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

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

On Sat Jun 6, 2026 at 1:43 AM CEST, Paul Moses wrote:
> btf_parse_struct_metas() walks user-supplied BTF during BPF_BTF_LOAD,
> and btf_repeat_fields() expands repeatable fields from array elements
> into the fixed BTF_FIELDS_MAX scratch array used by btf_parse_fields().
>
> The remaining-capacity check performs the expanded field count calculation
> in u32. A malformed BTF can wrap that calculation, causing the check to
> pass even when the expanded field count exceeds the scratch array
> capacity. The following memcpy() can then write past the end of the
> array.
>
> Use checked addition and multiplication before copying repeated fields
> and reject impossible counts.
>
> Fixes: 797d73ee232d ("bpf: Check the remaining info_cnt before repeating btf fields")
> Cc: stable@vger.kernel.org
> Signed-off-by: Paul Moses <p@1g4.org>
> ---

Do you have an example where this actually occurred in practice?

>  kernel/bpf/btf.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index a62d78581207..510aa32847da 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3668,7 +3668,7 @@ static int btf_get_field_type(const struct btf *btf, const struct btf_type *var_
>  static int btf_repeat_fields(struct btf_field_info *info, int info_cnt,
>  			     u32 field_cnt, u32 repeat_cnt, u32 elem_size)
>  {
> -	u32 i, j;
> +	u32 i, j, total_cnt, total_repeats;
>  	u32 cur;
>
>  	/* Ensure not repeating fields that should not be repeated. */
> @@ -3686,10 +3686,9 @@ static int btf_repeat_fields(struct btf_field_info *info, int info_cnt,
>  		}
>  	}
>
> -	/* The type of struct size or variable size is u32,
> -	 * so the multiplication will not overflow.
> -	 */
> -	if (field_cnt * (repeat_cnt + 1) > info_cnt)
> +	if (check_add_overflow(repeat_cnt, 1, &total_repeats) ||
> +	    check_mul_overflow(field_cnt, total_repeats, &total_cnt) ||
> +	    total_cnt > (u32)info_cnt)
>  		return -E2BIG;
>
>  	cur = field_cnt;