Re: [PATCH RFC 0/7] fs: Debug config option to disable filesystem checksum verification for fuzzing

[David Sterba]

On Fri, Oct 14, 2022 at 08:48:30AM +0000, Hrutvik Kanabar wrote:
> From: Hrutvik Kanabar <hrutvik@xxxxxxxxxx>
> 
> Fuzzing is a proven technique to discover exploitable bugs in the Linux
> kernel. But fuzzing filesystems is tricky: highly structured disk images
> use redundant checksums to verify data integrity. Therefore,
> randomly-mutated images are quickly rejected as corrupt, testing only
> error-handling code effectively.
> 
> The Janus [1] and Hydra [2] projects probe filesystem code deeply by
> correcting checksums after mutation. But their ad-hoc
> checksum-correcting code supports only a few filesystems, and it is
> difficult to support new ones - requiring significant duplication of
> filesystem logic which must also be kept in sync with upstream changes.
> Corrected checksums cannot be guaranteed to be valid, and reusing this
> code across different fuzzing frameworks is non-trivial.
> 
> Instead, this RFC suggests a config option:
> `DISABLE_FS_CSUM_VERIFICATION`. When it is enabled, all filesystems
> should bypass redundant checksum verification, proceeding as if
> checksums are valid. Setting of checksums should be unaffected. Mutated
> images will no longer be rejected due to invalid checksums, allowing
> testing of deeper code paths. Though some filesystems implement their
> own flags to disable some checksums, this option should instead disable
> all checksums for all filesystems uniformly. Critically, any bugs found
> remain reproducible on production systems: redundant checksums in
> mutated images can be fixed up to satisfy verification.
> 
> The patches below suggest a potential implementation for a few
> filesystems, though we may have missed some checksums. The option
> requires `DEBUG_KERNEL` and is not intended for production systems.
> 
> The first user of the option would be syzbot. We ran preliminary local
> syzkaller tests to compare behaviour with and without these patches.
> With the patches, we found a 19% increase in coverage, as well as many
> new crash types and increases in the total number of crashes:

I think the build-time option inflexible, but I see the point when
you're testing several filesystems that it's one place to set up the
environment. Alternatively I suggest to add sysfs knob available in
debuging builds to enable/disable checksum verification per filesystem.

As this may not fit to other filesystems I don't suggest to do that for
all but I am willing to do that for btrfs, with eventual extension to
the config option you propose. The increased fuzzing coverage would be
good to have.