Re: [Libguestfs] [nbdkit PATCH] tls: Gracefully handle early NBD_OPT_ payload

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(+)