rpc: Pass OPENSSL_CONF through to ssh invocations

It's no longer possible for libvirt to connect over the ssh transport
from RHEL 9 to RHEL 5.  This is because SHA1 signatures have been
effectively banned in RHEL 9 at the openssl level.  They are required
to check the RHEL 5 host key.  Note this is a separate issue from
openssh requiring additional configuration in order to connect to
older servers.

Connecting from a RHEL 9 client to RHEL 5 server:

$ cat ~/.ssh/config
Host 192.168.0.91
  KexAlgorithms            +diffie-hellman-group14-sha1
  MACs                     +hmac-sha1
  HostKeyAlgorithms        +ssh-rsa
  PubkeyAcceptedKeyTypes   +ssh-rsa
  PubkeyAcceptedAlgorithms +ssh-rsa

$ virsh -c 'qemu+ssh://root@192.168.0.91/system' list
error: failed to connect to the hypervisor
error: Cannot recv data: ssh_dispatch_run_fatal: Connection to 192.168.0.91 port 22: error in libcrypto: Connection reset by peer

"error in libcrypto: Connection reset by peer" is the characteristic
error of openssl having been modified to disable SHA1 by default.
(You will not see this on non-RHEL-derived distros.)

You could enable the legacy crypto policy which downgrades security on
the entire host, but a more fine-grained way to do this is to create
an alternate openssl configuration file that enables the "forbidden"
signatures.  However this requires passing the OPENSSL_CONF
environment variable through to ssh to specify the alternate
configuration.  Libvirt filters out this environment variable, but
this commit allows it through.  With this commit:

$ cat /var/tmp/openssl.cnf
.include /etc/ssl/openssl.cnf
[openssl_init]
alg_section = evp_properties
[evp_properties]
rh-allow-sha1-signatures = yes

$ OPENSSL_CONF=/var/tmp/openssl.cnf ./run virsh -c 'qemu+ssh://root@192.168.0.91/system' list
root@192.168.0.91's password:
 Id   Name   State
--------------------

Essentially my argument here is that OPENSSL_CONF is sufficiently
similar in nature to KRB5CCNAME, SSH* and XAUTHORITY that we should
permit it to be passed through.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2062360
Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/rpc/virnetsocket.c

@@ -855,6 +855,7 @@ int virNetSocketNewConnectSSH(const char *nodename,
     virCommandAddEnvPass(cmd, "KRB5CCNAME");
     virCommandAddEnvPass(cmd, "SSH_AUTH_SOCK");
     virCommandAddEnvPass(cmd, "SSH_ASKPASS");
+    virCommandAddEnvPass(cmd, "OPENSSL_CONF");
     virCommandAddEnvPass(cmd, "DISPLAY");
     virCommandAddEnvPass(cmd, "XAUTHORITY");
     virCommandClearCaps(cmd);