Fix a remotely triggerable assertion failure (CVE-2012-2250)

If we completed the handshake for the v2 link protocol but wound up
negotiating the wong protocol version, we'd become so confused about
what part of the handshake we were in that we'd promptly die with an
assertion.

This is a fix for CVE-2012-2250; it's a bugfix on 0.2.3.6-alpha.
All servers running that version or later should really upgrade.

Bug and fix from "some guy from France."  I tweaked his code slightly
to make it log the IP of the offending node, and to forward-port it to
0.2.4.

changes/link_negotiation_assert

@@ -0,0 +1,6 @@
+  o Major bugfixs (security):
+    - Fix a group of remotely triggerable assertion failures related to
+      incorrect link protocol negotiation. Found, diagnosed, and fixed
+      by "some guy from France." Fix for CVE-2012-2250; bugfix on
+      0.2.3.6-alpha.
+

src/or/channeltls.c

@@ -1229,6 +1229,15 @@ channel_tls_process_versions_cell(var_cell_t *cell, channel_tls_t *chan)
            "handshake. Closing connection.");
     connection_or_close_for_error(chan->conn, 0);
     return;
+  } else if (highest_supported_version != 2 &&
+             chan->conn->base_.state == OR_CONN_STATE_OR_HANDSHAKING_V2) {
+    /* XXXX This should eventually be a log_protocol_warn */
+    log_fn(LOG_WARN, LD_OR,
+           "Negotiated link with non-2 protocol after doing a v2 TLS "
+           "handshake with %s. Closing connection.",
+           fmt_addr(&chan->conn->base_.addr));
+    connection_or_close_for_error(chan->conn, 0);
+    return;
   }
 
   chan->conn->link_proto = highest_supported_version;