CVE-2022-50277: ext4: don't allow journal inode to have encrypt flag

In the Linux kernel, the following vulnerability has been resolved:

ext4: don't allow journal inode to have encrypt flag

Mounting a filesystem whose journal inode has the encrypt flag causes a NULL dereference in fscryptlimitioblocks() when the 'inlinecrypt' mount option is used.

The problem is that when jbd2journalinitinode() calls bmap(), it eventually finds its way into ext4iomapbegin(), which calls fscryptlimitioblocks(). fscryptlimitioblocks() requires that if the inode is encrypted, then its encryption key must already be set up. That's not the case here, since the journal inode is never "opened" like a normal file would be. Hence the crash.

A reproducer is:

mkfs.ext4 -F /dev/vdb debugfs -w /dev/vdb -R "setinodefield <8> flags 0x80808" mount /dev/vdb /mnt -o inlinecrypt

To fix this, make ext4 consider journal inodes with the encrypt flag to be invalid. (Note, maybe other flags should be rejected on the journal inode too. For now, this is just the minimal fix for the above issue.)

I've marked this as fixing the commit that introduced the call to fscryptlimitioblocks(), since that's what made an actual crash start being possible. But this fix could be applied to any version of ext4 that supports the encrypt feature.