Re: [RFC PATCH bpf-next 4/8] bpf: support GET_FD_BY_ID and GET_NEXT_ID for bpf_link

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: "Toke Høiland-Jørgensen" <toke@redhat.com>
Cc: Andrii Nakryiko <andriin@fb.com>, bpf <bpf@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [RFC PATCH bpf-next 4/8] bpf: support GET_FD_BY_ID and GET_NEXT_ID for bpf_link
Date: Mon, 6 Apr 2020 12:06:54 -0700	[thread overview]
Message-ID: <CAEf4BzYrW43EW_Uneqo4B6TLY4V9fKXJxWj+-gbq-7X0j7y86g@mail.gmail.com> (raw)
In-Reply-To: <87pnckc0fr.fsf@toke.dk>

On Mon, Apr 6, 2020 at 4:34 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Andrii Nakryiko <andriin@fb.com> writes:
>
> > Add support to look up bpf_link by ID and iterate over all existing bpf_links
> > in the system. GET_FD_BY_ID code handles not-yet-ready bpf_link by checking
> > that its ID hasn't been set to non-zero value yet. Setting bpf_link's ID is
> > done as the very last step in finalizing bpf_link, together with installing
> > FD. This approach allows users of bpf_link in kernel code to not worry about
> > races between user-space and kernel code that hasn't finished attaching and
> > initializing bpf_link.
> >
> > Further, it's critical that BPF_LINK_GET_FD_BY_ID only ever allows to create
> > bpf_link FD that's O_RDONLY. This is to protect processes owning bpf_link and
> > thus allowed to perform modifications on them (like LINK_UPDATE), from other
> > processes that got bpf_link ID from GET_NEXT_ID API. In the latter case, only
> > querying bpf_link information (implemented later in the series) will be
> > allowed.
>
> I must admit I remain sceptical about this model of restricting access
> without any of the regular override mechanisms (for instance, enforcing
> read-only mode regardless of CAP_DAC_OVERRIDE in this series). Since you
> keep saying there would be 'some' override mechanism, I think it would
> be helpful if you could just include that so we can see the full
> mechanism in context.

I wasn't aware of CAP_DAC_OVERRIDE, thanks for bringing this up.

One way to go about this is to allow creating writable bpf_link for
GET_FD_BY_ID if CAP_DAC_OVERRIDE is set. Then we can allow LINK_DETACH
operation on writable links, same as we do with LINK_UPDATE here.
LINK_DETACH will do the same as cgroup bpf_link auto-detachment on
cgroup dying: it will detach bpf_link, but will leave it alive until
last FD is closed.

We need to consider, though, if CAP_DAC_OVERRIDE is something that can
be disabled for majority of real-life applications to prevent them
from doing this. If every realistic application has/needs
CAP_DAC_OVERRIDE, then that's essentially just saying that anyone can
get writable bpf_link and do anything with it.

>
> -Toke
>