Re: [PATCH bpf-next v2 4/6] bpf: override the meaning of ARG_PTR_TO_MAP_VALUE for sockmap and sockhash

[Yonghong Song <yhs@fb.com>]

On 8/20/20 6:57 AM, Lorenz Bauer wrote:
> The verifier assumes that map values are simple blobs of memory, and
> therefore treats ARG_PTR_TO_MAP_VALUE, etc. as such. However, there are
> map types where this isn't true. For example, sockmap and sockhash store
> sockets. In general this isn't a big problem: we can just
> write helpers that explicitly requests PTR_TO_SOCKET instead of
> ARG_PTR_TO_MAP_VALUE.
> 
> The one exception are the standard map helpers like map_update_elem,
> map_lookup_elem, etc. Here it would be nice we could overload the
> function prototype for different kinds of maps. Unfortunately, this
> isn't entirely straight forward:
> We only know the type of the map once we have resolved meta->map_ptr
> in check_func_arg. This means we can't swap out the prototype
> in check_helper_call until we're half way through the function.
> 
> Instead, modify check_func_arg to treat ARG_PTR_TO_MAP_VALUE* to
> mean "the native type for the map" instead of "pointer to memory"
> for sockmap and sockhash. This means we don't have to modify the
> function prototype at all
> 
> Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
> ---
>   kernel/bpf/verifier.c | 37 +++++++++++++++++++++++++++++++++++++
>   1 file changed, 37 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index b6ccfce3bf4c..24feec515d3e 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3872,6 +3872,35 @@ static int int_ptr_type_to_size(enum bpf_arg_type type)
>   	return -EINVAL;
>   }
>   
> +static int resolve_map_arg_type(struct bpf_verifier_env *env,
> +				 const struct bpf_call_arg_meta *meta,
> +				 enum bpf_arg_type *arg_type)
> +{
> +	if (!meta->map_ptr) {
> +		/* kernel subsystem misconfigured verifier */
> +		verbose(env, "invalid map_ptr to access map->type\n");
> +		return -EACCES;
> +	}
> +
> +	switch (meta->map_ptr->map_type) {
> +	case BPF_MAP_TYPE_SOCKMAP:
> +	case BPF_MAP_TYPE_SOCKHASH:
> +		if (*arg_type == ARG_PTR_TO_MAP_VALUE) {
> +			*arg_type = ARG_PTR_TO_SOCKET;
> +		} else if (*arg_type == ARG_PTR_TO_MAP_VALUE_OR_NULL) {
> +			*arg_type = ARG_PTR_TO_SOCKET_OR_NULL;

Is this *arg_type == ARG_PTR_TO_MAP_VALUE_OR_NULL possible with
current implementation?

If not, we can remove this "else if" and return -EINVAL, right?

> +		} else {
> +			verbose(env, "invalid arg_type for sockmap/sockhash\n");
> +			return -EINVAL;
> +		}
> +		break;
> +
> +	default:
> +		break;
> +	}
> +	return 0;
> +}
> +
>   static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>   			  struct bpf_call_arg_meta *meta,
>   			  const struct bpf_func_proto *fn)
> @@ -3904,6 +3933,14 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>   		return -EACCES;
>   	}
>   
> +	if (arg_type == ARG_PTR_TO_MAP_VALUE ||
> +	    arg_type == ARG_PTR_TO_UNINIT_MAP_VALUE ||
> +	    arg_type == ARG_PTR_TO_MAP_VALUE_OR_NULL) {
> +		err = resolve_map_arg_type(env, meta, &arg_type);

I am okay with this to cover all MAP_VALUE types with func
name resolve_map_arg_type as a generic helper.

> +		if (err)
> +			return err;
> +	}
> +
>   	if (arg_type == ARG_PTR_TO_MAP_KEY ||
>   	    arg_type == ARG_PTR_TO_MAP_VALUE ||
>   	    arg_type == ARG_PTR_TO_UNINIT_MAP_VALUE ||
>