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 <[email protected]>
> ---
> 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;