Usage¶
oslo.privsep lets you define in your code specific functions that will run
in predefined privilege contexts. This lets you run functions with more (or
less) privileges than the rest of the code. Privsep functions live in a
specific privsep
submodule (for example, nova.privsep
for nova).
Defining a context¶
Contexts are defined in the privsep/__init__.py
file. For example, this
defines a sys_admin_pctxt with CAP_CHOWN
, CAP_DAC_OVERRIDE
,
CAP_DAC_READ_SEARCH
, CAP_FOWNER
, CAP_NET_ADMIN
, and
CAP_SYS_ADMIN
rights (equivalent to sudo
rights):
from oslo_privsep import capabilities
from oslo_privsep import priv_context
sys_admin_pctxt = priv_context.PrivContext(
'nova',
cfg_section='nova_sys_admin',
pypath=__name__ + '.sys_admin_pctxt',
capabilities=[capabilities.CAP_CHOWN,
capabilities.CAP_DAC_OVERRIDE,
capabilities.CAP_DAC_READ_SEARCH,
capabilities.CAP_FOWNER,
capabilities.CAP_NET_ADMIN,
capabilities.CAP_SYS_ADMIN],
)
Defining a context with timeout¶
It is possible to initialize PrivContext with timeout:
from oslo_privsep import capabilities
from oslo_privsep import priv_context
dhcp_release_cmd = priv_context.PrivContext(
__name__,
cfg_section='privsep_dhcp_release',
pypath=__name__ + '.dhcp_release_cmd',
capabilities=[caps.CAP_SYS_ADMIN,
caps.CAP_NET_ADMIN],
timeout=5
)
PrivsepTimeout
is raised if timeout is reached.
Warning
The daemon (the root process) task won’t stop when timeout is reached. That means we’ll have less available threads if the related thread never finishes.
Defining a privileged function¶
Functions are defined in files under the privsep/
subdirectory, for
example in a privsep/motd.py
file for functions touching the MOTD file.
They make use of a decorator pointing to the context we defined above:
import nova.privsep
@nova.privsep.sys_admin_pctxt.entrypoint
def update_motd(message):
with open('/etc/motd', 'w') as f:
f.write(message)
Privileged functions must be as simple, specialized and narrow as possible,
so as to prevent further escalation. In this example, update_motd(message)
is narrow: it only allows the service to overwrite the MOTD file. If a more
generic update_file(filename, content)
was created, it could be used to
overwrite any file in the filesystem, allowing easy escalation to root
rights. That would defeat the whole purpose of oslo.privsep.
Defining a privileged function with timeout¶
It is possible to use entrypoint_with_timeout
decorator:
from oslo_privsep import daemon
from neutron import privileged
@privileged.default.entrypoint_with_timeout(timeout=5)
def get_link_devices(namespace, **kwargs):
try:
with get_iproute(namespace) as ip:
return make_serializable(ip.get_links(**kwargs))
except OSError as e:
if e.errno == errno.ENOENT:
raise NetworkNamespaceNotFound(netns_name=namespace)
raise
except daemon.FailedToDropPrivileges:
raise
except daemon.PrivsepTimeout:
raise
PrivsepTimeout
is raised if timeout is reached.
Warning
The daemon (the root process) task won’t stop when timeout is reached. That means we’ll have less available threads if the related thread never finishes.
Using a privileged function¶
To use the privileged function in the regular code, you can just call it:
import nova.privsep.motd
...
nova.privsep.motd.update_motd('This node is currently idle')
It is better to import the complete path (import nova.privsep.motd
) rather
than the motd name (from nova.privsep import motd
) so that it is easier to
spot that the function runs in a different privileged context.
For more details, you can read the following blog post:
Converting from rootwrap to privsep¶
oslo.rootwrap is a precursor of oslo.privsep to allow code to run commands under sudo if they match a predefined filter. For example, you could define a filter that would allow you to run chmod as root using the following filter:
chmod: CommandFilter, chmod, root
Beyond the bad performance of calling full commands in order to accomplish simple tasks, rootwrap also led to bad security: it was difficult to filter commands in a way that would not easily allow privilege escalation.
Replacing rootwrap filters with privsep functions is easy. The chmod filter
above can be replaced with a function that calls os.chmod()
. However a
straight 1:1 filter:function replacement generally results in functions that
are still too broad for good security. It is better to replace each chmod
rootwrap call with a narrow privsep function that will limit it to specific
files.
Sometimes it is necessary to refactor the calling code: the rootwrap design discouraged the creation of new filters and therefore often resulted in the creation of overly-broad calling functions.
As an example, this patch series is work-in-progress to transition Nova from rootwrap to privsep.
For more details, you can read the following blog post: