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

[Dmitry Vyukov]

On Fri, 14 Oct 2022 at 11:15, David Sterba <dsterba@xxxxxxx> wrote:
>
> 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.

Hi David,

What usage scenarios do you have in mind for runtime changing of this option?
I see this option intended only for very narrow use cases which
require a specially built kernel in a number of other ways (lots of
which are not tunable at runtime, e.g. debugging configs).

> 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.