Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
MIME-Version: 1.0
References: <20231011-jmp-into-reserved-fields-v3-0-97d2aa979788@gmail.com> <20231011-jmp-into-reserved-fields-v3-1-97d2aa979788@gmail.com>
In-Reply-To: <20231011-jmp-into-reserved-fields-v3-1-97d2aa979788@gmail.com>
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Wed, 11 Oct 2023 06:38:56 -0700
Message-ID: <CAADnVQJnhfbALtNkCauS_ZwRfybcb_mryEvZW7Uu1uOSshQ9Ew@mail.gmail.com>
Subject: Re: [PATCH bpf-next v3 1/3] bpf: Detect jumping to reserved code
 during check_cfg()
To:     Hao Sun <sunhao.th@gmail.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        John Fastabend <john.fastabend@gmail.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>,
        Yonghong Song <yonghong.song@linux.dev>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Wed, Oct 11, 2023 at 2:01=E2=80=AFAM Hao Sun <sunhao.th@gmail.com> 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 verifi=
er
> gives the following log in such case:
>
> func#0 @0
> 0: R1=3Dctx(off=3D0,imm=3D0) R10=3Dfp0
> 0: (18) r4 =3D 0xffff888103436000       ; R4_w=3Dmap_ptr(off=3D0,ks=3D4,v=
s=3D128,imm=3D0)
> 2: (18) r1 =3D 0x1d                     ; R1_w=3D29
> 4: (55) if r4 !=3D 0x0 goto pc+4        ; R4_w=3Dmap_ptr(off=3D0,ks=3D4,v=
s=3D128,imm=3D0)
> 5: (1c) w1 -=3D w1                      ; R1_w=3D0
> 6: (18) r5 =3D 0x32                     ; R5_w=3D50
> 8: (56) if w5 !=3D 0xfffffff4 goto pc-2
> mark_precise: frame0: last_idx 8 first_idx 0 subseq_idx -1
> mark_precise: frame0: regs=3Dr5 stack=3D before 6: (18) r5 =3D 0x32
> 7: R5_w=3D50
> 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(). Fo=
r
> 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@gmail.com>
> ---
>  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 =3D env->cfg.insn_stack;
>         int *insn_state =3D env->cfg.insn_state;
> +       struct bpf_insn *insns =3D env->prog->insnsi;
>
>         if (e =3D=3D FALLTHROUGH && insn_state[t] >=3D (DISCOVERED | FALL=
THROUGH))
>                 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 =3D=3D BRANCH && insns[w].code =3D=3D 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.