More context is available in [0], but the TLDR; is that the verifier incorrectly assumes that any raw tracepoint argument will always be non-NULL. This means that even when users correctly check possible NULL arguments, the verifier can remove the NULL check due to incorrect knowledge of the NULL-ness of the pointer. Secondly, kernel helpers or kfuncs taking these trusted tracepoint arguments incorrectly assume that all arguments will always be valid non-NULL. In this set, we mark raw_tp arguments as PTR_MAYBE_NULL on top of PTR_TRUSTED, but special case their behavior when dereferencing them or pointer arithmetic over them is involved. When passing trusted args to helpers or kfuncs, raw_tp programs are permitted to pass possibly NULL pointers in such cases. Any loads into such maybe NULL trusted PTR_TO_BTF_ID is promoted to a PROBE_MEM load to handle emanating page faults. The verifier will ensure NULL checks on such pointers are preserved and do not lead to dead code elimination. This new behavior is not applied when ref_obj_id is non-zero, as those pointers do not belong to raw_tp arguments, but instead acquired objects. Since helpers and kfuncs already require attention for PTR_TO_BTF_ID (non-trusted) pointers, we do not implement any protection for such cases in this patch set, and leave it as future work for an upcoming series. A selftest is included with this patch set to verify the new behavior, and it crashes the kernel without the first patch. [0]: https://lore.kernel.org/bpf/CAADnVQLMPPavJQR6JFsi3dtaaLHB816JN4HCV_TFWohJ61D+wQ@xxxxxxxxxxxxxx Kumar Kartikeya Dwivedi (2): bpf: Mark raw_tp arguments with PTR_MAYBE_NULL selftests/bpf: Add tests for raw_tp null handling include/linux/bpf.h | 6 ++ kernel/bpf/btf.c | 5 +- kernel/bpf/verifier.c | 75 +++++++++++++++++-- .../bpf/bpf_testmod/bpf_testmod-events.h | 8 ++ .../selftests/bpf/bpf_testmod/bpf_testmod.c | 2 + .../selftests/bpf/prog_tests/raw_tp_null.c | 25 +++++++ .../testing/selftests/bpf/progs/raw_tp_null.c | 27 +++++++ .../bpf/progs/test_tp_btf_nullable.c | 6 +- 8 files changed, 145 insertions(+), 9 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/raw_tp_null.c create mode 100644 tools/testing/selftests/bpf/progs/raw_tp_null.c base-commit: e626a13f6fbb4697f8734333432dca577628d09a -- 2.43.5
