Re: [PATCH bpf-next v6 02/23] bpf/verifier: allow kfunc to read user provided context

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Tue, 12 Jul 2022 at 17:02, Benjamin Tissoires
<benjamin.tissoires@redhat.com> 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.
>
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
>
> ---
>
> new in v6
> ---
>  kernel/bpf/verifier.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 328cfab3af60..f6af57a84247 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 +5225,19 @@ 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) {
> +                       enum bpf_access_type access_t = meta->raw_mode ? BPF_WRITE : BPF_READ;

small nit: _t suffix is used for types, so you could probably rename
this. maybe atype?

> +
> +                       return check_mem_access(env, env->insn_idx, regno, access_size, BPF_B,
> +                                               access_t, -1, false);

If I read the code correctly, this makes the max_ctx_offset of prog
access_size + 1 (off + size_to_bytes(BPF_B)), which is 1 more than the
actual size being accessed.

This also messes up check_helper_mem_access when it allows NULL, 0
pair to pass (because check is against actual size + 1). We do allow
passing NULL when size is 0 for kfuncs (see zero_size_allowed is true
in check_mem_size_reg), so your hid_hw_request function is missing
that NULL check for buf too.

In the selftest that checks for failure in loading
+ bpf_kfunc_call_test_mem_len_pass1(&args->data, sizeof(*args) + 1);
so it will still fail with just sizeof(*args).

Also please add coverage for this case in the next version.

> +               }
> +
> +               fallthrough;
>         default: /* scalar_value or invalid ptr */
>                 /* Allow zero-byte read from NULL, regardless of pointer type */
>                 if (zero_size_allowed && access_size == 0 &&
> @@ -5335,6 +5350,7 @@ int check_kfunc_mem_size_reg(struct bpf_verifier_env *env, struct bpf_reg_state
>         WARN_ON_ONCE(regno < BPF_REG_2 || regno > BPF_REG_5);
>
>         memset(&meta, 0, sizeof(meta));
> +       meta.is_kfunc = true;
>
>         if (may_be_null) {
>                 saved_reg = *mem_reg;
> --
> 2.36.1
>