netdev - Re: [PATCH] bpf: Add deny list of btf ids check for tracing and ext programs

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Message-ID: <CAEf4BzY8m5v0LY7eC1p-_xHg8yZms5HCS6D5AyRL7uFZfbKkKw@mail.gmail.com>
Date:   Wed, 28 Apr 2021 12:41:34 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH] bpf: Add deny list of btf ids check for tracing and ext programs

On Wed, Apr 28, 2021 at 9:19 AM Jiri Olsa <jolsa@...nel.org> wrote:
>
> The recursion check in __bpf_prog_enter and __bpf_prog_exit
> leaves some (not inlined) functions unprotected:
>
> In __bpf_prog_enter:
>   - migrate_disable is called before prog->active is checked
>
> In __bpf_prog_exit:
>   - migrate_enable,rcu_read_unlock_strict are called after
>     prog->active is decreased
>
> When attaching trampoline to them we get panic like:
>
>   traps: PANIC: double fault, error_code: 0x0
>   double fault: 0000 [#1] SMP PTI
>   RIP: 0010:__bpf_prog_enter+0x4/0x50
>   ...
>   Call Trace:
>    <IRQ>
>    bpf_trampoline_6442466513_0+0x18/0x1000
>    migrate_disable+0x5/0x50
>    __bpf_prog_enter+0x9/0x50
>    bpf_trampoline_6442466513_0+0x18/0x1000
>    migrate_disable+0x5/0x50
>    __bpf_prog_enter+0x9/0x50
>    bpf_trampoline_6442466513_0+0x18/0x1000
>    migrate_disable+0x5/0x50
>    __bpf_prog_enter+0x9/0x50
>    bpf_trampoline_6442466513_0+0x18/0x1000
>    migrate_disable+0x5/0x50
>    ...
>
> Fixing this by adding deny list of btf ids for tracing
> and ext programs and checking btf id during program
> verification. Adding above functions to this list.
>
> Suggested-by: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  kernel/bpf/verifier.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 2579f6fbb5c3..4ffd64eaffda 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -13112,6 +13112,17 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>         return 0;
>  }
>
> +BTF_SET_START(btf_id_deny)
> +BTF_ID_UNUSED
> +#ifdef CONFIG_SMP
> +BTF_ID(func, migrate_disable)
> +BTF_ID(func, migrate_enable)
> +#endif
> +#if !defined CONFIG_PREEMPT_RCU && !defined CONFIG_TINY_RCU
> +BTF_ID(func, rcu_read_unlock_strict)
> +#endif
> +BTF_SET_END(btf_id_deny)
> +
>  static int check_attach_btf_id(struct bpf_verifier_env *env)
>  {
>         struct bpf_prog *prog = env->prog;
> @@ -13171,6 +13182,10 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>                 ret = bpf_lsm_verify_prog(&env->log, prog);
>                 if (ret < 0)
>                         return ret;
> +       } else if ((prog->type == BPF_PROG_TYPE_TRACING ||
> +                   prog->type == BPF_PROG_TYPE_EXT) &&

BPF_PROG_TYP_EXT can only replace other BPF programs/subprograms, it
can't replace kernel functions, so the deny list shouldn't be checked
for them.

> +                  btf_id_set_contains(&btf_id_deny, btf_id)) {
> +               return -EINVAL;
>         }
>
>         key = bpf_trampoline_compute_key(tgt_prog, prog->aux->attach_btf, btf_id);
> --
> 2.30.2
>