smack: Initialize the in-memory inode in smack_inode_init_security()

Currently, Smack initializes in-memory new inodes in three steps. It first
sets the xattrs in smack_inode_init_security(), fetches them in
smack_d_instantiate() and finally, in the same function, sets the in-memory
inodes depending on xattr values, unless they are in specially-handled
filesystems.

Other than being inefficient, this also prevents filesystems not supporting
xattrs from working properly since, without xattrs, there is no way to pass
the label determined in smack_inode_init_security() to
smack_d_instantiate().

Since the LSM infrastructure allows setting and getting the security field
without xattrs through the inode_setsecurity and inode_getsecurity hooks,
make the inode creation work too, by initializing the in-memory inode
earlier in smack_inode_init_security().

Also mark the inode as instantiated, to prevent smack_d_instantiate() from
overwriting the security field. As mentioned above, this potentially has
impact for inodes in specially-handled filesystems in
smack_d_instantiate(), if they are not handled in the same way in
smack_inode_init_security().

Filesystems other than tmpfs don't call security_inode_init_security(), so
they would be always initialized in smack_d_instantiate(), as before. For
tmpfs, the current behavior is to assign to inodes the label '*', but
actually that label is overwritten with the one fetched from the SMACK64
xattr, set in smack_inode_init_security() (default: '_').

Initializing the in-memory inode is straightforward: if not transmuting,
nothing more needs to be done; if transmuting, overwrite the current inode
label with the one from the parent directory, and set SMK_INODE_TRANSMUTE.
Finally, set SMK_INODE_INSTANT for all cases, to mark the inode as
instantiated.
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack_lsm.c

@@ -994,6 +994,7 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
 				     struct xattr *xattrs, int *xattr_count)
 {
 	struct task_smack *tsp = smack_cred(current_cred());
+	struct inode_smack *issp = smack_inode(inode);
 	struct smack_known *skp = smk_of_task(tsp);
 	struct smack_known *isp = smk_of_inode(inode);
 	struct smack_known *dsp = smk_of_inode(dir);
@@ -1029,7 +1030,9 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
 		 * smack_inode_alloc_security().
 		 */
 		if (tsp->smk_task != tsp->smk_transmuted)
-			isp = dsp;
+			isp = issp->smk_inode = dsp;
+
+		issp->smk_flags |= SMK_INODE_TRANSMUTE;
 		xattr_transmute = lsm_get_xattr_slot(xattrs,
 						     xattr_count);
 		if (xattr_transmute) {
@@ -1044,6 +1047,8 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
 		}
 	}
 
+	issp->smk_flags |= SMK_INODE_INSTANT;
+
 	if (xattr) {
 		xattr->value = kstrdup(isp->smk_known, GFP_NOFS);
 		if (!xattr->value)