Re: [PATCH bpf] bpf: Validate BTF repeated field counts before expansion

[Eduard Zingerman <eddyz87@gmail.com>]

On Sun, 2026-06-07 at 17:53 +0000, Paul Moses wrote:

[...]

The repro is legit.
Here is a somewhat minimized version as a selftest:

    diff --git a/tools/testing/selftests/bpf/prog_tests/btf.c b/tools/testing/selftests/bpf/prog_tests/btf.c
    index a9de328a8697..212ca4472a89 100644
    --- a/tools/testing/selftests/bpf/prog_tests/btf.c
    +++ b/tools/testing/selftests/bpf/prog_tests/btf.c
    @@ -4258,6 +4258,44 @@ static struct btf_raw_test raw_tests[] = {
            .max_entries = 1,
     },
    
    +{
    +#define N 0x1999999aU
    +       .descr = "repeat fields overflow",
    +       .raw_types = {
    +               /* int */                                               /* [1] */
    +               BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),
    +               /* struct target {} */                                  /* [2] */
    +               BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_STRUCT, 0, 0), 1),
    +               /* type_tag "kptr_untrusted" -> target */               /* [3] */
    +               BTF_TYPE_TAG_ENC(NAME_TBD, 2),
    +               /* target * (kptr) */                                   /* [4] */
    +               BTF_PTR_ENC(3),
    +               /* struct outer { target *kp; elem items[N]; } */       /* [5] */
    +               BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_STRUCT, 0, 2), (N * 8u + 8u)),
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* kp           */
    +               BTF_MEMBER_ENC(NAME_TBD, 6, 64),        /* items        */
    +               /* elem[N] */                                   /* [6] */
    +               BTF_TYPE_ARRAY_ENC(7, 1, N),
    +               /* struct elem { target *f0..f9; } */                   /* [7] */
    +               BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_STRUCT, 0, 10), 8),
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* f0           */
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* f1           */
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* f2           */
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* f3           */
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* f4           */
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* f5           */
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* f6           */
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* f7           */
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* f8           */
    +               BTF_MEMBER_ENC(NAME_TBD, 4, 0),         /* f9           */
    +               BTF_END_RAW,
    +       },
    +       BTF_STR_SEC("\0target\0kptr_untrusted\0outer\0kp\0items\0elem"
    +                   "\0f0\0f1\0f2\0f3\0f4\0f5\0f6\0f7\0f8\0f9"),
    +       .btf_load_err = true,
    +#undef N
    +},
    +
     }; /* struct btf_raw_test raw_tests[] */
    
     static const char *get_next_str(const char *start, const char *end)

However, as far as I understand the repro hits an overflow only
because `BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_STRUCT, 0, 10), 8)`
lies about `struct elem` size. It is specified as 8, while in reality it is 80.
The size of 80 would make `struct outer` unrepresentable in BTF,
because (N * 80 + 8) exceeds u32 range, and that's what btf_type->size uses.
Given that btf_repeat_fields() only traverses structs/arrays but not unions,
I suspect that overflow won't happen in `field_cnt * (repeat_cnt + 1)`
if proper size checks were implemented in btf_struct_check_meta() / btf_struct_resolve().
Even more, If I change "kptr_untrusted" to "kptr_untrusted11" to avoid fields parsing,
the kernel accepts the bogus BTF.

Paul, could you please investigate why is this happening?