[RFC PATCH v2 1/8] landlock: Fix non-TCP sockets restriction

[Mickaël Salaün]

On Mon, Dec 09, 2024 at 01:19:19PM +0300, Mikhail Ivanov wrote:
> On 12/4/2024 10:35 PM, Mickaël Salaün wrote:
> > On Wed, Dec 04, 2024 at 08:27:58PM +0100, Mickaël Salaün wrote:
> > > On Fri, Oct 18, 2024 at 08:08:12PM +0200, Mickaël Salaün wrote:
> > > > On Thu, Oct 17, 2024 at 02:59:48PM +0200, Matthieu Baerts wrote:
> > > > > Hi Mikhail and Landlock maintainers,
> > > > > 
> > > > > +cc MPTCP list.
> > > > 
> > > > Thanks, we should include this list in the next series.
> > > > 
> > > > > 
> > > > > On 17/10/2024 13:04, Mikhail Ivanov wrote:
> > > > > > Do not check TCP access right if socket protocol is not IPPROTO_TCP.
> > > > > > LANDLOCK_ACCESS_NET_BIND_TCP and LANDLOCK_ACCESS_NET_CONNECT_TCP
> > > > > > should not restrict bind(2) and connect(2) for non-TCP protocols
> > > > > > (SCTP, MPTCP, SMC).
> > > > > 
> > > > > Thank you for the patch!
> > > > > 
> > > > > I'm part of the MPTCP team, and I'm wondering if MPTCP should not be
> > > > > treated like TCP here. MPTCP is an extension to TCP: on the wire, we can
> > > > > see TCP packets with extra TCP options. On Linux, there is indeed a
> > > > > dedicated MPTCP socket (IPPROTO_MPTCP), but that's just internal,
> > > > > because we needed such dedicated socket to talk to the userspace.
> > > > > 
> > > > > I don't know Landlock well, but I think it is important to know that an
> > > > > MPTCP socket can be used to discuss with "plain" TCP packets: the kernel
> > > > > will do a fallback to "plain" TCP if MPTCP is not supported by the other
> > > > > peer or by a middlebox. It means that with this patch, if TCP is blocked
> > > > > by Landlock, someone can simply force an application to create an MPTCP
> > > > > socket -- e.g. via LD_PRELOAD -- and bypass the restrictions. It will
> > > > > certainly work, even when connecting to a peer not supporting MPTCP.
> > > > > 
> > > > > Please note that I'm not against this modification -- especially here
> > > > > when we remove restrictions around MPTCP sockets :) -- I'm just saying
> > > > > it might be less confusing for users if MPTCP is considered as being
> > > > > part of TCP. A bit similar to what someone would do with a firewall: if
> > > > > TCP is blocked, MPTCP is blocked as well.
> > > > 
> > > > Good point!  I don't know well MPTCP but I think you're right.  Given
> > > > it's close relationship with TCP and the fallback mechanism, it would
> > > > make sense for users to not make a difference and it would avoid bypass
> > > > of misleading restrictions.  Moreover the Landlock rules are simple and
> > > > only control TCP ports, not peer addresses, which seems to be the main
> > > > evolution of MPTCP.
> > > 
> > > Thinking more about this, this makes sense from the point of view of the
> > > network stack, but looking at external (potentially bogus) firewalls or
> > > malware detection systems, it is something different.  If we don't
> > > provide a way for users to differenciate the control of SCTP from TCP,
> > > malicious use of SCTP could still bypass this kind of bogus security
> > > appliances.  It would then be safer to stick to the protocol semantic by
> > > clearly differenciating TCP from MPTCP (or any other protocol).
> 
> You mean that these firewals have protocol granularity (e.g. different
> restrictions for MPTCP and TCP sockets)?

Yes, and more importantly they can miss the MTCP semantic and then not
properly filter such packet, which can be use to escape the network
policy.  See some issues here:
https://en.wikipedia.org/wiki/Multipath_TCP

The point is that we cannot assume anything about other networking
stacks, and if Landlock can properly differentiate between TCP and MTCP
(e.g. with new LANDLOCK_ACCESS_NET_CONNECT_MTCP) users of such firewalls
could still limit the impact of their firewall's bugs.  However, if
Landlock treats TCP and MTCP the same way, we'll not be able to only
deny MTCP.  In most use cases, the network policy should treat both TCP
and MTCP the same way though, but we should let users decide according
to their context.

>From an implementation point of view, adding MTCP support should be
simple, mainly tests will grow.

> 
> > > 
> > > Mikhail, could you please send a new patch series containing one patch
> > > to fix the kernel and another to extend tests?
> > 
> > No need to squash them in one, please keep the current split of the test
> > patches.  However, it would be good to be able to easily backport them,
> > or at least the most relevant for this fix, which means to avoid
> > extended refactoring.
> 
> No problem, I'll remove the fix of error consistency from this patchset.
> BTW, what do you think about second and third commits? Should I send the
> new version of them as well (in separate patch)?

According to the description, patch 2 may be included in this series if
it can be tested with any other LSM, but I cannot read these patches:
https://lore.kernel.org/all/20241017110454.265818-3-ivanov.mikhail1@huawei-partners.com/