linux-kernel - Re: [PATCH bpf 1/2] flow_dissector: reject invalid attach

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Date:   Mon, 15 Jun 2020 15:43:45 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Sitnicki <jakub@...udflare.com>,
        kernel-team <kernel-team@...udflare.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf 1/2] flow_dissector: reject invalid attach_flags

On Fri, 12 Jun 2020 at 23:36, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Fri, Jun 12, 2020 at 9:02 AM Lorenz Bauer <lmb@...udflare.com> wrote:
> >
> > Using BPF_PROG_ATTACH on a flow dissector program supports neither flags
> > nor target_fd but accepts any value. Return EINVAL if either are non-zero.
> >
> > Signed-off-by: Lorenz Bauer <lmb@...udflare.com>
> > Fixes: b27f7bb590ba ("flow_dissector: Move out netns_bpf prog callbacks")
> > ---
> >  kernel/bpf/net_namespace.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/bpf/net_namespace.c b/kernel/bpf/net_namespace.c
> > index 78cf061f8179..56133e78ae4f 100644
> > --- a/kernel/bpf/net_namespace.c
> > +++ b/kernel/bpf/net_namespace.c
> > @@ -192,6 +192,9 @@ int netns_bpf_prog_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> >         struct net *net;
> >         int ret;
> >
> > +       if (attr->attach_flags || attr->target_fd)
> > +               return -EINVAL;
> > +
>
> In theory it makes sense, but how did you test it?

Not properly it seems, sorry!

> test_progs -t flow
> fails 5 tests.

I spent today digging through this, and the issue is actually more annoying than
I thought. BPF_PROG_DETACH for sockmap and flow_dissector ignores
attach_bpf_fd. The cgroup and lirc2 attach point use this to make sure that the
program being detached is actually what user space expects. We actually have
tests that set attach_bpf_fd for these to attach points, which tells
me that this is
an easy mistake to make.

Unfortunately I can't come up with a good fix that seems backportable:
- Making sockmap and flow_dissector have the same semantics as cgroup
  and lirc2 requires a bunch of changes (probably a new function for sockmap)
- Returning EINVAL from BPF_PROG_DETACH if attach_bpf_fd is specified
  leads to a lot of churn in selftests

Is it worth just landing these fixes on bpf or bpf-next without
backporting them?

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

www.cloudflare.com