On 4/28/22 17:24, Chuck Lever III wrote:
On Apr 28, 2022, at 4:49 AM, Boris Pismenny <borispismenny@xxxxxxxxx> wrote:
On 18/04/2022 19:49, Chuck Lever wrote:
In-kernel TLS consumers need a way to perform a TLS handshake. In
the absence of a handshake implementation in the kernel itself, a
mechanism to perform the handshake in user space, using an existing
TLS handshake library, is necessary.
I've designed a way to pass a connected kernel socket endpoint to
user space using the traditional listen/accept mechanism. accept(2)
gives us a well-understood way to materialize a socket endpoint as a
normal file descriptor in a specific user space process. Like any
open socket descriptor, the accepted FD can then be passed to a
library such as openSSL to perform a TLS handshake.
This prototype currently handles only initiating client-side TLS
handshakes. Server-side handshakes and key renegotiation are left
to do.
Security Considerations
~~~~~~~~ ~~~~~~~~~~~~~~
This prototype is net-namespace aware.
The kernel has no mechanism to attest that the listening user space
agent is trustworthy.
Currently the prototype does not handle multiple listeners that
overlap -- multiple listeners in the same net namespace that have
overlapping bind addresses.
Thanks for posting this. As we discussed offline, I think this approach
is more manageable compared to a full in-kernel TLS handshake. A while
ago, I've hacked around TLS to implement the data-path for NVMe-TLS and
the data-path is indeed very simple provided an infrastructure such as
this one.
Making this more generic is desirable, and this obviously requires
supporting multiple listeners for multiple protocols (TLS, DTLS, QUIC,
PSP, etc.), which suggests that it will reside somewhere outside of net/tls.
Moreover, there is a need to support (TLS) control messages here too.
These will occasionally require going back to the userspace daemon
during kernel packet processing. A few examples are handling: TLS rekey,
TLS close_notify, and TLS keepalives. I'm not saying that we need to
support everything from day-1, but there needs to be a way to support these.
I agree that control messages need to be handled as well. For the
moment, the prototype simply breaks the connection when a control
message is encountered, and a new session is negotiated. That of
course is not the desired long-term solution.
If we believe that control messages are going to be distinct for
each transport security layer, then perhaps we cannot make the
handshake mechanism generic -- it will have to be specific to
each security layer. Just a thought.
A related kernel interface is the XFRM netlink where the kernel asks a
userspace daemon to perform an IKE handshake for establishing IPsec SAs.
This works well when the handshake runs on a different socket, perhaps
that interface can be extended to do handshakes on a given socket that
lives in the kernel without actually passing the fd to userespace. If we
avoid instantiating a full socket fd in userspace, then the need for an
accept(2) interface is reduced, right?
Certainly piping the handshake messages up to user space instead
of handing off a socket is possible. The TLS libraries would need
to tolerate this, and GnuTLS (at least) appears OK with performing
a handshake on an AF_TLSH socket.
Yeah, and I guess that'll be the hard part.
We would need to design an entirely data path for gnutls when going down
that path.
The beauty of the fd-passing idea is that gnutls (and openssl for that
matter) will 'just work' (tm), without us have to do larger surgery there.
Just for reference, I've raised an issue with gnutls to accept long
identifiers in TLS 1.3 (issue #1323), which is required for
NVMe-over-TLS support. That one is lingering for over two months now.
And that's a relatively simple change; I don't want to imagine how long
it'd take to try to push in a larger redesign...
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
hare@xxxxxxx +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
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