[PATCH v9 00/12] Network support for Landlock - allowed list of protocols

[Jeff Xu]

On Wed, Jun 28, 2023 at 12:03 PM Mickaël Salaün <mic at digikod.net> wrote:
>
>
> On 28/06/2023 04:33, Jeff Xu wrote:
> > On Mon, Jun 26, 2023 at 8:29 AM Mickaël Salaün <mic at digikod.net> wrote:
> >>
> >> Reviving Günther's suggestion to deny a set of network protocols:
> >>
> >> On 14/03/2023 14:28, Mickaël Salaün wrote:
> >>>
> >>> On 13/03/2023 18:16, Konstantin Meskhidze (A) wrote:
> >>>>
> >>>>
> >>>> 2/24/2023 1:17 AM, Günther Noack пишет:
> >>
> >> [...]
> >>
> >>>>>
> >>>>> * Given the list of obscure network protocols listed in the socket(2)
> >>>>>       man page, I find it slightly weird to have rules for the use of TCP,
> >>>>>       but to leave less prominent protocols unrestricted.
> >>>>>
> >>>>>       For example, a process with an enabled Landlock network ruleset may
> >>>>>       connect only to certain TCP ports, but at the same time it can
> >>>>>       happily use Bluetooth/CAN bus/DECnet/IPX or other protocols?
> >>>>
> >>>>          We also have started a discussion about UDP protocol, but it's
> >>>> more complicated since UDP sockets does not establish connections
> >>>> between each other. There is a performance problem on the first place here.
> >>>>
> >>>> I'm not familiar with Bluetooth/CAN bus/DECnet/IPX but let's discuss it.
> >>>> Any ideas here?
> >>>
> >>> All these protocols should be handled one way or another someday. ;)
> >>>
> >>>
> >>>>
> >>>>>
> >>>>>       I'm mentioning these more obscure protocols, because I doubt that
> >>>>>       Landlock will grow more sophisticated support for them anytime soon,
> >>>>>       so maybe the best option would be to just make it possible to
> >>>>>       disable these?  Is that also part of the plan?
> >>>>>
> >>>>>       (I think there would be a lot of value in restricting network
> >>>>>       access, even when it's done very broadly.  There are many programs
> >>>>>       that don't need network at all, and among those that do need
> >>>>>       network, most only require IP networking.
> >>>
> >>> Indeed, protocols that nobody care to make Landlock supports them will
> >>> probably not have fine-grained control. We could extend the ruleset
> >>> attributes to disable the use (i.e. not only the creation of new related
> >>> sockets/resources) of network protocol families, in a way that would
> >>> make sandboxes simulate a kernel without such protocol support. In this
> >>> case, this should be an allowed list of protocols, and everything not in
> >>> that list should be denied. This approach could be used for other kernel
> >>> features (unrelated to network).
> >>>
> >>>
> >>>>>
> >>>>>       Btw, the argument for more broad disabling of network access was
> >>>>>       already made at https://cr.yp.to/unix/disablenetwork.html in the
> >>>>>       past.)
> >>>
> >>> This is interesting but scoped to a single use case. As specified at the
> >>> beginning of this linked page, there must be exceptions, not only with
> >>> AF_UNIX but also for (the newer) AF_VSOCK, and probably future ones.
> >>> This is why I don't think a binary approach is a good one for Linux.
> >>> Users should be able to specify what they need, and block the rest.
> >>
> >> Here is a design to be able to only allow a set of network protocols and
> >> deny everything else. This would be complementary to Konstantin's patch
> >> series which addresses fine-grained access control.
> >>
> >> First, I want to remind that Landlock follows an allowed list approach
> >> with a set of (growing) supported actions (for compatibility reasons),
> >> which is kind of an allow-list-on-a-deny-list. But with this proposal,
> >> we want to be able to deny everything, which means: supported, not
> >> supported, known and unknown protocols.
> >>
> > I think this makes sense.  ChomeOS can use it at the process level:
> > disable network, allow VSOCK only, allow TCP only, etc.
> >
> >> We could add a new "handled_access_socket" field to the landlock_ruleset
> >> struct, which could contain a LANDLOCK_ACCESS_SOCKET_CREATE flag.
> >>
> >> If this field is set, users could add a new type of rules:
> >> struct landlock_socket_attr {
> >>       __u64 allowed_access;
> >>       int domain; // see socket(2)
>
> I guess "family" would also make sense. It's the name used in the
> kernel, the "AF" prefixes, and address_families(7). I'm not sure why
> "domain" was chosen for socket(2).
>
Agree also.

>
> >>       int type; // see socket(2)
> >> }
> >>
> > Do you want to add "int protocol" ? which is the third parameter of socket(2)
> > According to protocols(5), the protocols are defined in:
> > https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml
> >
> > It is part of IPv4/IPV6 header:
> > https://www.rfc-editor.org/rfc/rfc791.html#section-3.1
> > https://www.rfc-editor.org/rfc/rfc8200.html#section-3
>
> I understand the rationale but I'm not sure if this would be useful. Do
> you have use cases?
>
I agree this field is not commonly used, so might not be that useful.
In most cases, the protocol field will just be 0.

One case I thought of previously is building an icmp or DHCP packet
with raw socket,  but now I'm not sure what kind of support/enforce
the kernel has for the protocol field with raw socket.

We can drop this for now, if there is a clearer requirement in future,
it is easy to add a new rule.

>
> >
> >> The allowed_access field would only contain
> >> LANDLOCK_ACCESS_SOCKET_CREATE at first, but it could grow with other
> >> actions (which cannot be handled with seccomp):
> >> - use: walk through all opened FDs and mark them as allowed or denied
> >> - receive: hook on received FDs
> >> - send: hook on sent FDs
> >>
> > also bind, connect, accept.
>
> I don't think "accept" would be useful, and I'm not sure if "bind" and
> "connect" would not be redundant with LANDLOCK_ACCESS_NET_{CONNECT,BIND}_TCP
> Bind and connect for a datagram socket is optional, so this might lead
> to a false sense of security. If we want to support protocols other than
> TCP to restrict bind/connect, then they deserve to be controlled
> according to a port (or similar).
>
> >
> >> We might also use the same approach for non-socket objects that can be
> >> identified with some meaningful properties.
> >>
> >> What do you think?
> >
> > -Jeff