| Message ID | 1460671343-18485-1-git-send-email-eblake@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 15.04.2016 00:02, Eric Blake wrote: > Upstream NBD documents (as of commit 4feebc95) that servers MAY > choose to operate in a conditional mode, where it is up to the > client whether to use TLS. For qemu's case, we want to always be > in FORCEDTLS mode, because of the risk of man-in-the-middle > attacks, and since we never export more than one device; likewise, > the qemu client will ALWAYS send NBD_OPT_STARTTLS as its first > option. But now that SELECTIVETLS servers exist, it is feasible > to encounter a (non-qemu) client that is programmed to talk to > such a server, and does not do NBD_OPT_STARTTLS first, but rather > wants to probe if it can use a non-encrypted export. > > The NBD protocol documents that we should let such a client > continue trying, on the grounds that maybe the client will get the > hint to send NBD_OPT_STARTTLS, rather than immediately dropping > the connection. > > Note that NBD_OPT_EXPORT_NAME is a special case: since it is the > only option request that can't have an error return, we have to > (continue to) drop the connection on that one; rather, what we are > fixing here is that all other replies prior to TLS initiation tell > the client NBD_REP_ERR_TLS_REQD, but keep the connection alive. > > Signed-off-by: Eric Blake <eblake@redhat.com> > --- > > In v2: tweak commit message, continue to drop connection on > NBD_OPT_EXPORT_NAME > > nbd/server.c | 15 +++++++++++++-- > 1 file changed, 13 insertions(+), 2 deletions(-) Thanks Eric, applied to my block branch: https://github.com/XanClic/qemu/commits/block Max
diff --git a/nbd/server.c b/nbd/server.c index 2a4dd10..a13a691 100644 --- a/nbd/server.c +++ b/nbd/server.c @@ -449,11 +449,19 @@ static int nbd_negotiate_options(NBDClient *client) client->ioc = QIO_CHANNEL(tioc); break; + case NBD_OPT_EXPORT_NAME: + /* No way to return an error to client, so drop connection */ + TRACE("Option 0x%x not permitted before TLS", clientflags); + return -EINVAL; + default: TRACE("Option 0x%x not permitted before TLS", clientflags); + if (nbd_negotiate_drop_sync(client->ioc, length) != length) { + return -EIO; + } nbd_negotiate_send_rep(client->ioc, NBD_REP_ERR_TLS_REQD, clientflags); - return -EINVAL; + break; } } else if (fixedNewstyle) { switch (clientflags) { @@ -471,6 +479,9 @@ static int nbd_negotiate_options(NBDClient *client) return nbd_negotiate_handle_export_name(client, length); case NBD_OPT_STARTTLS: + if (nbd_negotiate_drop_sync(client->ioc, length) != length) { + return -EIO; + } if (client->tlscreds) { TRACE("TLS already enabled"); nbd_negotiate_send_rep(client->ioc, NBD_REP_ERR_INVALID, @@ -480,7 +491,7 @@ static int nbd_negotiate_options(NBDClient *client) nbd_negotiate_send_rep(client->ioc, NBD_REP_ERR_POLICY, clientflags); } - return -EINVAL; + break; default: TRACE("Unsupported option 0x%x", clientflags); if (nbd_negotiate_drop_sync(client->ioc, length) != length) {
Upstream NBD documents (as of commit 4feebc95) that servers MAY choose to operate in a conditional mode, where it is up to the client whether to use TLS. For qemu's case, we want to always be in FORCEDTLS mode, because of the risk of man-in-the-middle attacks, and since we never export more than one device; likewise, the qemu client will ALWAYS send NBD_OPT_STARTTLS as its first option. But now that SELECTIVETLS servers exist, it is feasible to encounter a (non-qemu) client that is programmed to talk to such a server, and does not do NBD_OPT_STARTTLS first, but rather wants to probe if it can use a non-encrypted export. The NBD protocol documents that we should let such a client continue trying, on the grounds that maybe the client will get the hint to send NBD_OPT_STARTTLS, rather than immediately dropping the connection. Note that NBD_OPT_EXPORT_NAME is a special case: since it is the only option request that can't have an error return, we have to (continue to) drop the connection on that one; rather, what we are fixing here is that all other replies prior to TLS initiation tell the client NBD_REP_ERR_TLS_REQD, but keep the connection alive. Signed-off-by: Eric Blake <eblake@redhat.com> --- In v2: tweak commit message, continue to drop connection on NBD_OPT_EXPORT_NAME nbd/server.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-)