netdev - Re: [PATCH bpf-next v5 5/8] bpf: Fix context type resolving for extension programs

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Date:   Wed, 16 Sep 2020 12:59:19 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 5/8] bpf: Fix context type resolving for
 extension programs

On Tue, Sep 15, 2020 at 5:50 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> From: Toke Høiland-Jørgensen <toke@...hat.com>
>
> Eelco reported we can't properly access arguments if the tracing
> program is attached to extension program.
>
> Having following program:
>
>   SEC("classifier/test_pkt_md_access")
>   int test_pkt_md_access(struct __sk_buff *skb)
>
> with its extension:
>
>   SEC("freplace/test_pkt_md_access")
>   int test_pkt_md_access_new(struct __sk_buff *skb)
>
> and tracing that extension with:
>
>   SEC("fentry/test_pkt_md_access_new")
>   int BPF_PROG(fentry, struct sk_buff *skb)
>
> It's not possible to access skb argument in the fentry program,
> with following error from verifier:
>
>   ; int BPF_PROG(fentry, struct sk_buff *skb)
>   0: (79) r1 = *(u64 *)(r1 +0)
>   invalid bpf_context access off=0 size=8
>
> The problem is that btf_ctx_access gets the context type for the
> traced program, which is in this case the extension.
>
> But when we trace extension program, we want to get the context
> type of the program that the extension is attached to, so we can
> access the argument properly in the trace program.
>
> This version of the patch is tweaked slightly from Jiri's original one,
> since the refactoring in the previous patches means we have to get the
> target prog type from the new variable in prog->aux instead of directly
> from the target prog.
>
> Reported-by: Eelco Chaudron <echaudro@...hat.com>
> Suggested-by: Jiri Olsa <jolsa@...nel.org>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>  kernel/bpf/btf.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 9228af9917a8..55f7b2ba1cbd 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3860,7 +3860,14 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>
>         info->reg_type = PTR_TO_BTF_ID;
>         if (tgt_prog) {
> -               ret = btf_translate_to_vmlinux(log, btf, t, tgt_prog->type, arg);
> +               enum bpf_prog_type tgt_type;
> +
> +               if (tgt_prog->type == BPF_PROG_TYPE_EXT)
> +                       tgt_type = tgt_prog->aux->tgt_prog_type;

what if tgt_prog->aux->tgt_prog_type is also BPF_PROG_TYPE_EXT? Should
this be a loop?

Which also brings up a few follow up questions. Now that we allow same
PROG_EXT program to be attached to multiple other programs:

1. what prevents us from attaching PROG_EXT to itself?
2. How do we prevent long chain of EXT programs or even loops?

Can you please add a few selftests testing such cases? I have a
feeling that with your changes in this patch set now it's possible to
break the kernel very easily. I don't know what the proper solution
is, but let's at least have a test that does show breakage, then try
to figure out the solution. See also comment in check_attach_btf_id()
about fentry/fexit and freplace interactions. That might not be
enough.


> +               else
> +                       tgt_type = tgt_prog->type;
> +
> +               ret = btf_translate_to_vmlinux(log, btf, t, tgt_type, arg);
>                 if (ret > 0) {
>                         info->btf_id = ret;
>                         return true;
>