Privilege Separation In Condor

Privilege separation in Condor

Bruce BecklesUniversity of Cambridge Computing

Service

What is privilege separation?

• Isolation of those parts of the code that run at different privilege levels

rootCondor daemons

Condor job

• No privilege separation:

rootCondor

daemonsCondor

job

• Privilege separation:

The principle of least privilege

• Code only ever runs at the minimum level of privilege required for its current task

rootCondor daemons

• Violation of principle of least privilege:

• Not in violation of principle of least privilege:

rootCondor

daemons

Condor asks root to perform privileged action

Upon completion of action control returns to Condor

How do we do this?• User context switching:

rootcontext switching daemon

Condor daemons

Condor job

• GNU userv allows one process to invoke another (in either the same or a different user context) in a secure fashion when only limited trust exists between them (see http://www.gnu.org/software/userv/).

Who does what?• Execute nodes:

Change ownership of any file (CAP_CHOWN capability) Switch to any user context (CAP_SETUID capability) Send signals to any process (CAP_KILL capability)

• Submit nodes: Switch to any user context (CAP_SETUID capability) Send signals to any process (CAP_KILL capability)

• Central manager: No privilege switching required (unless GSI

authentication is being used, in which case need to be able to switch to any user context (CAP_SETUID capability))

(Note that if you have the ability to switch to any user context you effectively have the ability to send signals to any process.)

Details – Central manager

• Unless using GSI authentication, Condor daemons don’t need to run as root, so…

• …don’t run them as root…(!)• If using GSI:

Flawed security paradigm, so… …waste of time trying to make the

system more “secure” until underlying paradigm is “fixed”…

stdout

stderr

Send signal to Condor job

Details – Execute noderoot

uservd

wrapper script

signal handler

Condor job

condor_startdCondor cron

job kill –KILL -1

Change file ownershipfork()

Condor jobChange file ownership

condor_starter

"Rogue"

process

Details – Execute node No Condor daemon or process runs as root Condor passes job to our wrapper script Our script installs a signal handler to intercept Condor’s

signals to the job Our script changes the ownership of the job’s files (via userv)

as necessary at beginning and end of job Our script fork()’s a userv process which runs the job in a

different (also non-privileged) user context On receipt of a signal from Condor, our script calls userv to

send the signal to the job userv uses pipes to pass the job’s standard output and

standard error back to our script (and so to Condor) A cron job (using Condor’s STARTD_CRON mechanism) runs

once a minute to make sure that if there is no job executing then there are no processes running in our dedicated “Condor job” user context

What does this gain us?• If there is a vulnerability in Condor then the entire

machine is not compromised as a result.• We do not have any Condor processes, whose real

user ID is root, listening to the network.• Much greater control over the job’s execution

environment – could run the job chroot’d if desired.• Our wrapper script can examine the job and make

more sophisticated decisions about whether or not to run it (restrict to local executables, etc.)

• We now have a “hook” for executing arbitrary tasks on job completion, a feature “ordinary” Condor lacks.

• If a Condor job leaves behind any processes running after job completion they will be killed – normally, Condor only does this properly if specially configured (EXECUTE_LOGIN_IS_DEDICATED).

What do we lose?• Can no longer suspend jobs (cannot catch SIGSTOP)• We now need to handle passing the job

environment variables, setting resource limits and scheduling priority, etc., which Condor would normally handle

• Condor can no longer correctly distinguish between the load Condor processes and Condor jobs are putting on the machine, and the load generated by other processes

• Information returned by Condor about the job’s CPU utilization, etc. is incorrect

• Cannot work with GSI authentication• Does not yet work with Condor’s Standard universe

(although this may not be difficult to fix)• Adds of the order of 5 seconds to job execution time

Details – Submit node• If not using “strong” authentication

(Kerberos, etc.), can use a similar technique

• If using strong authentication then this won’t work: condor_shadow processes need to run as the

submitting user, but… …machine credentials need to not be

accessible to ordinary users… Damn!

• So…

IPCe.g. Unix domain

sockets

Fully privilege separated architecture (1)

rootcontext switching daemon

Condor daemon

Network

network listener daemon

credential on filesystem

Request privileged services as needed chroot()

dedicated condor user

dedicated different user

Fully privilege separated architecture (1) – details

• Requires extensive changes to Condor code• No Condor daemon or process runs as root• User context switching daemon provides services to

Condor components that would otherwise need a higher level of privilege

• Long running Condor daemons that communicate with the network are privilege separated “OpenBSD-style”: Split into two components that run in different user contexts:

one that exclusively handles network communication, one that does everything else

Network component chroot()’d to somewhere “safe” Components communicate via some form of IPC mechanism

(e.g. Unix domain sockets) Allows authentication credentials to be protected but still

used for authentication (network component acts as a relay between other component and the remote daemon)

IPC

"network so

cket"

Fully privilege separated architecture (2)

rootcontext switching daemon

Condor daemon

(long running)

Network

network listener daemon

credential

chroot()

dedicated condor user

dedicated different user

Condor daemon

(short running)

IPC

possibly different user context, e.g. user who submitted job e.g. condor_shadow

Fully privilege separated architecture (2) – details

• For “short” running Condor daemons (condor_starter, condor_shadow, etc): Typically instantiated by a long running

daemon Parent daemon could set up secure network

channel which child then inherits This gives the child a session key When session key is due to expire (or if re-

negotiation of secure channel is required):• Use session key to “authenticate” to parent

daemon• Now act as a relay for passing authentication

messages between remote daemon and parent daemon to re-negotiate secure channel

Future work• Architecture described on the last

two slides is just a proposal• Details may (will!) change as

design evolves…• Some form of privilege separation

should appear in the Condor 6.9 development series

Further Details

• See:

Implementing privilege separation in the Condor® system (2005):

http://www.allhands.org.uk/2005/proceedings/papers/568.pdf