[bpf-next,v5,0/4] verify scalar ids mapping in regsafe()

Message

Eduard Zingerman June 12, 2023, 4:07 p.m. UTC

Update regsafe() to use check_ids() for scalar values.
Otherwise the following unsafe pattern is accepted by verifier:

  1: r9 = ... some pointer with range X ...
  2: r6 = ... unbound scalar ID=a ...
  3: r7 = ... unbound scalar ID=b ...
  4: if (r6 > r7) goto +1
  5: r6 = r7
  6: if (r6 > X) goto ...
  --- checkpoint ---
  7: r9 += r7
  8: *(u64 *)r9 = Y

This example is unsafe because not all execution paths verify r7 range.
Because of the jump at (4) the verifier would arrive at (6) in two states:
I.  r6{.id=b}, r7{.id=b} via path 1-6;
II. r6{.id=a}, r7{.id=b} via path 1-4, 6.

Currently regsafe() does not call check_ids() for scalar registers,
thus from POV of regsafe() states (I) and (II) are identical.

The change is split in two parts:
- patches #1,2: update for mark_chain_precision() to propagate
  precision marks through scalar IDs.
- patches #3,4: update for regsafe() to use a special version of
  check_ids() for precise scalar values.

Changelog:
- V4 -> V5 (all changes are based on feedback for V4 from Andrii):
  - mark_precise_scalar_ids() error code is updated to EFAULT;
  - bpf_verifier_env::idmap_scratch field type is changed to struct
    bpf_idmap to encapsulate temporary ID generation counter;
  - regsafe() is updated to call scalar_regs_exact() only for
    env->explore_alu_limits case (this had no measurable impact on
    verification duration when tested using veristat).
- V3 -> V4:
  - check_ids() in regsafe() is replaced by check_scalar_ids(),
    as discussed with Andrii in [3],
    Note: I did not transfer Andrii's ack for patch #3 from V3 because
          of the changes to the algorithm.
  - reg_id_scratch is renamed to idset_scratch;
  - mark_precise_scalar_ids() is modified to propagate error from
    idset_push();
  - test cases adjusted according to feedback from Andrii for V3.
- V2 -> V3:
  - u32_hashset for IDs used for range transfer is removed;
  - mark_chain_precision() is updated as discussed with Andrii in [2].
- V1 -> v2:
  - 'rold->precise' and 'rold->id' checks are dropped as unsafe
    (thanks to discussion with Yonghong);
  - patches #3,4 adding tracking of ids used for range transfer in
    order to mitigate performance impact.
- RFC -> V1:
  - Function verifier.c:mark_equal_scalars_as_read() is dropped,
    as it was an incorrect fix for problem solved by commit [3].
  - check_ids() is called only for precise scalar values.
  - Test case updated to use inline assembly.

[V1]  https://lore.kernel.org/bpf/[e-mail]/
[V2]  https://lore.kernel.org/bpf/[e-mail]/T/
[V3]  https://lore.kernel.org/bpf/[e-mail]/T/
[V4]  https://lore.kernel.org/bpf/[e-mail]/
[RFC] https://lore.kernel.org/bpf/[e-mail]/
[1]   https://gist.github.com/eddyz87/a32ea7e62a27d3c201117c9a39ab4286
[2]   https://lore.kernel.org/bpf/[e-mail]/T/#mc21009dcd8574b195c1860a98014bb037f16f450
[3]   https://lore.kernel.org/bpf/[e-mail]/T/#m89da8eeb2fa8c9ca1202c5d0b6660e1f72e45e04

Eduard Zingerman (4):
  bpf: use scalar ids in mark_chain_precision()
  selftests/bpf: check if mark_chain_precision() follows scalar ids
  bpf: verify scalar ids mapping in regsafe() using check_ids()
  selftests/bpf: verify that check_ids() is used for scalars in
    regsafe()

 include/linux/bpf_verifier.h                  |  25 +-
 kernel/bpf/verifier.c                         | 223 +++++-
 .../selftests/bpf/prog_tests/verifier.c       |   2 +
 .../selftests/bpf/progs/verifier_scalar_ids.c | 657 ++++++++++++++++++
 .../testing/selftests/bpf/verifier/precise.c  |   8 +-
 5 files changed, 883 insertions(+), 32 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_scalar_ids.c