[PATCH v2 2/2] selinux: Implement mptcp_add_subflow hook

[Paolo Abeni]

On Wed, 2022-12-21 at 20:21 -0500, Paul Moore wrote:
> On Wed, Dec 21, 2022 at 2:24 PM Paolo Abeni <pabeni at redhat.com> wrote:
> > I just tested the other option and there is another problem :(
> 
> It's never easy, is it? ;)
> 
> > The first subflow creations happens inside af_inet->create, via the sk-
> > > sk_prot->init() hook. The security_socket_post_create() call on the
> > owning MPTCP sockets happens after that point. So we copy data from a
> > not yet initialized security context (and the test fail badly).
> 
> Hmmm.  Let's come back to this later on down this email.
> 
> > There are a few options to cope with that:
> > - [ugly hack] call  security_socket_post_create() on the mptcp code
> > before creating the subflow. I experimented this just to double the
> > problem and a possible solution.
> 
> I'm guessing "[ugly hack]" is probably a bit of an understatement.
> Let's see if we can do better before we explore this option too much
> further.

Yup, I compiled the list in "brainstom-mode", trying to include
whatever would be possible even if clearly not suitable. 

[...]

> > WDYT?
> 
> Let's go back to the the inet_create() case for a little bit.  I'm
> thinking we might be able to do something by leveraging the
> sk_alloc()->sk_prot_alloc()->security_sk_alloc() code path.  As
> inet_create() is going to be called from task context here, it seems
> like we could do the sock's sid/sclass determination here, cached in
> separate fields in the sk_security_struct if necessary, and use those
> in a new MPTCP subflow hook.  We could also update
> selinux_socket_post_create() to take advantage of this as well.  We
> could also possibly pass the proto struct into security_sk_alloc() if
> we needed to identify IPPROTO_MPTCP there as well.
> 
> I'll admit to not chasing down all the details, but I suspect this may
> be the cleanest option - thoughts?

Thanks, I did not consider such possibility!

I think we should be careful to avoid increasing sk_security_struct
size. Currently it is 16 bytes, nicely matching a kmalloc slab, any
increase will move it on kmalloc-32 bytes slab possibly causing
performance and memory regressions).

More importantly, I think there is a problem with the 
sk_clone_lock() -> sk_prot_alloc() -> security_sk_alloc()
code path. 

sk_clone_lock() happens in BH context, if security_transition_sid()
needs process context that would be a problem - quickly skimming the
code it does not look so, I need to double check.

sk_clone_lock() is in a very critical path - socket creation for
incoming connections. The sid-related operation there will be
unnecessary/discarded by later the selinux_inet_csk_clone(), this will
likelly cause performance regressions even for plain TCP sockets.

Perhaps the cleanest option could be the one involving the mptcp
refactoring, moving subflow creation at a later stage. It could have
some minor side benefit for MPTCP, too - solving:

https://github.com/multipath-tcp/mptcp_net-next/issues/290

but I'm not fond of that option because it will require quite a bit of
time: we need first to have the mptcp refactor in place and then cook
the lsm patches. I guess such process will require at least 2 release
cycles, due to the needed mptcp(netdev)/lsm trees synchronization.

If that would prove to be the most reasonable option, could we consider
to transiently merge first something alike:

https://lore.kernel.org/mptcp/CAHC9VhSQnhH3UL4gqzu+YiA1Q3YyLLCv88gLJOvw-0+uw5Lvkw@mail.gmail.com/T/#m06c612f84f6b6fe759e670573b2c8092df71607b

to have a workable short-term solution, and later revert it when the
final solution would be in place?

Thanks,

Paolo