Re: [PATCH bpf] bpf: verifier: check for packet data access based on target prog

[Yonghong Song]

On 8/18/20 6:12 PM, Udip Pant wrote:
> While using dynamic program extension (of type BPF_PROG_TYPE_EXT), we
> need to check the program type of the target program to grant the read /
> write access to the packet data.
>
> The BPF_PROG_TYPE_EXT type can be used to extend types such as XDP, SKB
> and others. Since the BPF_PROG_TYPE_EXT program type on itself is just a
> placeholder for those, we need this extended check for those target
> programs to actually work while using this option.
>
> Tested this with a freplace xdp program. Without this patch, the
> verifier fails with error 'cannot write into packet'.

Could you add a selftest for this? FYI, current selftests/bpf have
4 freplace programs:
  fexit_bpf2bpf.c:SEC("freplace/get_skb_len")
  fexit_bpf2bpf.c:SEC("freplace/get_skb_ifindex")
  fexit_bpf2bpf.c:SEC("freplace/get_constant")
  freplace_connect4.c:SEC("freplace/do_bind")


> Signed-off-by: Udip Pant <udippant@xxxxxx>
> ---
>   kernel/bpf/verifier.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index ef938f17b944..4d7604430994 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2629,7 +2629,11 @@ static bool may_access_direct_pkt_data(struct bpf_verifier_env *env,
>   				       const struct bpf_call_arg_meta *meta,
>   				       enum bpf_access_type t)
>   {
> -	switch (env->prog->type) {
> +	struct bpf_prog *prog = env->prog;
> +	enum bpf_prog_type prog_type = prog->aux->linked_prog ?
> +	      prog->aux->linked_prog->type : prog->type;
> +
> +	switch (prog_type) {
>   	/* Program types only with direct read access go here! */
>   	case BPF_PROG_TYPE_LWT_IN:
>   	case BPF_PROG_TYPE_LWT_OUT: