Insecure /proc/net/unix parsing

Got a slightly better version of the patch. It does a chdir() to fix a TOCTOU problem.

Hi Stéphane - Tavis pointed out[1] two additional issues with the patch in comment #2.

1) The owner of the /proc/PID directory is controllable by executing a setuid binary. You'll have to check the real UID of the process. That's doable by parsing /proc/PID/status. The real UID is the first UID in the Uid: row.

2) There's a race between getting the ppid and changing into the /proc/ppid/ directory and the ppid could be recycled. It is best if you call get_ppid() again, after the chdir(), and verify that the ppid hasn't changed (meaning that it has been recycled).

[1] http://www.openwall.com/lists/oss-security/2015/04/15/11

Attaching an updated diff which should be fixing those two concerns.

Thanks! It all looks good to me except for the now unneeded 'ppid_stat = os.stat(".")' line.

This bug was fixed in the package apport - 2.14.1-0ubuntu3.10

---------------
apport (2.14.1-0ubuntu3.10) trusty-security; urgency=medium

  * SECURITY UPDATE: insecure /proc/net/unix parsing (LP: #1444518)
    - data/apport: temporarily disable container support until it can be
      re-written in a secure manner.
    - CVE number pending
 -- Marc Deslauriers <email address hidden> Thu, 16 Apr 2015 07:56:02 -0400

This bug was fixed in the package apport - 2.14.7-0ubuntu8.4

---------------
apport (2.14.7-0ubuntu8.4) utopic-security; urgency=medium

  * SECURITY UPDATE: insecure /proc/net/unix parsing (LP: #1444518)
    - data/apport: temporarily disable container support until it can be
      re-written in a secure manner.
    - CVE number pending
 -- Marc Deslauriers <email address hidden> Thu, 16 Apr 2015 07:40:49 -0400

Ritesh: Please package the upcoming 2.17.2 instead, which will disable this feature entirely and thus fix that bug too.

Fixed (aka disabled) upstream: http://bazaar.launchpad.net/~apport-hackers/apport/trunk/revision/2948

Fixed upstream in https://launchpad.net/apport/trunk/2.17.2

This bug was fixed in the package apport - 2.17.2-0ubuntu1

---------------
apport (2.17.2-0ubuntu1) vivid; urgency=medium

  * New upstream bug fix release:
    - SECURITY UPDATE: Disable crash forwarding to containers. The previous
      fix in 2.17.1 was not sufficient against all attack scenarios. By
      binding to specially crafted sockes, a normal user program could forge
      arbitrary entries in /proc/net/unix. We cannot currently rely on a
      kernel-side solution for this; this feature will be re-enabled once it
      gets re-done to be secure. (LP: #1444518)
    - apport-kde: Fix crash when showing byte array values. Thanks Jonathan
      Riddell. (LP: #1443659)
    - Really create a better duplicate signature for recoverable problems,
      using ExecutablePath. Thanks Brian Murray. (LP: #1316763)
  * Disable Launchpad crash upload for final Ubuntu 15.04.
 -- Martin Pitt <email address hidden> Thu, 16 Apr 2015 17:51:18 -0500