evm: add file system uuid to EVM hmac

EVM uses the same key for all file systems to calculate the HMAC,
making it possible to paste inodes from one file system on to another
one, without EVM being able to detect it. To prevent such an attack,
it is necessary to make the EVM HMAC file system specific.

This patch uses the file system UUID, a file system unique identifier,
to bind the EVM HMAC to the file system. The value inode->i_sb->s_uuid
is used for the HMAC hash calculation, instead of using it for deriving
the file system specific key. Initializing the key for every inode HMAC
calculation is a bit more expensive operation than adding the uuid to
the HMAC hash.

Changing the HMAC calculation method or adding additional info to the
calculation, requires existing EVM labeled file systems to be relabeled.
This patch adds a Kconfig HMAC version option for backwards compatability.

Changelog v1:
- squash "hmac version setting"
Changelog v0:
- add missing Kconfig depends (Mimi)

Signed-off-by: Dmitry Kasatkin <[e-mail]>
Signed-off-by: Mimi Zohar <[e-mail]>

security/integrity/evm/Kconfig

···
 	  integrity attacks.
 
 	  If you are unsure how to answer this question, answer N.
+
+config EVM_HMAC_VERSION
+	int "EVM HMAC version"
+	depends on EVM
+	default 2
+	help
+	  This options adds EVM HMAC version support.
+	  1 - original version
+	  2 - add per filesystem unique identifier (UUID) (default)
+
+	  WARNING: changing the HMAC calculation method or adding 
+	  additional info to the calculation, requires existing EVM
+	  labeled file systems to be relabeled.  

security/integrity/evm/evm.h

···
 extern int evm_initialized;
 extern char *evm_hmac;
 extern char *evm_hash;
+extern int evm_hmac_version;
 
 extern struct crypto_shash *hmac_tfm;
 extern struct crypto_shash *hash_tfm;

security/integrity/evm/evm_crypto.c

···
 	hmac_misc.gid = from_kgid(&init_user_ns, inode->i_gid);
 	hmac_misc.mode = inode->i_mode;
 	crypto_shash_update(desc, (const u8 *)&hmac_misc, sizeof hmac_misc);
+	if (evm_hmac_version > 1)
+		crypto_shash_update(desc, inode->i_sb->s_uuid,
+				    sizeof(inode->i_sb->s_uuid));
 	crypto_shash_final(desc, digest);
 }
 

security/integrity/evm/evm_main.c

···
 
 char *evm_hmac = "hmac(sha1)";
 char *evm_hash = "sha1";
+int evm_hmac_version = CONFIG_EVM_HMAC_VERSION;
 
 char *evm_config_xattrnames[] = {
 #ifdef CONFIG_SECURITY_SELINUX