Add design doc for Custom CA support for S3 BSLs and Velero Installat…

…ion (#2259)

* Add design doc for Custom CA support for S3

Signed-off-by: Dylan Murray <dymurray@redhat.com>

design/custom-ca-support.md

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+# Custom CA Bundle Support for S3 Object Storage
+
+It is desired that Velero performs SSL verification on the Object Storage
+endpoint (BackupStorageLocation), but it is not guaranteed that the Velero
+container has the endpoints' CA bundle in it's system store. Velero needs to
+support the ability for a user to specify custom CA bundles at installation
+time and Velero needs to support a mechanism in the BackupStorageLocation
+Custom Resource to allow a user to specify a custom CA bundle. This mechanism
+needs to also allow Restic to access and use this custom CA bundle.
+
+## Goals
+
+- Enable Velero to be configured with a custom CA bundle at installation
+- Enable Velero support for custom CA bundles with S3 API BackupStorageLocations
+- Enable Restic to use the custom CA bundles whether it is configured at installation time or on the BackupStorageLocation
+- Enable Velero client to take a CA bundle as an argument
+
+## Non Goals
+
+- Support non-S3 providers
+
+## Background
+
+Currently, in order for Velero to perform SSL verification of the object
+storage endpoint the user must manually set the `AWS_CA_BUNDLE` environment
+variable on the Velero deployment. If the user is using Restic, the user has to
+either:
+1. Add the certs to the Restic container's system store
+1. Modify Velero to pass in the certs as a CLI parameter to Restic - requiring
+   a custom Velero deployment
+
+## High-Level Design
+
+There are really 2 methods of using Velero with custom certificates:
+1. Including a custom certificate at Velero installation
+1. Specifying a custom certificate to be used with a `BackupStorageLocation`
+
+### Specifying a custom cert at installation
+
+On the Velero deployment at install time, we can set the AWS environment variable
+`AWS_CA_BUNDLE` which will allow Velero to communicate over https with the
+proper certs when communicating with the S3 bucket. This means we will add the
+ability to specify a custom CA bundle at installation time. For more
+information, see "Install Command Changes".
+
+On the Restic daemonset, we will want to also mount this secret at a pre-defined
+location. In the `restic` pkg, the command to invoke restic will need to be
+updated to pass the path to the cert file that is mounted if it is specified in
+the config.
+
+This is good, but doesn't allow us to specify different certs when
+`BackupStorageLocation` resources are created.
+
+### Specifying a custom cert on BSL
+
+In order to support custom certs for object storage, Velero will add an
+additional field to the `BackupStorageLocation`'s provider `Config` resource to
+provide a secretRef which will contain the coordinates to a secret containing
+the relevant cert file for object storage. 
+
+In order for Restic to be able to consume and use this cert, Velero will need
+the ability to write the CA bundle somewhere in memory for the Restic pod to
+consume it.
+
+To accomplish this, we can look at the code for managing restic repository
+credentials. The way this works today is that the key is stored in a secret in
+the Velero namespace, and each time Velero executes a restic command, the
+contents of the secret are read and written out to a temp file. The path to
+this file is then passed to restic and removed afterwards. pass the path of the
+temp file to restic, and then remove the temp file afterwards. See ref #1 and #2.
+
+This same approach can be taken for CA bundles. The bundle can be stored in a
+secret which is referenced on the BSL and written to a temp file prior to
+invoking Restic.
+
+[1](https://github.com/vmware-tanzu/velero/blob/master/pkg/restic/repository_manager.go#L238-L245)
+[2](https://github.com/vmware-tanzu/velero/blob/master/pkg/restic/common.go#L168-L203)
+
+## Detailed Design
+
+The `AWS_CA_BUNDLE` environment variable works for the Velero deployment
+because this environment variable is passed into the AWS SDK which is used in
+the [plugin][1] to build up the config object. This means that a user can
+simply define the CA bundle in the deployment as an env var. This can be
+utilized for the installation of Velero with a custom cert by simply setting
+this env var to the contents of the CA bundle, or the env var can be mapped to
+a secret which is controlled at installation time. I recommend using a secret
+as it makes the Restic integration easier as well.
+
+At installation time, if a user has specified a custom cert then the Restic
+daemonset should be updated to include the secret mounted at a predefined path.
+We could optionally use the system store for all custom certs added at
+installation time. Restic supports using the custom certs [in addition][3] to
+the root certs.
+
+In the case of the BSL being created with a secret reference, then at runtime
+the secret will need to be consumed. This secret will be read and applied to
+the AWS `session` object. The `getSession()` function will need to be updated
+to take in the custom CA bundle so it can be passed [here][4].
+
+The Restic controller will need to be updated to write the contents of the CA
+bundle secret out to a temporary file inside of the restic pod.The restic
+[command invocation][2] will need to be updated to include the path to the file
+as an argument to the restic server using `--cacert`. For the path when a user
+defines a custom cert on the BSL, Velero will be responsible for updating the
+daemonset to include the secret mounted as a volume at a predefined path.
+
+Where we mount the secret is a fine detail, but I recommend mounting the certs
+to `/certs` to keep it in line with the other volume mount paths being used.
+
+### Install command changes
+
+The installation flags should be updated to include the ability to pass in a
+cert file. Then the install command would do the heavy lifting of creating a
+secret and updating the proper fields on the deployment and daemonset to mount
+the secret at a well defined path.
+
+### Velero client changes
+
+Since the Velero client is responsible for gathering logs and information about
+the Object Storage, this implementation should include a new flag `--cacert`
+which can be used when communicating with the Object Storage. Additionally, the
+user should be able to set this in their client configuration. The command
+would look like:
+```
+$ velero client config set cacert PATH
+```
+
+[1]: https://github.com/vmware-tanzu/velero-plugin-for-aws/blob/master/velero-plugin-for-aws/object_store.go#L135
+[2]: https://github.com/vmware-tanzu/velero/blob/master/pkg/restic/command.go#L47
+[3]: https://github.com/restic/restic/blob/master/internal/backend/http_transport.go#L81
+[4]: https://github.com/vmware-tanzu/velero-plugin-for-aws/blob/master/velero-plugin-for-aws/object_store.go#L154