Libguestfs - Aug 2021 - [PATCH] spec: Clarify STARTTLS vs. SET_META_CONTEXT interaction

On Thu, Aug 12, 2021 at 10:20:40AM -0500, Eric Blake
wrote:
> Consider a SELECTIVETLS server and a MitM attacker, under the
> following NBD_OPT_ handshake scenario:
> 
>  client:                  mitm:                    server:
>  > _STARTTLS
>                           > _SET_META_CONTEXT("A")
>                                                    < _REP_META_CONTEXT
>                                                    < _REP_ACK
>                           > _STARTTLS
>                                                    < _REP_ACK
>                           < _REP_ACK
>  > _SET_META_CONTEXT("B")
>                           < _REP_META_CONTEXT
>                           < _REP_ACK
>  > _GO
>                           > _GO
>                                                    < _REP_ACK
>                           < _REP_ACK
>  > NBD_CMD_BLOCK_STATUS
> 
> While this scenario requires the MitM to be able to use encryption to
> speak to the client (and thus a less likely scenario than a true
> protocol downgrade or plaintext buffering attack), it results in a
> situation where the client is asking for information on context
"B",
> but where the server only saw a request for context "A", which
may
> result in the client interpreting the results of BLOCK_STATUS
> incorrectly even though it is coming over an encrypted connection.
> 
> The safest fix to this is to require that a server cannot use any meta
> context requests from prior to enabling encryption with any successful
> NBD_OPT_GO after encryption.  At this point, the spec already states
> that the server should then return an error (the client is asking for
> block status without proper negotiation), which is better than letting
> the client blindly misinterpret a response sent for a different meta
> context.
> 
> To date, the only known server that has implemented TLS with
> SELECTIVETLS mode as well as support for NBD_OPT_SET_META_CONTEXT is
> nbdkit (qemu-nbd only has FORCEDTLS mode, and nbd-server lacks meta
> context support); thankfully, that implementation is in already line
> with this stricter requirement.
> ---
>  doc/proto.md | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/doc/proto.md b/doc/proto.md
> index 61d57b5..7155b42 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -1165,6 +1165,14 @@ of the newstyle negotiation.
>      permitted by this document (for example, `NBD_REP_ERR_INVALID` if
>      the client included data).
> 
> +    When this command succeeds, the server MUST NOT preserve any
> +    per-export state (such as metadata contexts from
> +    `NBD_OPT_SET_META_CONTEXT`) issued before this command.  The
> +    server MAY preserve global state such as a client request for
> +    `NBD_OPT_STRUCTURED_REPLY`; however, a client SHOULD defer other
> +    stateful option requests until after it determines whether
> +    encryption is available.
I'm not sure I think that makes sense. It's safer to require that
STARTTLS wipes out everything.

There are two reasonable ways of communicating with a server that
supports opportunistic TLS: either you enable TLS as soon as possible
after opening the connection (and then you should do any state
modification after the STARTTLS command), or you don't do any STARTTLS
at all, ever (and then all state settings are done in the unencrypted
connection). Anything else seems like a silly idea.

As such, I think trying to support ways in which you configure things
before STARTTLS, then do STARTTLS, and then expect things to retain
state, is bound to invite security issues, and we should not even try to
support it.

-- 
     w at uter.{be,co.za}
wouter@{grep.be,fosdem.org,debian.org}

On Mon, Aug 16, 2021 at 05:31:10PM +0200, Wouter Verhelst wrote:
> > +++ b/doc/proto.md
> > @@ -1165,6 +1165,14 @@ of the newstyle negotiation.
> >      permitted by this document (for example, `NBD_REP_ERR_INVALID` if
> >      the client included data).
> > 
> > +    When this command succeeds, the server MUST NOT preserve any
> > +    per-export state (such as metadata contexts from
> > +    `NBD_OPT_SET_META_CONTEXT`) issued before this command.  The
> > +    server MAY preserve global state such as a client request for
> > +    `NBD_OPT_STRUCTURED_REPLY`; however, a client SHOULD defer other
> > +    stateful option requests until after it determines whether
> > +    encryption is available.
> 
> I'm not sure I think that makes sense. It's safer to require that
> STARTTLS wipes out everything.
> 
> There are two reasonable ways of communicating with a server that
> supports opportunistic TLS: either you enable TLS as soon as possible
> after opening the connection (and then you should do any state
> modification after the STARTTLS command), or you don't do any STARTTLS
> at all, ever (and then all state settings are done in the unencrypted
> connection). Anything else seems like a silly idea.
Concur.
> 
> As such, I think trying to support ways in which you configure things
> before STARTTLS, then do STARTTLS, and then expect things to retain
> state, is bound to invite security issues, and we should not even try to
> support it.
Okay, how about this wording:

+When this command succeeds, the server MUST NOT preserve any
+negotiation state (such as a request for `NBD_OPT_STRUCTURED_REPLY`,
+or metadata contexts from `NBD_OPT_SET_META_CONTEXT`) issued before
+this command.  A client SHOULD defer all stateful option requests
+until after it determines whether encryption is available.

Unfortunately, nbdkit in opportunistic mode does not (yet) obey that:
a request for structured replies prior to starttls is currently
preserved across the jump into encryption - which may result in the
server sending structured replies to a client not expecting them due
to a MitM plaintext injection.  I'll be submitting a patch for that
soon; sounds like we found a CVE in nbdkit.

On the other hand, qemu as NBD client always sends NBD_OPT_STARTTLS
first (disconnecting rather than falling back to plaintext).  libnbd
as NBD client does not (yet) expose any way to attempt starttls after
other negotiation commands: in opportunistic mode, it currently probes
for encryption before giving the user any control over other NBD_OPT_
commands.  At one point, I had the idea of expanding the libnbd API to
make it easier to try even NBD_OPT_STARTTLS under user control (which
would make it easier to test issues like how servers behave with
clients that don't lead off with STARTTLS), but that has not been
implemented yet, so at the moment, we don't have clients in the wild
that were relying on nbdkit's stateful behavior.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org