Re: [PATCH bpf-next 1/3] bpf: verifier: Do not extract constant map keys for irrelevant maps

[Eduard Zingerman]

On Sat, 2025-02-01 at 12:58 -0700, Daniel Xu wrote:
> Previously, we were trying to extract constant map keys for all
> bpf_map_lookup_elem(), regardless of map type. This is an issue if the
> map has a u64 key and the value is very high, as it can be interpreted
> as a negative signed value. This in turn is treated as an error value by
> check_func_arg() which causes a valid program to be incorrectly
> rejected.
> 
> Fix by only extracting constant map keys for relevant maps. See next
> commit for an example via selftest.
> 
> Reported-by: Marc Hartmayer <mhartmay@xxxxxxxxxxxxx>
> Reported-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> Tested-by: Marc Hartmayer <mhartmay@xxxxxxxxxxxxx>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>

Acked-by: Eduard Zingerman <eddyz87@xxxxxxxxx>

Nit:
  would be good if commit message said something along the lines:
  ... the fix works because nullness elision is only allowed for
      {PERCPU_}ARRAY maps, and keys for these are within u32 range ...

> ---
>  kernel/bpf/verifier.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9971c03adfd5..e9176a5ce215 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9206,6 +9206,8 @@ static s64 get_constant_map_key(struct bpf_verifier_env *env,
>  	return reg->var_off.value;
>  }
>  
> +static bool can_elide_value_nullness(enum bpf_map_type type);
> +
>  static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>  			  struct bpf_call_arg_meta *meta,
>  			  const struct bpf_func_proto *fn,
> @@ -9354,9 +9356,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>  		err = check_helper_mem_access(env, regno, key_size, BPF_READ, false, NULL);
>  		if (err)
>  			return err;
> -		meta->const_map_key = get_constant_map_key(env, reg, key_size);
> -		if (meta->const_map_key < 0 && meta->const_map_key != -EOPNOTSUPP)
> -			return meta->const_map_key;
> +		if (can_elide_value_nullness(meta->map_ptr->map_type)) {
> +			meta->const_map_key = get_constant_map_key(env, reg, key_size);
> +			if (meta->const_map_key < 0 && meta->const_map_key != -EOPNOTSUPP)
> +				return meta->const_map_key;
> +		}
>  		break;
>  	case ARG_PTR_TO_MAP_VALUE:
>  		if (type_may_be_null(arg_type) && register_is_null(reg))