[PATCH bpf v2 0/2] libbpf: Reject out-of-range linker relocation offsets

[HyeongJun An <sammiee5311@gmail.com>]

The libbpf static linker validates relocation type, symbol index and
instruction alignment in linker_sanity_check_elf_relos(), but does not
check that the relocation offset is inside the relocated section.  A
malformed BPF object processed by the static linker (e.g. via
"bpftool gen object") can therefore carry an out-of-range r_offset that
linker_append_elf_relos() then uses to index the section data, reading
and writing past the buffer.

The normal object-loading path already rejects such offsets (libbpf.c,
rel->r_offset >= scn_data->d_size); the static linker path is the
missing sibling.  Patch 1 adds the same bound.  Patch 2 adds a selftest
that builds a tiny object with an out-of-range relocation offset and
checks that the linker now rejects it, with a valid relocation as a
positive control.

Reproduced with ASAN: before patch 1 the out-of-range relocation is
accepted (and triggers a heap-buffer-overflow); after, it is rejected
with -EINVAL.

Changes in v2:
- selftest: set the generated ELF object's EI_DATA from the host byte
  order instead of hardcoding little-endian, so it works on big-endian
  hosts (e.g. s390x).
- selftest: add fallback definitions for EM_BPF and R_BPF_64_64 for
  older system headers.
Patch 1 (the fix) is unchanged.

HyeongJun An (2):
  libbpf: Reject out-of-range linker relocation offsets
  selftests/bpf: Test linker rejects out-of-range relocation offset

 tools/lib/bpf/linker.c                        |   6 +
 .../selftests/bpf/prog_tests/libbpf_linker.c  | 231 ++++++++++++++++++
 2 files changed, 237 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/libbpf_linker.c

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2.43.0