Re: [PATCH bpf-next 2/4] bpf: verifier: explain opcode check in check_ld_imm()

[Yonghong Song <yhs@fb.com>]

On 5/20/22 4:37 AM, Shung-Hsi Yu wrote:
> The BPF_SIZE check in the beginning of check_ld_imm() actually guard
> against program with JMP instructions that goes to the second
> instruction of BPF_LD_IMM64, but may be easily dismissed as an simple
> opcode check that's duplicating the effort of bpf_opcode_in_insntable().
> 
> Add comment to better reflect the importance of the check.
> 
> Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
> ---
>   kernel/bpf/verifier.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 79a2695ee2e2..133929751f80 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9921,6 +9921,10 @@ static int check_ld_imm(struct bpf_verifier_env *env, struct bpf_insn *insn)
>   	struct bpf_map *map;
>   	int err;
>   
> +	/* checks that this is not the second part of BPF_LD_IMM64, which is
> +	 * skipped over during opcode check, but a JMP with invalid offset may
> +	 * cause check_ld_imm() to be called upon it.
> +	 */

The check_ld_imm() call context is:

                 } else if (class == BPF_LD) {
                         u8 mode = BPF_MODE(insn->code);

                         if (mode == BPF_ABS || mode == BPF_IND) {
                                 err = check_ld_abs(env, insn);
                                 if (err)
                                         return err;

                         } else if (mode == BPF_IMM) {
                                 err = check_ld_imm(env, insn);
                                 if (err)
                                         return err;

                                 env->insn_idx++;
                                 sanitize_mark_insn_seen(env);
                         } else {
                                 verbose(env, "invalid BPF_LD mode\n");
                                 return -EINVAL;
                         }
                 }

which is a normal checking of LD_imm64 insn.

I think the to-be-added comment is incorrect and unnecessary.

>   	if (BPF_SIZE(insn->code) != BPF_DW) {
>   		verbose(env, "invalid BPF_LD_IMM insn\n");
>   		return -EINVAL;