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.
