Re: [PATCH v2 bpf-next 01/16] bpf: Introduce bpf_sys_bpf() helper and program type.

On Fri, Apr 23, 2021 at 11:16 AM Yonghong Song [off-list ref] wrote:

quoted

+
+static bool syscall_prog_is_valid_access(int off, int size,
+                                      enum bpf_access_type type,
+                                      const struct bpf_prog *prog,
+                                      struct bpf_insn_access_aux *info)
+{
+     if (off < 0 || off >= U16_MAX)
+             return false;

Is this enough? If I understand correctly, the new program type
allows any arbitrary context data from user as long as its size
meets the following constraints:
    if (ctx_size_in < prog->aux->max_ctx_offset ||
            ctx_size_in > U16_MAX)
                return -EINVAL;

So if user provides a ctx with size say 40 and inside the program looks
it is still able to read/write to say offset 400.
Should we be a little more restrictive on this?

At the load time the program can have a read/write at offset 400,
but it will be rejected at prog_test_run time.
That's similar to tp and raw_tp test_run-s and attach-es.
That's why test_run has that check you've quoted.
It's a two step verification.
The verifier rejects <0 || > u16_max right away and
keeps the track of max_ctx_offset.
Then at attach/test_run the final check is done with an actual ctx_size_in.