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 check_ids() for precise
scalar values.
Changelog:
- 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/20230526184126.3104040-1-eddyz87@xxxxxxxxx/
[V2] https://lore.kernel.org/bpf/20230530172739.447290-1-eddyz87@xxxxxxxxx/T/
[RFC] https://lore.kernel.org/bpf/20221128163442.280187-1-eddyz87@xxxxxxxxx/
[1] https://gist.github.com/eddyz87/a32ea7e62a27d3c201117c9a39ab4286
[2] https://lore.kernel.org/bpf/20230530172739.447290-1-eddyz87@xxxxxxxxx/T/#mc21009dcd8574b195c1860a98014bb037f16f450
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 | 10 +-
kernel/bpf/verifier.c | 148 ++++-
.../selftests/bpf/prog_tests/verifier.c | 2 +
.../selftests/bpf/progs/verifier_scalar_ids.c | 508 ++++++++++++++++++
.../bpf/progs/verifier_search_pruning.c | 3 +-
.../testing/selftests/bpf/verifier/precise.c | 8 +-
6 files changed, 669 insertions(+), 10 deletions(-)
create mode 100644 tools/testing/selftests/bpf/progs/verifier_scalar_ids.c
--
2.40.1
