YARN-1063 adds the necessary infrasturcture to launch a process as a domain user as a solution for the problem of having a security boundary between processes executed in YARN containers and the Hadoop services. The WCE is a container executor that leverages the winutils capabilities introduced in YARN-1063 and launches containers as an OS process running as the job submitter user. A description of the S4U infrastructure used by YARN-1063 alternatives considered can be read on that JIRA.
The WCE is based on the DefaultContainerExecutor. It relies on the DCE to drive the flow of execution, but it overwrrides some emthods to the effect of:
- change the DCE created user cache directories to be owned by the job user and by the nodemanager group.
- changes the actual container run command to use the 'createAsUser' command of winutils task instead of 'create'
- runs the localization as standalone process instead of an in-process Java method call. This in turn relies on the winutil createAsUser feature to run the localization as the job user.
When compared to LinuxContainerExecutor (LCE), the WCE has some minor differences:
- it does no delegate the creation of the user cache directories to the native implementation.
- it does no require special handling to be able to delete user files
The approach on the WCE came from a practical trial-and-error approach. I had to iron out some issues around the Windows script shell limitations (command line length) to get it to work, the biggest issue being the huge CLASSPATH that is commonplace in Hadoop environment container executions. The job container itself is already dealing with this via a so called 'classpath jar', see
HADOOP-8899 and YARN-316 for details. For the WCE localizer launch as a separate container the same issue had to be resolved and I used the same 'classpath jar' approach.
To use the WCE one needs to set the `yarn.nodemanager.container-executor.class` to `org.apache.hadoop.yarn.server.nodemanager.WindowsSecureContainerExecutor` and set the `yarn.nodemanager.windows-secure-container-executor.group` to a Windows security group name that is the nodemanager service principal is a member of (equivalent of LCE `yarn.nodemanager.linux-container-executor.group`). Unlike the LCE the WCE does not require any configuration outside of the Hadoop own's yar-site.xml.
For WCE to work the nodemanager must run as a service principal that is member of the local Administrators group or LocalSystem. this is derived from the need to invoke LoadUserProfile API which mention these requirements in the specifications. This is in addition to the SE_TCB privilege mentioned in
YARN-1063, but this requirement will automatically imply that the SE_TCB privilege is held by the nodemanager. For the Linux speakers in the audience, the requirement is basically to run NM as root.
Due to the high privilege required by the WCE we had discussed the need to isolate the high privilege operations into a separate process, an 'executor' service that is solely responsible to start the containers (incloding the localizer). The NM would have to authenticate, authorize and communicate with this service via an IPC mechanism and use this service to launch the containers. I still believe we'll end up deploying such a service, but the effort to onboard such a new platfrom specific new service on the project are not trivial.