[OLPC Security] Thoughts on bitfrost capabilities, enforcement, and ACLs

[Marcus Leech]

Here's a skeletal python snippet for ACLs in support of bitfrost
capabilities.  Thoughts would be most welcome.

#
# Thoughts on ACLs and bitfrost capabilities
#
# Some of the bitfrost capabilities could be implemented by adding ACLs to
#  the appropriate filesystem bits, including relevant entries under /dev
#
# For example, /dev/video0 could be owned by root, mode 0600, with ACLs
added
#   and removed dynamically as activities come and go that require video
#   access ('use_camera').
#
# The appropriate audio device (/dev/dsp?) could be mode 0622, which
would only
#   allow opening for write for any random process, and an ACL could be
added
#   when an activity that has 'use_microphone' enabled is launched.  The ACL
#   would allow read/write access, rather than the usual write-only.
#
# The alternative is to have activity launch simply change the ownership
#   and/or permissions of the relevant device(s) on launch.  But this
precludes
#   sharing (which, I admit, may not make sense for devices).
#
# A downside to the ACL approach is that the rainbow daemon would need to
#   keep track for purposes of "garbage collection" on ACL entries.
#
import subprocess

def adduser_acl(path, uid, perms):
    acl = "u:%d:%s" % (uid, perms)
    args = ['setfacl', '-m', acl, path]
    subprocess.check_call(args)

def deluser_acl(path, uid):
    acl = "u:%d" % uid
    args = ['setfacl', '-x', acl, path]
    subprocess.check_call(args)


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