upload-pack: disable object filtering when disabled by config

When upload-pack gained partial clone support (v2.17.0-rc0~132^2~12,
2017-12-08), it was guarded by the uploadpack.allowFilter config item
to allow server operators to control when they start supporting it.

That config item didn't go far enough, though: it controls whether the
'filter' capability is advertised, but if a (custom) client ignores
the capability advertisement and passes a filter specification anyway,
the server would handle that despite allowFilter being false.

This is particularly significant if a security bug is discovered in
this new experimental partial clone code.  Installations without
uploadpack.allowFilter ought not to be affected since they don't
intend to support partial clone, but they would be swept up into being
vulnerable.

Simplify and limit the attack surface by making uploadpack.allowFilter
disable the feature, not just the advertisement of it.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jrn

Documentation/config.txt

@@ -3270,7 +3270,7 @@ uploadpack.packObjectsHook::
 	stdout.
 
 uploadpack.allowFilter::
-	If this option is set, `upload-pack` will advertise partial
+	If this option is set, `upload-pack` will support partial
 	clone and partial fetch object filtering.
 +
 Note that this configuration variable is ignored if it is seen in the

upload-pack.c

@@ -68,7 +68,7 @@ static int stateless_rpc;
 static const char *pack_objects_hook;
 
 static int filter_capability_requested;
-static int filter_advertise;
+static int allow_filter;
 static struct list_objects_filter_options filter_options;
 
 static void reset_timeout(void)
@@ -845,7 +845,7 @@ static void receive_needs(void)
 			no_progress = 1;
 		if (parse_feature_request(features, "include-tag"))
 			use_include_tag = 1;
-		if (parse_feature_request(features, "filter"))
+		if (allow_filter && parse_feature_request(features, "filter"))
 			filter_capability_requested = 1;
 
 		o = parse_object(&oid_buf);
@@ -975,7 +975,7 @@ static int send_ref(const char *refname, const struct object_id *oid,
 				     " allow-reachable-sha1-in-want" : "",
 			     stateless_rpc ? " no-done" : "",
 			     symref_info.buf,
-			     filter_advertise ? " filter" : "",
+			     allow_filter ? " filter" : "",
 			     git_user_agent_sanitized());
 		strbuf_release(&symref_info);
 	} else {
@@ -1055,7 +1055,7 @@ static int upload_pack_config(const char *var, const char *value, void *unused)
 		if (!strcmp("uploadpack.packobjectshook", var))
 			return git_config_string(&pack_objects_hook, var, value);
 	} else if (!strcmp("uploadpack.allowfilter", var)) {
-		filter_advertise = git_config_bool(var, value);
+		allow_filter = git_config_bool(var, value);
 	}
 	return parse_hide_refs_config(var, value, "uploadpack");
 }