[PATCH] dbus: explicitly handle messages with NULL interface

[James Prestwood <prestwoj@gmail.com>]

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It was assumed that the DBus daemon would filter messages with
no interfaces set, but some daemons do not (dbus-broker). This
leads to the potential for a crash if the method call has no
interface set. A crash can be seen in IWD with a few lines of
python:

bus = dbus.SystemBus()
obj = bus.get_object("net.connman.iwd", "/")
print(obj.FooBar())

The above isn't necissarily a 'valid' way of doing things, but
it does result in a crash which traces back to ELL. The actual
method call (FooBar in this case) is arbitrary and could be
anything.

++++++++ backtrace ++++++++
0  0x7f532cda6a70 in /lib64/libc.so.6
1  0x47c4d2 in _dbus_object_tree_dispatch() at ell/dbus-service.c:1755
2  0x473f23 in message_read_handler() at ell/dbus.c:284
3  0x46be0c in io_callback() at ell/io.c:118
4  0x46b12d in l_main_iterate() at ell/main.c:471 (discriminator 2)
5  0x46b1dc in l_main_run() at ell/main.c:520
6  0x46b3ec in l_main_run_with_signal() at ell/main.c:648
7  0x403ea9 in main() at src/main.c:490
8  0x7f532cd91042 in /lib64/libc.so.6
+++++++++++++++++++++++++++

The DBus spec does mention the possibility of the interface field
being empty. It does not recommend doing this, but does not
explicitly forbid it. Handling of this case is left up to the
implementation.

The fix is simple: check that the message has an interface set and
if not return an error.
---
 ell/dbus-service.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/ell/dbus-service.c b/ell/dbus-service.c
index 4976b43..a7d6236 100644
--- a/ell/dbus-service.c
+++ b/ell/dbus-service.c
@@ -1749,6 +1749,16 @@ bool _dbus_object_tree_dispatch(struct _dbus_object_tree *tree,
 	member = l_dbus_message_get_member(message);
 	msg_sig = l_dbus_message_get_signature(message);
 
+	/*
+	 * Nothing in the spec explicitly forbids this, but handling of such
+	 * messages is left up to the implementation.
+	 *
+	 * TODO: Another route is to go looking for a matching method under this
+	 * object and call it.
+	 */
+	if (!interface)
+		return false;
+
 	if (!msg_sig)
 		msg_sig = "";
 
-- 
2.26.2