Re: Potential issues (security and otherwise) with the current cgroup-bpf API

On Mon, Dec 19, 2016 at 07:50:01PM -0800, Andy Lutomirski wrote:

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net.socket_create_filter = "none": no filter
net.socket_create_filter = "bpf:baadf00d": bpf filter

i'm assuming 'baadf00d' is bpf program fd expressed a text string?
and kernel needs to parse above? will you allow capital and lower
case for 'bpf:' ? and mixed case too? spaces and tabs allowed or not?
can program fd expressed as decimal or hex or both?
how do you return the error? as a text string for user space
to parse?

No.  The kernel does not parse it because you cannot write this to the
file.  You set a bpf filter with ioctl and pass an fd.  If you *read*
the file, you get the same bpf program hash that fdinfo on the bpf
object would show -- this is for debugging and (eventually) CRIU.

my understanding that cgroup is based on kernfs and both don't support
ioctl, so you'd need quite some hacking to introduce such concepts
and buy-in from a bunch of people first.

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net.socket_create_filter = "iptables:foobar": some iptables thingy
net.socket_create_filter = "nft:blahblahblah": some nft thingy

iptables/nft are not applicable to 'bpf_socket_create' which
looks into:
struct bpf_sock {
        __u32 bound_dev_if;
        __u32 family;
        __u32 type;
        __u32 protocol;
};
so don't fit as an example.

The code that takes a 'struct sock' and sets up bpf_sock is
bpf-specific and would obviously not be used for non-bpf filter.  But
if you had a filter that was just a like of address families, that
filter would look at struct sock and do its own thing.  iptables
wouldn't make sense for a socket creation filter, but it would make
perfect sense for an ingress filter.  Obviously the bpf-specific state
object wouldn't be used, but it would still be a hook, invoked from
the same network code, guarded by the same static key, looking at the
same skb.

I strongly suggest to go back and read my first reply where
I think I explained well enough that something like iptables
will not able to reuse the ioctl mechanism you're proposing here,
hook ids will be different, attachment mechanism will be different too.
So your proposed cgroup ioctl is already dead as a reusable interface.

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net.socket_create_filter = "disallow": no sockets created period
net.socket_create_filter = "address_family_list:1,2,3": allow AF 1, 2, and 3

so you're proposing to add a bunch of hard coded logic to the kernel.
First to parse such text into some sort of syntax tree or list/set
and then have hard coded logic specifically for these two use cases?
While above two can be implemented as trivial bpf programs already?!
That goes 180% degree vs bpf philosophy. bpf is about moving
the specific code out of the kernel and keeping kernel generic that
it can solve as many use cases as possible by being programmable.

I'm not seriously proposing implementing these.  My point is that
*bpf*, while wonderful, is not the be-all-and-end-all of kernel
configurability, and other types of hooks might want to be hooked in
here.

Then please let's talk about real use cases.
This daydreaming of some future llvm in the kernel that you were
bringing up during LPC doesn't help the discussion.
Just like these artificial examples.

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What exactly isn't sensible about using cgroup_bpf for containers?

my use case is close to zero overhead application network monitoring.

So if I set up a cgroup that's monitored and call it /cgroup/a and
enable delegation and if the program running there wants to do its own
monitoring in /cgroup/a/b (via delegation), then you really want the
outer monitor to silently drop events coming from /cgroup/a/b?

yes. both are root and must talk to each other if they want
to co-exist. When root process is asking kernel to do X, this X has
to happen.

Then disallow nesting.  You're welcome to not rush the decision, but
that's not what you've done.  If 4.10 is released as is, you've made
the decision and you're going to have a hard time changing it.

Nothing needs to be changed.

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No. As was pointed out before only root can load BPF_PROG_TYPE_CGROUP_[SKB|SOCK]
type programs and only root can attach them.

Why?  It really seems to me that you expect that future namespaceable
bpf hooks will use a totally different API.  At KS, I sat in a room
full of people arguing about cgroup v2 and a lot of them pointed out
that there are valid, paying use cases that want to stick cgroup v1 in
a container, because the code that uses cgroup v1 in the container is
called systemd and the contained OS is called RHEL (or SuSE or Ubuntu
or Gentoo or whatever), and that code is *already written* and needs
to be contained.

bpf in general is not namespace aware. It's global and this cgroup scoping
of bpf programs is the first of this kind. Namespacing of bpf is completely
different topic.

The current approach to bpf hooks will bite you down the road.  David
Ahern is already proposing using it for something that is not tracing
at all, and someone will want that in a container, and there will be a
problem.

vrf use case already supported by existing code.

How about slowing down a wee bit and trying to come up with cgroup
hook semantics that work for all of these use cases?  I think my
proposal is quite close to workable.

you've started the topic by claiming that things are broken
and non-extensible. At the course of this thread it was explained
that the interface is extensible and not broken for the use case
it was designed for. The 'security' use case like lsm+bpf+cgroup
is not supported by the current model yet and that's what we need
to discuss in the future. So, yes, please slow down.