Re: [PATCH bpf 1/2] flow_dissector: reject invalid attach_flags

[Lorenz Bauer <lmb@cloudflare.com>]

On Wed, 17 Jun 2020 at 07:49, John Fastabend <john.fastabend@gmail.com> wrote:
>
> Alexei Starovoitov wrote:
> > On Tue, Jun 16, 2020 at 1:30 AM Lorenz Bauer <lmb@cloudflare.com> wrote:
> > >
> > > On Tue, 16 Jun 2020 at 04:55, Alexei Starovoitov
> > > <alexei.starovoitov@gmail.com> wrote:
> > > >
> > > > On Mon, Jun 15, 2020 at 7:43 AM Lorenz Bauer <lmb@cloudflare.com> wrote:
> > > > >
> > > > > On Fri, 12 Jun 2020 at 23:36, Alexei Starovoitov
> > > > > <alexei.starovoitov@gmail.com> wrote:
> > > > > >
> > > > > > On Fri, Jun 12, 2020 at 9:02 AM Lorenz Bauer <lmb@cloudflare.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@cloudflare.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.
>
> In sockmap case I didn't manage to think what multiple programs of the same type
> on the same map would look like so we can just remove whatever program is there.
> Is there a problem with this or is it that we just want the sanity check.
>
> > > > >
> > > > > 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)
> > > >
> > > > making flow dissector pass prog_fd as cg and lirc is certainly my preference.
> > > > Especially since tests are passing fd user code is likely doing the same,
> > > > so breakage is unlikely. Also it wasn't done that long ago, so
> > > > we can backport far enough.
> > > > It will remove cap_net_admin ugly check in bpf_prog_detach()
> > > > which is the only exception now in cap model.
> > >
> > > SGTM. What about sockmap though? The code for that has been around for ages.
> >
> > you mean the second patch that enforces sock_map_get_from_fd doesn't
> > use attach_flags?
> > I think it didn't break anything, so enforcing is fine.
>
> I'm ok with enforcing it.
>
> >
> > or the detach part that doesn't use prog_fd ?
> > I'm not sure what's the best here.
> > At least from cap perspective it's fine because map_fd is there.
> >
> > John, wdyt?
>
> I think we can keep the current detach without the prog_fd as-is. And
> then add logic so that if the prog_fd is included we check it?

Do you know of users that rely on this? FWIW all of the selftests actually
pass attach_bpf_fd when detaching from sockmap (on a recent bpf-next at least).

It'd be nice if I could make sockmap require this to be present, just
so that it's consistent with flow_dissector and other BPF_PROG_DETACH
users.

OTOH I'm not sure if this is backport material after all.

Lorenz

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Lorenz Bauer  |  Systems Engineer
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