Re: ext4's dependency on crc32c

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

On Tue, Feb 01, 2022 at 03:19:54PM +0100, Jan Beulich wrote:
> Hello,
> 
> in 5.16, due to (afaict) adad556efcdd ("crypto: api - Fix built-in
> testing dependency failures") booting a system with cryptmgr.ko not
> (perhaps manually) put in the initrd doesn't work when ext4.ko is
> responsible for / . I've contacted Herbert already after finding
> this issue with btrfs, but in the case of ext4 another aspect plays
> into it: I've observed the problem on a system where ext4.ko is used
> solely to service ext3 partitions (including / ), but aiui crc32c
> isn't used at all in this case. Yet it's the attempt of loading it
> which actually causes the mount (and hence booting) to fail.
> 
> If my understanding is correct, wouldn't it make sense to skip the
> call to crypto_alloc_shash() unless an ext4 superblock is being
> processed?

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.

	       	       				- Ted