Re: Trust intermediate CA for client certificates

On 03/18/2013 01:07 PM, Craig Ringer wrote:
> System wide installation of the root may allow OpenSSL to discover it
> and use it for verification back to the root without having to trust it
> to sign clients. I'll do some more checking to see if this is possible
> with how Pg uses OpenSSL but I'm inclined to doubt it.
It looks like we aren't reading the system-wide certs or looking them up
when certs aren't resolved in the files Pg explicitly passes to OpenSSL,
so a system-wide install doesn't look like it'll work.

I've had a look at how Apache handles this and it took me a while to
work out what's going on. Apache uses SSLCACertificateFile (concatenated
certs) / SSLCACertificatePath (hashed dir) to look up trusted
certificate signers. It According to the docs it doesn't appear to make
any provision there for trusting intermediate certificates but not their
parents as signers of client certificates, but it looks like support is
there, the docs just don't explain it well.

Apache has SSLCADNRequestFile / SSLCADNRequestPath which are described
as controlling the acceptable certificate names sent to clients in a
client cert request. The docs don't make it clear if Apache will trust
only client certs with these certs in their chains or whether this only
controls the list of certificate DNs presented to the client rather than
what's accepted in response. The code suggests that they control trust
not just the cert list presented.

In Apache's modules/ssl/ssl_engine_init.c it calls
SSL_CTX_load_verify_locations on the SSLCACertificateFile and
SSLCACertificatePath.

It then calls ssl_init_FindCAList with the
SSLCADNRequestFile/SSLCADNRequestPath if they're specified in the
configuration, otherwise it calls it with the
SSLCACertificateFile/SSLCACertificatePath . That calls
ssl_init_PushCAList on all of the certs it finds in the File and Path
variants. For each cert file that calls SSL_load_client_CA_file and for
each cert within each file pushes the cert onto a STACK_OF(X509_NAME) if
a cert with the same DN isn't already in the stack. It passes the stack
to OpenSSL's SSL_CTX_set_client_CA_list .

So what Apache does appears to boil down to:

SSL_CTX_load_verify_locations(ca_file,ca_path);
if (ca_dn_file || ca_dn_path) {
SSL_CTX_set_client_CA_list( ... STACK_OF unique certs on ca_dn_file
and ca_dn_path ... );
} else {
SSL_CTX_set_client_CA_list( ... STACK_OF unique certs on ca_file and
ca_path );
}

This appears to match Ian's description of having a validation-only cert
list and a separate list of certs used to verify clients. I'd like to
follow Apache's model:

in postgresql.conf, if ssl_ca_file is set then pass it to
SSL_CTX_load_verify_locations . If the proposed new parameter
ssl_ca_valid_client_signers_file is set then pass the certs in that to
SSL_CTX_set_client_CA_list ; otherwise pass the certs in ssl_ca_file to
SSL_CTX_set_client_CA_list and thus retain the current behaviour.
Hopefully we can avoid the ugly read-and-deduplicate stuff Apache has to
do because we currently only support a certfile anyway, we don't read
certdirs, so I'll look for helper functions that wrap
SSL_CTX_set_client_CA_list.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services