHER_NAME_SIZE to 31 and includes the + 1 for
the end of string terminator.

NOTE: An overflow is not possible in practice since the value copied
into global_default_cipher_name is validated by
ecryptfs_code_for_cipher_string() at mount time. None of the allowed
cipher strings are long enough to cause the potential buffer overflow
fixed by this patch.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
[tyhicks: Added the NOTE about the overflow not being triggerable]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>

fs/ecryptfs/ecryptfs_kernel.h

@@ -124,7 +124,7 @@ ecryptfs_get_key_payload_data(struct key *key)
 }
 
 #define ECRYPTFS_MAX_KEYSET_SIZE 1024
-#define ECRYPTFS_MAX_CIPHER_NAME_SIZE 32
+#define ECRYPTFS_MAX_CIPHER_NAME_SIZE 31
 #define ECRYPTFS_MAX_NUM_ENC_KEYS 64
 #define ECRYPTFS_MAX_IV_BYTES 16	/* 128 bits */
 #define ECRYPTFS_SALT_BYTES 2
@@ -237,7 +237,7 @@ struct ecryptfs_crypt_stat {
 	struct crypto_ablkcipher *tfm;
 	struct crypto_hash *hash_tfm; /* Crypto context for generating
 				       * the initialization vectors */
-	unsigned char cipher[ECRYPTFS_MAX_CIPHER_NAME_SIZE];
+	unsigned char cipher[ECRYPTFS_MAX_CIPHER_NAME_SIZE + 1];
 	unsigned char key[ECRYPTFS_MAX_KEY_BYTES];
 	unsigned char root_iv[ECRYPTFS_MAX_IV_BYTES];
 	struct list_head keysig_list;

fs/ecryptfs/keystore.c

@@ -891,7 +891,7 @@ struct ecryptfs_parse_tag_70_packet_silly_stack {
 	struct blkcipher_desc desc;
 	char fnek_sig_hex[ECRYPTFS_SIG_SIZE_HEX + 1];
 	char iv[ECRYPTFS_MAX_IV_BYTES];
-	char cipher_string[ECRYPTFS_MAX_CIPHER_NAME_SIZE];
+	char cipher_string[ECRYPTFS_MAX_CIPHER_NAME_SIZE + 1];
 };
 
 /**

fs/ecryptfs/main.c

@@ -407,7 +407,7 @@ static int ecryptfs_parse_options(struct ecryptfs_sb_info *sbi, char *options,
 	if (!cipher_name_set) {
 		int cipher_name_len = strlen(ECRYPTFS_DEFAULT_CIPHER);
 
-		BUG_ON(cipher_name_len >= ECRYPTFS_MAX_CIPHER_NAME_SIZE);
+		BUG_ON(cipher_name_len > ECRYPTFS_MAX_CIPHER_NAME_SIZE);
 		strcpy(mount_crypt_stat->global_default_cipher_name,
 		       ECRYPTFS_DEFAULT_CIPHER);
 	}