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(+)
diff --git a/server/protocol-handshake-newstyle.c
b/server/protocol-handshake-newstyle.c
index 5ac45efa..4e16b72c 100644
--- a/server/protocol-handshake-newstyle.c
+++ b/server/protocol-handshake-newstyle.c
@@ -424,8 +424,16 @@ negotiate_handshake_newstyle_options (void)
*/
if (tls == 2 && !conn->using_tls &&
!(option == NBD_OPT_ABORT || option == NBD_OPT_STARTTLS)) {
+ if (option == NBD_OPT_EXPORT_NAME) {
+ debug ("newstyle negotiation: can't reply NBD_REP_ERR_TLS_REQD to %s",
+ name_of_nbd_opt (option));
+ return -1;
+ }
if (send_newstyle_option_reply (option, NBD_REP_ERR_TLS_REQD))
return -1;
+ if (conn_recv_full (data, optlen,
+ "waiting for starttls: %m") == -1)
+ return -1;
continue;
}
--
2.37.3
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