Re: ext4's dependency on crc32c

["Theodore Ts'o" <tytso@mit.edu>]

On Wed, Feb 02, 2022 at 09:08:03AM +0100, Jan Beulich wrote:
> > Sure, there are some subtleties, though.  For example, we would need
> > to make sure that sbi->s_chksum_driver() is initialized before we
> > attempt to use it.  That's because an malicious attacker (or syzbot
> > fuzzer --- is there a difference? :-) could force the file system
> > feature bits to be set after we decide whether or not to allocate the
> > crypto handle.  This can happen by having a maliciously corrupted file
> > system image which sets the file system feature bits as part of the
> > journal replay, or simply by writing to the superblock after it is
> > mounted.
> 
> Can any of this happen for an ext3 partition (without destroying its
> ext3 nature)? IOW would it be possible to set sbi->s_chksum_driver
> depending on just file system type rather than individual features?

The idea of "an ext3 partition" is not well defined, at least in terms
of the on-disk format.  The ext2/ext3/ext4 superblock has a set of
feature flags, the compat, r/o, and incompat feature flags.  You can
take an "ext3" file system, and enable the "extents" feature, and on
modern kernels (where "mount -t ext3" is handled by the ext4 file
system), new files which are created will be extent-mapped.

You can look at what "mke2fs -t ext3" and "mke2fs -t ext4" will do ---
although that will change over time as you install new versions of
e2fsprogs, but it can also be modified by editing the /etc/mke2fs.conf
file, either becaue a distribution wants to be more aggressive about
enabling a bleeding edge feature (such as fast commits), or because a
particular system adminsitrator or company wants to explicitly enable
or disable some features for their workload:

[defaults]
	base_features = sparse_super,large_file,filetype,resize_inode,dir_index,ext_attr
	default_mntopts = acl,user_xattr
	enable_periodic_fsck = 0
	blocksize = 4096
	inode_size = 256
	...
	
[fs_types]
	ext3 = {
		features = has_journal
	}
	ext4 = {
		features = has_journal,extent,huge_file,flex_bg,metadata_csum,64bit,dir_nlink,extra_isize
	}
	....

Cheers,

					- Ted