Security for Scheduler Partitions

By default, anyone on the system can add partitions and modify their attributes. We recommend that you use the SCHED_APS_ADD_SECURITY command to SchedCtl(), or the aps command to specify the level of security that suits your system.

The following table shows the main security options (including both the security policy for the -s option for the aps command and the corresponding SchedCtl() flag), in increasing order of security. For information about enabling PROCMGR_AID_APS_ROOT, see the entry for procmgr_ability() in the C Library Reference.

aps SchedCtl() Description
none SCHED_APS_SEC_OFF Anyone on the system can add partitions and modify their attributes.
basic SCHED_APS_SEC_BASIC Only processes with PROCMGR_AID_APS_ROOT enabled and running in the System partition can change the overall scheduling parameters. Processes with PROCMGR_AID_APS_ROOT enabled and running in any partition can set critical budgets.
flexible SCHED_APS_SEC_FLEXIBLE Only processes with PROCMGR_AID_APS_ROOT enabled and running in the System partition can change scheduling parameters. However, processes with PROCMGR_AID_APS_ROOT enabled and running in any partition can create subpartitions, join threads into their own subpartitions, modify subpartitions, and change critical budgets. This lets applications create their own local subpartitions out of their own budgets. The percentage for budgets must not be zero.
recommended SCHED_APS_SEC_RECOMMENDED Only processes with PROCMGR_AID_APS_ROOT enabled and running in the System partition can create partitions or change parameters. This creates a two-level hierarchy of partitions: the System partition and its children. Only processes with PROCMGR_AID_APS_ROOT enabled and running in the System partition can join their own threads to partitions. The percentage for budgets must not be zero.
Note: Unless you're testing the partitioning aspects and want to change all of the parameters without restarting, you should set at least basic security.

After setting up the scheduler partitions, you can use SCHED_APS_SEC_PARTITIONS_LOCKED to prevent further unauthorized changes. For example: 

sched_aps_security_parms p;

APS_INIT_DATA( &p );
p.sec_flags = SCHED_APS_SEC_PARTITIONS_LOCKED;
SchedCtl( SCHED_APS_ADD_SECURITY, &p, sizeof(p));
Note: Before you call SchedCtl(), ensure that you initialize all the members of the data structure associated with the command. You can use the APS_INIT_DATA() macro to do this.

The security options listed above are combinations of individual options (but it's more convenient to use the compound options): 

#define SCHED_APS_SEC_BASIC       (SCHED_APS_SEC_ROOT0_OVERALL | SCHED_APS_SEC_ROOT_MAKES_CRITICAL)

#define SCHED_APS_SEC_FLEXIBLE    (SCHED_APS_SEC_BASIC | SCHED_APS_SEC_NONZERO_BUDGETS |\
                                   SCHED_APS_SEC_ROOT_MAKES_PARTITIONS |\
                                   SCHED_APS_SEC_PARENT_JOINS | SCHED_APS_SEC_PARENT_MODIFIES )

#define SCHED_APS_SEC_RECOMMENDED (SCHED_APS_SEC_FLEXIBLE | SCHED_APS_SEC_SYS_MAKES_PARTITIONS |\
                                   SCHED_APS_SEC_SYS_JOINS | SCHED_APS_SEC_JOIN_SELF_ONLY)

#define SCHED_APS_SEC_OFF         0x00000000

The individual options are as follows:

aps SchedCtl() Description
root0_overall SCHED_APS_SEC_ROOT0_OVERALL You must have PROCMGR_AID_APS_ROOT enabled and be in the System partition in order to change the overall scheduling parameters, such as the averaging window size.
root_makes_partitions SCHED_APS_SEC_ROOT_MAKES_PARTITIONS You must have PROCMGR_AID_APS_ROOT enabled in order to create or modify partitions.
sys_makes_partitions SCHED_APS_SEC_SYS_MAKES_PARTITIONS You must be running in the System partition in order to create or modify partitions.
parent_modifies SCHED_APS_SEC_PARENT_MODIFIES Allows partitions to be modified (SCHED_APS_MODIFY_PARTITION), but you must be running in the parent partition of the partition being modified. Modify means to change a partition's percentage or critical budget.
nonzero_budgets SCHED_APS_SEC_NONZERO_BUDGETS A partition may not be created with, or modified to have, a zero budget. Unless you know your partition needs to run only in response to client requests, i.e. receipt of messages, you should set this option.
root_makes_critical SCHED_APS_SEC_ROOT_MAKES_CRITICAL You must have PROCMGR_AID_APS_ROOT enabled in order to create a nonzero critical budget or change an existing critical budget.
sys_makes_critical SCHED_APS_SEC_SYS_MAKES_CRITICAL You must be running in the System partition to create a nonzero critical budget or change an existing critical budget.
root_joins SCHED_APS_SEC_ROOT_JOINS You must have PROCMGR_AID_APS_ROOT enabled in order to join a thread to a partition.
sys_joins SCHED_APS_SEC_SYS_JOINS You must be running in the System partition in order to join a thread.
parent_joins SCHED_APS_SEC_PARENT_JOINS You must be running in the parent partition of the partition you wish to join.
join_self_only SCHED_APS_SEC_JOIN_SELF_ONLY A process may join only itself to a partition.
partitions_locked SCHED_APS_SEC_PARTITIONS_LOCKED Prevent further changes to any partition's budget, or overall scheduling parameters, such as the window size. Set this after you've set up your partitions.

Security and critical threads

A thread might be able to set its priority to a partition's critical priority, but this isn't a security issue. That's because a thread marked as critical has no effect on the thread scheduler unless the thread is in a partition that has a critical budget. The thread scheduler has security options that control who may set or change a partition's critical budget.

For the system to be secure against possible critical thread abuse, it's important to: