[Proposal] Allow a Pod Security Policy to managing access to the Flexvolumes

contributors/design-proposals/flex-volumes-drivers-psp.md

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+# Allow Pod Security Policy to manage access to the Flexvolumes
+
+## Current state
+
+Cluster admins can control the usage of specific volume types by using Pod
+Security Policy (PSP). Admins can allow the use of Flexvolumes by listing the
+`flexVolume` type in the `volumes` field. The only thing that can be managed is
+allowance or disallowance of Flexvolumes.
+
+Technically, Flexvolumes are implemented as vendor drivers. They are executable
+files that must be placed on every node at
+`/usr/libexec/kubernetes/kubelet-plugins/volume/exec/<vendor~driver>/<driver>`.
+In most cases they are scripts. Limiting driver access means not only limiting
+an access to the volumes that this driver can provide, but also managing access
+to executing a driver’s code (that is arbitrary, in fact).
+
+It is possible to have many flex drivers for the different storage types. In
+essence, Flexvolumes represent not a single volume type, but the different
+types that allow usage of various vendor volumes.
+
+## Desired state
+
+In order to further improve security and to provide more granular control for
+the usage of the different Flexvolumes, we need to enhance PSP. When such a
+change takes place, cluster admins will be able to grant access to any
+Flexvolumes of a particular driver (in contrast to any volume of all drivers).
+
+For example, if we have two drivers for Flexvolumes (`cifs` and
+`digitalocean`), it will become possible to grant access for one group to use
+only volumes from DigitalOcean and grant access for another group to use
+volumes from all Flexvolumes.
+
+## Proposed changes
+
+It has been suggested to add a whitelist of allowed Flexvolume drivers to the
+PSP. It should behave similar to [the existing
+`allowedHostPaths`](https://github.com/kubernetes/kubernetes/pull/50212) except
+that:
+
+1) comparison of equality will be used instead of comparison of prefixes.
+2) Flexvolume’s driver field will be inspected rather than `hostPath`’s path field.
+
+### PodSecurityPolicy modifications
+
+```go
+// PodSecurityPolicySpec defines the policy enforced.
+type PodSecurityPolicySpec struct {
+    ...
+    // AllowedFlexVolumes is a whitelist of allowed Flexvolumes.  Empty or nil indicates that all
+    // Flexvolumes may be used.  This parameter is effective only when the usage of the Flexvolumes
+    // is allowed in the "Volumes" field.
+    // +optional
+    AllowedFlexVolumes []AllowedFlexVolume
+}
+
+// AllowedFlexVolume represents a single Flexvolume that is allowed to be used.
+type AllowedFlexVolume struct {
+       // Driver is the name of the Flexvolume driver.
+       Driver string
+}
+```
+
+Empty `AllowedFlexVolumes` allows usage of Flexvolumes with any driver. It must
+behave as before and provide backward compatibility.
+
+Non-empty `AllowedFlexVolumes` changes the behavior from "all allowed" to "all
+disallowed except those that are explicitly listed here".
+
+### Admission controller modifications
+
+Admission controller should be updated accordingly to inspect a Pod's volumes.
+If it finds a `flexVolume`, it should ensure that its driver is allowed to be
+used.
+
+### Validation rules
+
+Flexvolume driver names must be non-empty.
+
+If a PSP disallows to pods to request volumes of type `flexVolume` then
+`AllowedFlexVolumes` must be empty. In case it is not empty, API server must
+report an error.
+
+API server should allow granting an access to Flexvolumes that do not exist at
+time of PSP creation.
+
+## Notes
+It is possible to have even more flexible control over the Flexvolumes and take
+into account options that have been passed to a driver. We decided that this is
+a desirable feature but outside the scope of this proposal.
+
+The current change could be enough for many cases. Also, when cluster admins
+are able to manage access to particular Flexvolume drivers, it becomes possible
+to "emulate" control over the driver’s options by using many drivers with
+hard-coded options.