On Mon, Oct 03, 2022 at 03:13:33PM -0500, Eric Blake wrote:
> When using --tls=require, we need to consume any payload sent by the
> client on an option other than NBD_OPT_STARTTLS (for example,
> NBD_OPT_INFO); otherwise we will be out of sync and unable to read the
> next NBD_OPT_, most likely preventing us from being able to establish
> TLS if the client next attempts NBD_OPT_STARTTLS. Also, if the client
> sends NBD_OPT_EXPORT_NAME, they are expecting us to disconnect on
> failure, rather than trying to reply with an error (most clients use
> NBD_OPT_GO these days because of that).
>
> While it is unfortunate that a MitM proxy attacker can use this
> weakness in nbdkit to prevent a TLS connection, it is not quite the
> same as the prior fix for CVE-2021-3716 (commit 09a13daf) where a
> plaintext NBD_OPT_STRUCTURED_REPLY could confuse a client; this is
> because failure to establish TLS is easily detected at the client as a
> proxy attack, and not a situation where the plaintext injection can
> break client behavior without detection.
>
> This bug has been latent for some time; that is because most people do
> not try to connect a plaintext client to a server that requires TLS;
> and even if it is done accidentally, clients like qemu or libnbd that
> give a payload-free option request of NBD_OPT_STRUCTURED_REPLY first
> don't tickle the problem, especially if they disconnect on getting the
> NBD_REP_ERR_TLS_REQD rather than trying to proceed with another
> NBD_OPT_*. But with an upcoming libnbd release adding a new API
> nbd_opt_starttls() to write a SELECTIVETLS client, it becomes easier
> to tickle. See also qemu commit d1129a8a.
>
> Fixes: c5e76492 ("Add TLS support.", v1.1.15)
> ---
> server/protocol-handshake-newstyle.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
This one is now in as 282c3b03
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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