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SCC proposal edits for clarity and correctness

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This commit fixes a few typographical and wording nits, adds formatting
for keywords where appropriate, and tweaks punctuation for clarity
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dobbymoodge 2016-01-12 16:26:11 -05:00 committed by Paul Weil
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docs/proposals/security-context-constraints.md
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@ -57,7 +57,7 @@ a user runs.
Use case 1: Use case 1:
As an administrator, I can create a namespace for a person that can't create privileged containers As an administrator, I can create a namespace for a person that can't create privileged containers
AND enforces that the UID of the containers is set to a certain value AND enforce that the UID of the containers is set to a certain value
Use case 2: Use case 2:
As a cluster operator, an infrastructure component should be able to create a pod with elevated As a cluster operator, an infrastructure component should be able to create a pod with elevated
@ -76,7 +76,7 @@ pods and service accounts within a project
## Requirements ## Requirements
1. Provide a set of restrictions that controls how a security context is created for pods and containers 1. Provide a set of restrictions that controls how a security context is created for pods and containers
as a new, cluster-scoped, object called PodSecurityPolicy. as a new cluster-scoped object called `PodSecurityPolicy`.
1. User information in `user.Info` must be available to admission controllers. (Completed in 1. User information in `user.Info` must be available to admission controllers. (Completed in
https://github.com/GoogleCloudPlatform/kubernetes/pull/8203) https://github.com/GoogleCloudPlatform/kubernetes/pull/8203)
1. Some authorizers may restrict a user’s ability to reference a service account. Systems requiring 1. Some authorizers may restrict a user’s ability to reference a service account. Systems requiring
@ -88,7 +88,7 @@ referencing specific service accounts themselves.
### Model ### Model
PodSecurityPolicy objects exists in the root scope, outside of a namespace. The PodSecurityPolicy objects exist in the root scope, outside of a namespace. The
PodSecurityPolicy will reference users and groups that are allowed PodSecurityPolicy will reference users and groups that are allowed
to operate under the constraints. In order to support this, `ServiceAccounts` must be mapped to operate under the constraints. In order to support this, `ServiceAccounts` must be mapped
to a user name or group list by the authentication/authorization layers. This allows the security to a user name or group list by the authentication/authorization layers. This allows the security
@ -96,10 +96,10 @@ context to treat users, groups, and service accounts uniformly.
Below is a list of PodSecurityPolicies which will likely serve most use cases: Below is a list of PodSecurityPolicies which will likely serve most use cases:
1. A default policy object. This object is permissioned to something covers all actors such 1. A default policy object. This object is permissioned to something which covers all actors, such
as a `system:authenticated` group and will likely be the most restrictive set of constraints. as a `system:authenticated` group, and will likely be the most restrictive set of constraints.
1. A default constraints object for service accounts. This object can be identified as serving 1. A default constraints object for service accounts. This object can be identified as serving
a group identified by `system:service-accounts` which can be imposed by the service account authenticator / token generator. a group identified by `system:service-accounts`, which can be imposed by the service account authenticator / token generator.
1. Cluster admin constraints identified by `system:cluster-admins` group - a set of constraints with elevated privileges that can be used 1. Cluster admin constraints identified by `system:cluster-admins` group - a set of constraints with elevated privileges that can be used
by an administrative user or group. by an administrative user or group.
1. Infrastructure components constraints which can be identified either by a specific service 1. Infrastructure components constraints which can be identified either by a specific service
@ -259,27 +259,27 @@ that the administrator has defined for the groups they are assigned.
## Default PodSecurityPolicy And Overrides ## Default PodSecurityPolicy And Overrides
In order to establish policy for service accounts and users there must be a way In order to establish policy for service accounts and users, there must be a way
to identify the default set of constraints that is to be used. This is best accomplished by using to identify the default set of constraints that is to be used. This is best accomplished by using
groups. As mentioned above, groups may be used by the authentication/authorization layer to ensure groups. As mentioned above, groups may be used by the authentication/authorization layer to ensure
that every user maps to at least one group (with a default example of `system:authenticated`) and it that every user maps to at least one group (with a default example of `system:authenticated`) and it
is up to the cluster administrator to ensure that a PodSecurityPolicy object exists that is up to the cluster administrator to ensure that a `PodSecurityPolicy` object exists that
references the group. references the group.
If an administrator would like to provide a user with a changed set of security context permissions If an administrator would like to provide a user with a changed set of security context permissions,
they may do the following: they may do the following:
1. Create a new PodSecurityPolicy object and add a reference to the user or a group 1. Create a new `PodSecurityPolicy` object and add a reference to the user or a group
that the user belongs to. that the user belongs to.
1. Add the user (or group) to an existing PodSecurityPolicy object with the proper 1. Add the user (or group) to an existing `PodSecurityPolicy` object with the proper
elevated privileges. elevated privileges.
## Admission ## Admission
Admission control using an authorizer allows the ability to control the creation of resources Admission control using an authorizer provides the ability to control the creation of resources
based on capabilities granted to a user. In terms of the PodSecurityPolicy it means based on capabilities granted to a user. In terms of the `PodSecurityPolicy`, it means
that an admission controller may inspect the user info made available in the context to retrieve that an admission controller may inspect the user info made available in the context to retrieve
and appropriate set of policies for validation. an appropriate set of policies for validation.
