linux-ext4 - Re: ext4's dependency on crc32c

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Date:   Wed, 2 Feb 2022 09:08:03 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: ext4's dependency on crc32c

On 01.02.2022 16:47, Theodore Ts'o wrote:
> 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.

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?

Jan