Monday, 6 October 2014

Improving the security of D-Bus

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.

Security issues fixed in D-Bus

  • CVE-2014-3477 (Bug #78979): dbus-daemon sent an AccessDenied error to the service instead of a client when the client is prohibited from accessing the service, which allowed local users to cause a denial of service (initialization failure and exit) or possibly conduct a side-channel attack via a D-Bus message to an inactive service.
  • CVE-2014-3532 (Bug #80163): when running on Linux 2.6.37-rc4 or later, local users could cause a denial of service (system-bus disconnect of other services or applications) by sending a message containing a file descriptor, then exceeding the maximum recursion depth before the initial message is forwarded.
  • CVE-2014-3533 (Bug #80469): dbus-daemon allowed local users to cause a denial of service (disconnect) via a certain sequence of crafted messages that caused the dbus-daemon to forward a message containing an invalid file descriptor.
  • CVE-2014-3635 (Bug #83622): an off-by-one error in dbus-daemon allowed remote attackers to cause a denial of service (dbus-daemon crash) or possibly execute arbitrary code by sending one more file descriptor than the limit, which triggered a heap-based buffer overflow or an assertion failure.
  • CVE-2014-3636 (Bug #82820): a denial-of-service vulnerability in dbus-daemon allowed local attackers to prevent new connections to dbus-daemon, or disconnect existing clients, by exhausting descriptor limits.
  • CVE-2014-3637 (Bug #80559): malicious local users could create D-Bus connections to dbus-daemon which could not be terminated by killing the participating processes, resulting in a denial-of-service vulnerability.
  • CVE-2014-3638 (Bug #81053): dbus-daemon suffered from a denial-of-service vulnerability in the code which tracks which messages expect a reply, allowing local attackers to reduce the performance of dbus-daemon.
  • CVE-2014-3639 (Bug #80919): dbus-daemon did not properly reject malicious connections from local users, resulting in a denial-of-service vulnerability.

Other fixes

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/cgroup was accepted in Linux 3.17.

Identify bogus D-Bus match rules

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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