The appropriate set of PodSecurityPolicies is defined as all of the policies The appropriate set of PodSecurityPolicies is defined as all of the policies
available that have reference to the user or groups that the user belongs to. available that have reference to the user or groups that the user belongs to.
@ -287,26 +287,27 @@ available that have reference to the user or groups that the user belongs to.
Admission will use the PodSecurityPolicy to ensure that any requests for a Admission will use the PodSecurityPolicy to ensure that any requests for a
specific security context setting are valid and to generate settings using the following approach: specific security context setting are valid and to generate settings using the following approach:
1. Determine all the available PodSecurityPolicy objects that are allowed to be used 1. Determine all the available `PodSecurityPolicy` objects that are allowed to be used
1. Sort the PodSecurityPolicy objects in a most restrictive to least restrictive order. 1. Sort the `PodSecurityPolicy` objects in a most restrictive to least restrictive order.
1. For each PodSecurityPolicy, generate a SecurityContext for each container. The generation phase will not override 1. For each `PodSecurityPolicy`, generate a `SecurityContext` for each container. The generation phase will not override
and user requested settings in the SecurityContext and will rely on the validation phase to ensure that any user requested settings in the `SecurityContext`, and will rely on the validation phase to ensure that
the user requests are valid. the user requests are valid.
1. Validate the generated SecurityContext to ensure it falls within the boundaries of the PodSecurityPolicy 1. Validate the generated `SecurityContext` to ensure it falls within the boundaries of the `PodSecurityPolicy`
1. If all containers validate under a single PodSecurityPolicy then the pod will be admitted 1. If all containers validate under a single `PodSecurityPolicy` then the pod will be admitted
1. If all containers DO NOT validate under the PodSecurityPolicy then try the next PodSecurityPolicy 1. If all containers DO NOT validate under the `PodSecurityPolicy` then try the next `PodSecurityPolicy`
1. If no PodSecurityPolicy validates for the pod then the pod will not be admitted 1. If no `PodSecurityPolicy` validates for the pod then the pod will not be admitted
## Creation of a SecurityContext Based on PodSecurityPolicy ## Creation of a SecurityContext Based on PodSecurityPolicy
The creation of a SecurityContext based on a PodSecurityPolicy is based upon the configured The creation of a `SecurityContext` based on a `PodSecurityPolicy` is based upon the configured
settings of the PodSecurityPolicy. settings of the `PodSecurityPolicy`.
There are three scenarios under which a PodSecurityPolicy field may fall: There are three scenarios under which a `PodSecurityPolicy` field may fall:
1. Governed by a boolean: fields of this type will be defaulted to the most restrictive value. 1. Governed by a boolean: fields of this type will be defaulted to the most restrictive value.
For instance, `AllowPrivileged` will always be set to false if unspecified. For instance, `AllowPrivileged` will always be set to false if unspecified.
1. Governed by an allowable set: fields of this type will be checked against the set to ensure 1. Governed by an allowable set: fields of this type will be checked against the set to ensure
their value is allowed. For example, `AllowCapabilities` will ensure that only capabilities their value is allowed. For example, `AllowCapabilities` will ensure that only capabilities
that are allowed to be requested are considered valid. `HostNetworkSources` will ensure that that are allowed to be requested are considered valid. `HostNetworkSources` will ensure that
@ -346,7 +347,7 @@ type RunAsUserStrategy interface {
An administrator may wish to create a resource in a namespace that runs with An administrator may wish to create a resource in a namespace that runs with
escalated privileges. By allowing security context escalated privileges. By allowing security context
constraints to operate on both the requesting user and pod's service account administrators are able to constraints to operate on both the requesting user and the pod's service account, administrators are able to
create pods in namespaces with elevated privileges based on the administrator's security context create pods in namespaces with elevated privileges based on the administrator's security context
constraints. constraints.
@ -354,7 +355,7 @@ This also allows the system to guard commands being executed in the non-conformi
instance, an `exec` command can first check the security context of the pod against the security instance, an `exec` command can first check the security context of the pod against the security
context constraints of the user or the user's ability to reference a service account. context constraints of the user or the user's ability to reference a service account.
If it does not validate then it can block users from executing the command. Since the validation If it does not validate then it can block users from executing the command. Since the validation
will be user aware administrators would still be able to run the commands that are restricted to normal users. will be user aware, administrators would still be able to run the commands that are restricted to normal users.
## Interaction with the Kubelet ## Interaction with the Kubelet
@ -362,11 +363,11 @@ In certain cases, the Kubelet may need provide information about
the image in order to validate the security context. An example of this is a cluster the image in order to validate the security context. An example of this is a cluster
that is configured to run with a UID strategy of `MustRunAsNonRoot`. that is configured to run with a UID strategy of `MustRunAsNonRoot`.
In this case the admission controller can set the existing `MustRunAsNonRoot` flag on the SecurityContext In this case the admission controller can set the existing `MustRunAsNonRoot` flag on the `SecurityContext`
based on the UID strategy of the SecurityPolicy. It should still validate any requests on the pod based on the UID strategy of the `SecurityPolicy`. It should still validate any requests on the pod
for a specific UID and fail early if possible. However, if the `RunAsUser` is not set on the pod for a specific UID and fail early if possible. However, if the `RunAsUser` is not set on the pod
it should still admit the pod and allow the Kubelet to ensure that the image does not run as it should still admit the pod and allow the Kubelet to ensure that the image does not run as
root with the existing non-root checks. `root` with the existing non-root checks.