wrong environment when using python subprocess as another user

Question

I have this simple python script that run as root, but will execute subprocesses as some other user:

#!/usr/bin/env python2
import subprocess
import os

def demote(user_uid):
   def result():
      os.setuid(user_uid)
   return result

cmd = "echo $USER"
proc = subprocess.Popen(cmd, preexec_fn=demote(1000), stdout=subprocess.PIPE, shell=True)
output = proc.communicate()[0]

print output

If cmd is "sleep 60" then the script spawns sleep as the user:

# ps -ef | grep sleep
dave   17812 17811  0 17:05 pts/5    00:00:00 /usr/bin/sleep 60

However, if cmd is 'echo $USER', the outout is "root". Is there someone I need to do to get the user's env if I spawn the subprocess as a nother user?

Answer 1

You don't really have a problem with subprocess here. Subprocess is getting passed the environment from your current environment, so when you "echo $USER", it's reporting the user from the current environment.

See what happens when you change the command to whoami, which actually checks your UID:

My python shell is running as root:

>>> print os.getuid()
0

But changing the command being run outputs the correct new user:

>>> cmd = "whoami"
>>> proc = subprocess.Popen(cmd, preexec_fn=demote(1000), stdout=subprocess.PIPE, shell=True)
>>> output = proc.communicate()[0]
>>> print output
voodoonofx

If you really want to modify the environment to be that user, you could pass a new dictionary to the Popen call. See the env keyword passed with help(subprocess.Popen):

  __init__(self, args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=False, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0)
      Create new Popen instance.