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

[Hrutvik Kanabar]

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:

Filesystem | % new crash types | % increase in crashes
—----------|-------------------|----------------------
  ext4     |        60%        |         1400%
  btrfs    |        25%        |         185%
  f2fs     |        63%        |         16%


[1] Fuzzing file systems via two-dimensional input space exploration,
    Xu et al., 2019, IEEE Symposium on Security and Privacy,
    doi: 10.1109/SP.2019.00035
[2] Finding semantic bugs in file systems with an extensible fuzzing
    framework, Kim et al., 2019, ACM Symposium on Operating Systems
    Principles, doi: 10.1145/3341301.3359662


Hrutvik Kanabar (7):
  fs: create `DISABLE_FS_CSUM_VERIFICATION` config option
  fs/ext4: support `DISABLE_FS_CSUM_VERIFICATION` config option
  fs/btrfs: support `DISABLE_FS_CSUM_VERIFICATION` config option
  fs/exfat: support `DISABLE_FS_CSUM_VERIFICATION` config option
  fs/xfs: support `DISABLE_FS_CSUM_VERIFICATION` config option
  fs/ntfs: support `DISABLE_FS_CSUM_VERIFICATION` config option
  fs/f2fs: support `DISABLE_FS_CSUM_VERIFICATION` config option

 fs/Kconfig.debug            | 20 ++++++++++++++++++++
 fs/btrfs/check-integrity.c  |  3 ++-
 fs/btrfs/disk-io.c          |  6 ++++--
 fs/btrfs/free-space-cache.c |  3 ++-
 fs/btrfs/inode.c            |  3 ++-
 fs/btrfs/scrub.c            |  9 ++++++---
 fs/exfat/nls.c              |  3 ++-
 fs/exfat/super.c            |  3 +++
 fs/ext4/bitmap.c            |  6 ++++--
 fs/ext4/extents.c           |  3 ++-
 fs/ext4/inode.c             |  3 ++-
 fs/ext4/ioctl.c             |  3 ++-
 fs/ext4/mmp.c               |  3 ++-
 fs/ext4/namei.c             |  6 ++++--
 fs/ext4/orphan.c            |  3 ++-
 fs/ext4/super.c             |  6 ++++--
 fs/ext4/xattr.c             |  3 ++-
 fs/f2fs/checkpoint.c        |  3 ++-
 fs/f2fs/compress.c          |  3 ++-
 fs/f2fs/f2fs.h              |  2 ++
 fs/f2fs/inode.c             |  3 +++
 fs/ntfs/super.c             |  3 ++-
 fs/xfs/libxfs/xfs_cksum.h   |  5 ++++-
 lib/Kconfig.debug           |  6 ++++++
 24 files changed, 86 insertions(+), 25 deletions(-)
 create mode 100644 fs/Kconfig.debug

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2.38.0.413.g74048e4d9e-goog