October 06, 2014
In the last months, I have been working on improving the security of D-Bus, mainly to make it more resistant to denial of service attacks. This work was sponsored by Collabora.
Eight security issues were discovered, fixed and attributed a CVE. They were found by looking at the source code (in D-Bus and Linux' af_unix implementation), checking existing issues in the D-Bus bugzilla and a bit of luck.
In addition to fixing specific bugs, I also explored ideas to restrict the number of D-Bus connections a process or a cgroup could create. After discussions with upstream, those ideas were not retained upstream. But while working on cgroups, my patch for parsing /proc/pid/cgroupwas accepted in Linux 3.17.
D-Bus security issues are not all in dbus-daemon: they could be in applications misusing D-Bus. One common mistake done by applications is to receive a D-Bus signal and handle it without checking it was really sent by the expected sender. It seems impossible to check the code of all applications potentially using D-Bus in order to see if such a mistake is done. Instead of looking the code of random applications, my approach was to add a new method GetAllMatchRules in dbus-daemon to retrieve all match rules and look for suspicious patterns. For example, a match rule for NameOwnerChanged signals that does not filter on the sender of such signals is suspicious and it worth checking the source code of the applications to see if it is legitimate. With this method, I was able to fix bugs in Bluez, ConnMan, Pacrunner, Ofono and Avahi.
GetAllMatchRules is released in dbus 1.9.0 and it is now possible to try it without recompiling D-Bus to enable the feature. I have used a script to tell me which processes register suspicious match rules. I would like if there was a way to do that in a graphical interface. It's not ready yet, but I started a patch in D-Feet.
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