Re: [PATCH bpf-next v3 1/3] bpf: Detect jumping to reserved code during check_cfg()

[Alexei Starovoitov]

On Wed, Oct 11, 2023 at 2:01 AM Hao Sun <sunhao.th@xxxxxxxxx> wrote:
>
> Currently, we don't check if the branch-taken of a jump is reserved code of
> ld_imm64. Instead, such a issue is captured in check_ld_imm(). The verifier
> gives the following log in such case:
>
> func#0 @0
> 0: R1=ctx(off=0,imm=0) R10=fp0
> 0: (18) r4 = 0xffff888103436000       ; R4_w=map_ptr(off=0,ks=4,vs=128,imm=0)
> 2: (18) r1 = 0x1d                     ; R1_w=29
> 4: (55) if r4 != 0x0 goto pc+4        ; R4_w=map_ptr(off=0,ks=4,vs=128,imm=0)
> 5: (1c) w1 -= w1                      ; R1_w=0
> 6: (18) r5 = 0x32                     ; R5_w=50
> 8: (56) if w5 != 0xfffffff4 goto pc-2
> mark_precise: frame0: last_idx 8 first_idx 0 subseq_idx -1
> mark_precise: frame0: regs=r5 stack= before 6: (18) r5 = 0x32
> 7: R5_w=50
> 7: BUG_ld_00
> invalid BPF_LD_IMM insn
>
> Here the verifier rejects the program because it thinks insn at 7 is an
> invalid BPF_LD_IMM, but such a error log is not accurate since the issue
> is jumping to reserved code not because the program contains invalid insn.
> Therefore, make the verifier check the jump target during check_cfg(). For
> the same program, the verifier reports the following log:
>
> func#0 @0
> jump to reserved code from insn 8 to 7
>
> Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
> ---
>  kernel/bpf/verifier.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index eed7350e15f4..725ac0b464cf 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -14980,6 +14980,7 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
>  {
>         int *insn_stack = env->cfg.insn_stack;
>         int *insn_state = env->cfg.insn_state;
> +       struct bpf_insn *insns = env->prog->insnsi;
>
>         if (e == FALLTHROUGH && insn_state[t] >= (DISCOVERED | FALLTHROUGH))
>                 return DONE_EXPLORING;
> @@ -14993,6 +14994,12 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
>                 return -EINVAL;
>         }
>
> +       if (e == BRANCH && insns[w].code == 0) {
> +               verbose_linfo(env, t, "%d", t);
> +               verbose(env, "jump to reserved code from insn %d to %d\n", t, w);
> +               return -EINVAL;
> +       }

I don't think we should be changing the verifier to make
fuzzer logs more readable.

Same with patch 2. The code is fine as-is.