Fix SCRAM authentication via SSL when mixing versions of OpenSSL

When using a libpq client linked with OpenSSL 1.0.1 or older to connect
to a backend linked with OpenSSL 1.0.2 or newer, the server would send
SCRAM-SHA-256-PLUS and SCRAM-SHA-256 as valid mechanisms for the SASL
exchange, and the client would choose SCRAM-SHA-256-PLUS even if it does
not support channel binding, leading to a confusing error.  In this
case, what the client ought to do is switch to SCRAM-SHA-256 so as the
authentication can move on and succeed.

So for a SCRAM authentication over SSL, here are all the cases present
and how we deal with them using libpq:
1) Server supports channel binding, it sends SCRAM-SHA-256-PLUS and
SCRAM-SHA-256 as allowed mechanisms.
1-1) Client supports channel binding, chooses SCRAM-SHA-256-PLUS.
1-2) Client does not support channel binding, chooses SCRAM-SHA-256.
2) Server does not support channel binding, sends SCRAM-SHA-256 as
allowed mechanism.
2-1) Client supports channel binding, still it has no choice but to
choose SCRAM-SHA-256.
2-2) Client does not support channel binding, it chooses SCRAM-SHA-256.
In all these scenarios the connection should succeed, and the one which
was handled incorrectly prior this commit is 1-2), causing the
connection attempt to fail because client chose SCRAM-SHA-256-PLUS over
SCRAM-SHA-256.

Reported-by: Hugh Ranalli
Diagnosed-by: Peter Eisentraut
Author: Michael Paquier
Reviewed-by: Peter Eisentraut
Discussion: https://postgr.es/m/CAAhbUMO89SqUk-5mMY+OapgWf-twF2NA5sCucbHEzMfGbvcepA@mail.gmail.com
Backpatch-through: 11

diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index 08a5a9c1f306184eced695ddd44669f61a9bd50e..4cbe64ceb58469200d0c2ee5a8df7eeb7c4176c8 100644 (file)
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -526,14 +526,24 @@ pg_SASL_init(PGconn *conn, int payloadlen)
 
                /*
                 * Select the mechanism to use.  Pick SCRAM-SHA-256-PLUS over anything
-                * else if a channel binding type is set.  Pick SCRAM-SHA-256 if
-                * nothing else has already been picked.  If we add more mechanisms, a
-                * more refined priority mechanism might become necessary.
+                * else if a channel binding type is set and if the client supports
+                * it. Pick SCRAM-SHA-256 if nothing else has already been picked.  If
+                * we add more mechanisms, a more refined priority mechanism might
+                * become necessary.
                 */
                if (strcmp(mechanism_buf.data, SCRAM_SHA_256_PLUS_NAME) == 0)
                {
                        if (conn->ssl_in_use)
+                       {
+                               /*
+                                * The server has offered SCRAM-SHA-256-PLUS, which is only
+                                * supported by the client if a hash of the peer certificate
+                                * can be created.
+                                */
+#ifdef HAVE_PGTLS_GET_PEER_CERTIFICATE_HASH
                                selected_mechanism = SCRAM_SHA_256_PLUS_NAME;
+#endif
+                       }
                        else
                        {
                                /*