Re: selinux: support IPPROTO_SMC in socket_type_to_security_class()

[Paul Moore]

On Wed, Aug 21, 2024 at 9:38 AM Stephen Smalley
<stephen.smalley.work@xxxxxxxxx> wrote:
> On Tue, Aug 20, 2024 at 3:51 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:

...

> > Without passing any judgement on the patches Ondrej submitted (I tend
> > to ignore patches as attachments for various reasons), I do share
> > Ondrej's concerns that this may not be as simple as suggested in the
> > original patch in this thread.  I saw the same thing as Ondrej
> > regarding the TCP fallback and that immediately raised a number of
> > questions that I don't believe have been properly addressed yet.
> >
> > Someone needs to dig into how the standard SMC protocol works first to
> > ensure we have the necessary access controls for the current code; my
> > guess is that we are probably okay since the socket-level controls are
> > fairly generic, but I'm not sure we've actually seen proper
> > confirmation that everything is good from a conceptual standpoint.
> > Once that is done, we need to examine how the TCP fallback works,
> > specifically how are connections managed and are the existing TCP
> > hooks sufficient for SMC (the early connection state stuff can be
> > tricky) and how to distinguish between normal-TCP and SMC-TCP.
> >
> > Basically I'm looking for some basic design concepts and not simply a
> > passing test without any understanding of why/how it passed.
>
> At present, we are already applying the general socket layer access
> controls to AF_SMC sockets; hence, existing policies can prevent or
> allow use of AF_SMC sockets through that mechanism. This is useful for
> reducing kernel attack surface, e.g. prevent all use of AF_SMC by
> untrusted code, or to limit use of AF_SMC to specific
> processes/programs.

That's true.  I'm not suggesting we revert what we currently have, I'm
only expressing some caution about moving forward with
AF_INET/IPPROTO_SMC without a better understanding.  Ideally we would
have done so before adding AF_SMC support, but we didn't, or at least
I don't recall much discussion at the time.

> Since kernel commit d25a92ccae6bed02327b63d138e12e7806830f78
> ("net/smc: Introduce IPPROTO_SMC"), there is a way to bypass such
> controls by creating such sockets via (AF_INET, SOCK_STREAM,
> IPPROTO_SMC) instead of AF_SMC. In that situation, any process that is
> allowed the socket layer permissions to the generic socket class would
> be allowed to create/use SMC sockets.
>
> Jeongjun's patch closes this bypass and ensures consistent application
> of the general socket layer access controls for SMC sockets. Given
> that, I don't see why we would defer merging it until someone figures
> out a more complete solution for SMC sockets. It's more of a bug fix
> than an enhancement.

SCTP, that's why.  Granted, SCTP is likely a far more complicated
protocol than SMC, but the TCP fallback raises all sorts of complexity
red flags in my mind.  Before we go further with SMC I want to see
some evidence that someone has looked through the SMC protocol and can
write a few coherent paragraphs about how the SELinux access controls
for the SMC protocol should work.

... and yes, labeled SCTP is still broken.  Perhaps someday soon I'll
have the time to finish the patchset to fix it.

-- 
paul-moore.com