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

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Date:   Thu, 17 Sep 2020 11:06:18 -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 Thu, Sep 17, 2020 at 10:10 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>
> > On Wed, Sep 16, 2020 at 12:59 PM Andrii Nakryiko
> > <andrii.nakryiko@...il.com> wrote:
> >>
> >> 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?
> >
> > ok, never mind this specifically. there is an explicit check
> >
> > if (tgt_prog->type == prog->type) {
> >     verbose(env, "Cannot recursively attach\n");
> >     return -EINVAL;
> > }
> >
> > that will prevent this.
> >
> > But, I think we still will be able to construct a long chain of
> > fmod_ret -> freplace -> fmod_ret -> freplace -> and so on ad
> > infinitum. Can you please construct such a selftest? And then we
> > should probably fix those checks to also disallow FMOD_RET, in
> > addition to BPF_TRACE_FENTRY/FEXIT (and someone more familiar with LSM
> > prog type should check if that can cause any problems).
>
> Huh, I thought fmod_ret was supposed to be for kernel functions only?

Yeah, I realized that afterwards, but didn't want to ramble on forever :)

> However, I can't really point to anywhere in the code that ensures this,
> other than check_attach_modify_return(), but I think that will allow a
> bpf function as long as its name starts with "security_" ?

I think error_injection_list check will disallow anything that's not a
specially marked kernel function. So we are probably safe as is, even
though a bit implicitly.

>
> Is there actually any use case for modify_return being attached to a BPF
> function (you could just use freplace instead, couldn't you?). Or should
> we just disallow that entirely (if I'm not missing somewhere it's
> already blocked)?

No idea, but I think it works as is right now, so I wouldn't touch it.

>
> -Toke
>