My Security Planet » Dominic White's .tHE pRODUCT » Dropping Privileges in Python (pattern)

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Dominic White's .tHE pRODUCT

• 

  Dropping Privileges in Python (pattern)

  Posted: November 14th, 2011, 8:08am CST

  Tags:  Geek   [edit]

  Recently, I had a simple python program that created a listening socket, and was uncomfortable running it as root (required to access a port below 1000). I had a quick look around, and found a good blog entry on doing exactly this. However, when running this on OSX, which uses negative UID and GID, I ran into a problem. It turns out that the negative ID is an offset from UINT32_MAX, i.e. 2^32+(-ve UID). The problem is, Python 2.7.1 (Lion's default) os.setgid() was returning an OverFlowError (but not in 2.7.2). I made a mod to the code to handle that case, and figured this pattern may be useful to others wanting to drop privs in a python app.
  

  import os, pwd, grp
  def safe_setgid(running_gid):   try:            os.setgid(running_gid)  except OSError, e:              print('Could not set effective group id: %s' % e)
  def safe_setuid(running_uid):   try:            os.setuid(running_uid)  except OSError, e:              print('Could not set effective group id: %s' % e)
  # Taken from http://antonym.org/2005/12/dropping-privileges-in-python.htmldef drop_privileges(uid_name='nobody', gid_name='nogroup'):   starting_uid = os.getuid()      starting_gid = os.getgid()      starting_uid_name = pwd.getpwuid(starting_uid)[0]
          if os.getuid() != 0:            # We're not root so don't drop, you may want to change this             print("drop_privileges: already running as '%s'"%starting_uid_name)             return
          # If we started as root, drop privs and become the specified user/group if starting_uid == 0:           # Get the uid/gid from the name         running_uid = pwd.getpwnam(uid_name)[2]         running_gid = grp.getgrnam(gid_name)[2]
                  # Try setting the new uid/gid           try:                    safe_setgid(running_gid)                except OverflowError, e:                        if (running_gid > 4294967290):                               running_gid = -4294967296 + running_gid                         safe_setgid(running_gid)
                  try:                    safe_setuid(running_uid)                except OverflowError, e:                        if (running_uid > 4294967290):                               running_uid = -4294967296 + running_uid                         safe_setuid(running_uid)
                  # Ensure a very conservative umask              new_umask = 077         old_umask = os.umask(new_umask)         print('drop_privileges: Old umask: %s, new umask: %s' % (oct(old_umask), oct(new_umask)))
          final_uid = os.getuid() final_gid = os.getgid() print('drop_privileges: running as %s/%s' % (pwd.getpwuid(final_uid)[0],grp.getgrgid(final_gid)[0]))