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))
