fscrypto: require write access to mount to set encryption policy

BugLink: http://bugs.launchpad.net/bugs/1729107

commit ba63f23d upstream.

[Please apply to 4.4-stable.  Note: this was already backported, but
only to ext4; it was missed that it should go to f2fs as well.  This is
needed to make xfstest generic/395 pass on f2fs.]

Since setting an encryption policy requires writing metadata to the
filesystem, it should be guarded by mnt_want_write/mnt_drop_write.
Otherwise, a user could cause a write to a frozen or readonly
filesystem.  This was handled correctly by f2fs but not by ext4.  Make
fscrypt_process_policy() handle it rather than relying on the filesystem
to get it right.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/f2fs/file.c

@@ -1541,12 +1541,18 @@ static int f2fs_ioc_set_encryption_policy(struct file *filp, unsigned long arg)
 				sizeof(policy)))
 		return -EFAULT;
 
+	err = mnt_want_write_file(filp);
+	if (err)
+		return err;
+
 	mutex_lock(&inode->i_mutex);
 
 	err = f2fs_process_policy(&policy, inode);
 
 	mutex_unlock(&inode->i_mutex);
 
+	mnt_drop_write_file(filp);
+
 	return err;
 #else
 	return -EOPNOTSUPP;