Re: [PATCH bpf-next v8 02/24] bpf/verifier: allow kfunc to read user provided context

[Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>]

On Fri, Jul 22, 2022 at 1:46 AM Benjamin Tissoires
<benjamin.tissoires@xxxxxxxxxx> wrote:
>
> When a kfunc was trying to access data from context in a syscall eBPF
> program, the verifier was rejecting the call.
> This is because the syscall context is not known at compile time, and
> so we need to check this when actually accessing it.
>
> Check for the valid memory access and allow such situation to happen.
>
> Acked-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
>
> ---
>
> changes in v8:
> - fixup comment
> - return -EACCESS instead of -EINVAL for consistency
>
> changes in v7:
> - renamed access_t into atype
> - allow zero-byte read
> - check_mem_access() to the correct offset/size
>
> new in v6
> ---
>  kernel/bpf/verifier.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 7c1e056624f9..c807c5d7085a 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -248,6 +248,7 @@ struct bpf_call_arg_meta {
>         struct bpf_map *map_ptr;
>         bool raw_mode;
>         bool pkt_access;
> +       bool is_kfunc;
>         u8 release_regno;
>         int regno;
>         int access_size;
> @@ -5170,6 +5171,7 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
>                                    struct bpf_call_arg_meta *meta)
>  {
>         struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
> +       enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
>         u32 *max_access;
>
>         switch (base_type(reg->type)) {
> @@ -5223,6 +5225,24 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
>                                 env,
>                                 regno, reg->off, access_size,
>                                 zero_size_allowed, ACCESS_HELPER, meta);
> +       case PTR_TO_CTX:
> +               /* in case of a kfunc called in a program of type SYSCALL, the context is
> +                * user supplied, so not computed statically.
> +                * Dynamically check it now
> +                */
> +               if (prog_type == BPF_PROG_TYPE_SYSCALL && meta && meta->is_kfunc) {

prog_type check looks a bit odd here.
Can we generalize with
if (!env->ops->convert_ctx_access

In other words any program type that doesn't have ctx rewrites can
use helpers to access ctx fields ?

Also why kfunc only?
It looks safe to allow normal helpers as well.