Hi Mickaël,

On 29/01/2025 15:51, Mickaël Salaün wrote:

On Wed, Jan 29, 2025 at 02:47:19PM +0300, Mikhail Ivanov wrote:

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On 1/29/2025 2:33 PM, Matthieu Baerts wrote:

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On 29/01/2025 12:02, Mikhail Ivanov wrote:

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On 1/29/2025 1:25 PM, Matthieu Baerts wrote:

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Hi Mikhail,

On 29/01/2025 10:52, Mikhail Ivanov wrote:

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On 1/28/2025 9:14 PM, Matthieu Baerts wrote:

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Hi Mikhail,

Sorry, I didn't follow all the discussions in this thread, but here are
some comments, hoping this can help to clarify the MPTCP case.

Thanks a lot for sharing your knowledge, Matthieu!

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On 28/01/2025 11:56, Mikhail Ivanov wrote:

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On 1/27/2025 10:48 PM, Mickaël Salaün wrote:

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I'm a bit worried that we miss some of these places (now or in future
kernel versions).  We'll need a new LSM hook for that.

Could you list the current locations?

Currently, I know only about TCP-related transformations:

* SMC can fallback to TCP during connection. TCP connection is used
     (1) to exchange CLC control messages in default case and (2)
for the
     communication in the case of fallback. If socket was connected or
     connection failed, socket can not be reconnected again. There
is no
     existing security hook to control the fallback case,

* MPTCP uses TCP for communication between two network interfaces
in the
     default case and can fallback to plain TCP if remote peer does not
     support MPTCP. AFAICS, there is also no security hook to
control the
     fallback transformation,

There are security hooks to control the path creation, but not to
control the "fallback transformation".

Technically, with MPTCP, the userspace will create an IPPROTO_MPTCP
socket. This is only used "internally": to communicate between the
userspace and the kernelspace, but not directly used between network
interfaces. This "external" communication is done via one or multiple
kernel TCP sockets carrying extra TCP options for the mapping. The
userspace cannot directly control these sockets created by the kernel.

In case of fallback, the kernel TCP socket "simply" drop the extra TCP
options needed for MPTCP, and carry on like normal TCP. So on the wire
and in the Linux network stack, it is the same TCP connection, without
the MPTCP options in the TCP header. The userspace continue to
communicate with the same socket.

I'm not sure if there is a need to block the fallback: it means only
one
path can be used at a time.

Thanks Matthieu.

So user space needs to specific IPPROTO_MPTCP to use MPTCP, but on the
network this socket can translate to "augmented" or plain TCP.

Correct. On the wire, you will only see packet with the IPPROTO_TCP
protocol. When MPTCP is used, extra MPTCP options will be present in the
TCP headers, but the protocol is still IPPROTO_TCP on the network.

From Landlock point of view, what matters is to have a consistent policy
that maps to user space code.  The fear was that a malicious user space
that is only allowed to use MPTCP could still transform an MPTCP socket
to a TCP socket, while it wasn't allowed to create a TCP socket in the
first place.  I now think this should not be an issue because:
1. MPTCP is kind of a superset of TCP
2. user space legitimately using MPTCP should not get any error related
   to a Landlock policy because of TCP/any automatic fallback.  To say
   it another way, such fallback is independent of user space requests
   and may not be predicted because it is related to the current network
   path.  This follows the principle of least astonishment (at least
   from user space point of view).

So, if I understand correctly, this should be simple for the Landlock
socket creation control:  we only check socket properties at creation
time and we ignore potential fallbacks.  This should be documented
though.

It depends on the restrictions that are put in place: are the user and
kernel sockets treated the same way? If yes, blocking TCP means that
even if it will be possible for the userspace to create an IPPROTO_MPTCP
socket, the kernel will not be allowed to IPPROTO_TCP ones to
communicate with the outside world. So blocking TCP will implicitly
block MPTCP.

On the other hand, if only TCP user sockets are blocked, then it will be
possible to use MPTCP to communicate to any TCP sockets: with an
IPPROTO_MPTCP socket, it is possible to communicate with any IPPROTO_TCP
sockets, but without the extra features supported by MPTCP.

As an example, if a Landlock policies only allows MPTCP: socket(...,
IPPROTO_MPTCP) should be allowed and any legitimate use of the returned
socket (according to MPTCP) should be allowed, including TCP fallback.
However, socket(..., IPPROTO_TCP/0), should only be allowed if TCP is
explicitly allowed.  This means that we might end up with an MPTCP
socket only using TCP, which is OK.

Would it not be confusing for the person who set the Landlock policies?
Especially for the ones who had policies to block TCP, and thought they
were "safe", no?

If only TCP is blocked on the userspace side, simply using IPPROTO_MPTCP
instead of IPPROTO_TCP will allow any users to continue to talk with the
outside world. Also, it is easy to force apps to use IPPROTO_MPTCP
instead of IPPROTO_TCP, e.g. using 'mptcpize' which set LD_PRELOAD in
order to change the parameters of the socket() call.

   mptcpize run curl https://check.mptcp.dev

I guess this should be the same for other protocols, except if user
space can explicitly transform a specific socket type to use an
*arbitrary* protocol, but I think this is not possible.

I'm sorry, I don't know what is possible with the other ones. But again,
blocking both user and kernel sockets the same way might make more sense
here.

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You mean that users always rely on a plain TCP communication in the case
the connection of MPTCP multipath communication fails?

Yes, that's the same TCP connection, just without extra bit to be able
to use multiple TCP connections associated to the same MPTCP one.

Indeed, so MPTCP communication should be restricted the same way as TCP.
AFAICS this should be intuitive for MPTCP users and it'll be better
to let userland define this dependency.

Yes, I think that would make more sense.

I guess we can look at MPTCP as TCP with extra features.

Yeap

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So if TCP is blocked, MPTCP should be blocked as well. (And eventually
having the possibility to block only TCP but not MPTCP and the opposite,
but that's a different topic: a possible new feature, but not a bug-fix)

What do you mean by the "bug fix"?

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* IPv6 -> IPv4 transformation for TCP and UDP sockets withon
     IPV6_ADDRFORM. Can be controlled with setsockopt() security hook.

According to the man page: "It is allowed only for IPv6 sockets that are
connected and bound to a v4-mapped-on-v6 address."

This compatibility feature makes sense from user space point of view and
should not result in an error because of Landlock.

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As I said before, I wonder if user may want to use SMC or MPTCP and
deny
TCP communication, since he should rely on fallback transformation
during the connection in the common case. It may be unexpected for
connect(2) to fail during the fallback due to security politics.

With MPTCP, fallbacks can happen at the beginning of a connection, when
there is only one path. This is done after the userspace's
connect(). If

A remaining question is then, can we repurpose an MPTCP socket that did
fallback to TCP, to (re)connect to another destination (this time
directly with TCP)?

If the socket was created with the IPPROTO_MPTCP protocol, the protocol
will not change after a disconnection. But still, with an MPTCP socket,
it is by design possible to connect to a TCP one no mater how the socket
was used before.

I guess this is possible.  If it is the case, I think it should be OK
anyway.  That could be used by an attacker, but that should not give
more access because of the MPTCP fallback mechanism anyway.  We should
see MPTCP as a superset of TCP.  At the end, security policy is in the
hands of user space.

As long as it is documented and not seen as a regression :)

To me, it sounds strange to have to add extra rules for MPTCP if TCP is
blocked, but that's certainly because I see MPTCP like it is seen on the
wire: as an extension to TCP, not as a different protocol.

(...)

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.