securityfs: Extend securityfs with namespacing support

Extend 'securityfs' for support of IMA namespacing so that each
IMA (user) namespace can have its own front-end for showing the currently
active policy, the measurement list, number of violations and so on.

Drop the addition dentry reference to enable simple cleanup of dentries
upon umount.

Prevent mounting of an instance of securityfs in another user namespace
than it belongs to. Also, prevent accesses to directories when another
user namespace is active than the one that the instance of securityfs
belongs to.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

security/inode.c

@@ -26,6 +26,29 @@
 static struct vfsmount *init_securityfs_mount;
 static int init_securityfs_mount_count;
 
+static int securityfs_permission(struct user_namespace *mnt_userns,
+				 struct inode *inode, int mask)
+{
+	int err;
+
+	err = generic_permission(&init_user_ns, inode, mask);
+	if (!err) {
+		if (inode->i_sb->s_user_ns != current_user_ns())
+			err = -EACCES;
+	}
+
+	return err;
+}
+
+const struct inode_operations securityfs_dir_inode_operations = {
+	.permission	= securityfs_permission,
+	.lookup		= simple_lookup,
+};
+
+const struct inode_operations securityfs_file_inode_operations = {
+	.permission	= securityfs_permission,
+};
+
 static void securityfs_free_inode(struct inode *inode)
 {
 	if (S_ISLNK(inode->i_mode))
@@ -41,20 +64,25 @@ static const struct super_operations securityfs_super_operations = {
 static int securityfs_fill_super(struct super_block *sb, struct fs_context *fc)
 {
 	static const struct tree_descr files[] = {{""}};
+	struct user_namespace *ns = fc->user_ns;
 	int error;
 
+	if (WARN_ON(ns != current_user_ns()))
+		return -EINVAL;
+
 	error = simple_fill_super(sb, SECURITYFS_MAGIC, files);
 	if (error)
 		return error;
 
 	sb->s_op = &securityfs_super_operations;
+	sb->s_root->d_inode->i_op = &securityfs_dir_inode_operations;
 
 	return 0;
 }
 
 static int securityfs_get_tree(struct fs_context *fc)
 {
-	return get_tree_single(fc, securityfs_fill_super);
+	return get_tree_keyed(fc, securityfs_fill_super, fc->user_ns);
 }
 
 static const struct fs_context_operations securityfs_context_ops = {
@@ -72,6 +100,7 @@ static struct file_system_type fs_type = {
 	.name =		"securityfs",
 	.init_fs_context = securityfs_init_fs_context,
 	.kill_sb =	kill_litter_super,
+	.fs_flags =	FS_USERNS_MOUNT,
 };
 
 /**
@@ -157,18 +186,18 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode,
 	inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
 	inode->i_private = data;
 	if (S_ISDIR(mode)) {
-		inode->i_op = &simple_dir_inode_operations;
+		inode->i_op = &securityfs_dir_inode_operations;
 		inode->i_fop = &simple_dir_operations;
 		inc_nlink(inode);
 		inc_nlink(dir);
 	} else if (S_ISLNK(mode)) {
 		inode->i_op = iops ? iops : &simple_symlink_inode_operations;
 		inode->i_link = data;
 	} else {
+		inode->i_op = &securityfs_file_inode_operations;
 		inode->i_fop = fops;
 	}
 	d_instantiate(dentry, inode);
-	dget(dentry);
 	inode_unlock(dir);
 	return dentry;
 
@@ -316,10 +345,12 @@ void securityfs_remove(struct dentry *dentry)
 	dir = d_inode(dentry->d_parent);
 	inode_lock(dir);
 	if (simple_positive(dentry)) {
+		dget(dentry);
 		if (d_is_dir(dentry))
 			simple_rmdir(dir, dentry);
 		else
 			simple_unlink(dir, dentry);
+		d_delete(dentry);
 		dput(dentry);
 	}
 	inode_unlock(dir);