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Multi Party Authorization

This module enables multi-party authorization for any sansshell command. Approval data is stored in-memory in sansshell-server.

User flow

MPA must be explicitly requested. When requested, the MPA flow will be used regardless of whether a policy would allow a command without MPA.

  1. A user issues commands.

    $ sanssh -mpa -targets=1.2.3.4 -justification emergency exec run /bin/echo hi
Waiting for multi-party approval on all targets, ask an approver to run:
  sanssh --targets 1.2.3.4 mpa approve 244407fc-6b9b338a-db0760b8

  2. The approver views the commands and approves it.

    $ sanssh -targets=1.2.3.4 mpa list
244407fc-6b9b338a-db0760b8 /Exec.Exec/Run from sanssh for emergency
$ sanssh -targets=1.2.3.4 mpa get 244407fc-6b9b338a-db0760b8
{
"action": {
   "user": "sanssh",
   "justification": "emergency",
   "method": "/Exec.Exec/Run",
   "message": {
      "@type": "type.googleapis.com/Exec.ExecRequest",
      "command": "/bin/echo",
      "args": [
      "hi"
      ]
   }
}
}
$ sanssh -targets=1.2.3.4 mpa approve 244407fc-6b9b338a-db0760b8

  3. If the user's command is still running, it will complete. If the user had stopped their command, they can rerun it and the approval will still be valid as long as the command's input remains the same and the sansshell-server still has the approval in memory. Approvals are lost if the server restarts, if the server evicts the approval due to age or staleness, or if a user calls sanssh mpa clear oon the request id.

Enabling MPA

SansShell is built on a principle of "Don't pay for what you don't use". MPA is a more invasive than the typical sansshell module, so it requires updating more places than most modules. The reference sanssh, sansshell-server, and proxy-server binaries implement these changes.

  1. In sanssh, in addition to importing the module to get the mpa subcommand, you should conditionally add interceptors when a -mpa flag is provided to the cli. This will let other sanssh commands use MPA with the -mpa flag and get the user experience mentioned above. There are four interceptors.

    1. Unary interceptor for direct calls
    2. Stream interceptor for direct calls
    3. Unary interceptor for proxied calls
    4. Stream interceptor for proxied calls.

    The interceptors for direct calls get added when dialing out.

    proxy.DialContext(ctx, proxy, targets,
   grpc.WithStreamInterceptor(mpahooks.StreamClientIntercepter),
   grpc.WithUnaryInterceptor(mpahooks.MPAUnaryClientInterceptor))

    The interceptors for proxied calls are added as fields in the *proxy.Conn struct.

    conn.UnaryInterceptors = []proxy.UnaryInterceptor{mpahooks.ProxyClientUnaryInterceptor(state)}
conn.StreamInterceptors = []proxy.StreamInterceptor{mpahooks.ProxyClientStreamInterceptor(state)}

  2. In sansshell-server, import the MPA service and add an authz hook to consult the local datastore for MPA info.

    mpa "github.com/Snowflake-Labs/sansshell/services/mpa/server"
server.WithAuthzHook(mpa.ServerMPAAuthzHook)

  3. If using the proxy-server, add an authz hook to consult the server for MPA info.

    proxy.WithAuthzHook(mpa.ProxyMPAAuthzHook)

    You'll also need to update the server's rego policies to reject requests that unexpectedly set proxied-sansshell-identity metadata if it allows callers other than the proxy to make direct calls. If you fail to do so, a direct caller can manipulate the metadata to approve their own request. For example, the policy below will reject calls if proxied identity information is in the metadata and the caller is something other than a peer with an identity of "proxy".

    package sansshell.authz
default authz = false
authz {
   allow
   not deny
}
deny {
   input.metadata["proxied-sansshell-identity"]
   not input.peer.principal.id = "proxy"
}

  4. Any approvers must be able to call /Mpa.Mpa/Approve and any requestor must be able to call /Mpa.Mpa/Store. It's highly recommended to additionally let potential approvers call /Mpa.Mpa/Get and potential requestors call /Mpa.Mpa/WaitForApproval for better user experiences. /Mpa.Mpa/Clear can be used for cancelling MPA requests.

Approvers will show up in RPCAuthInput. Match on these in the OPA policies.

allow if {
    input.approvers[_].principal.id == 'superuser'
}

Design details

MPA requests and approvals are stored in memory in sansshell-server. The id for a request is generated as a hash of the request information, allowing us to reuse the same id for multiple identical requests across multiple commands or across multiple machines.

To support proxying, there are multiple ways of populating the user identity used in /Mpa.Mpa/Store and /Mpa.Mpa/Approve.

  1. From the proxied-sansshell-identity key in the gRPC metadata, used if the identity is set and the server has been configured to accept a proxied identity. The value of this is a JSON representation of rpcauth.Principal.
  2. From the peer identity of the call, used in all other cases.

Justification information can be provided via a sansshell-justification key in the gRPC metadata, available as a constant at rpcauth.ReqJustKey.

The values in RPCAuthInput are populated by authz hooks that look up a MPA request based on the sansshell-mpa-request-id key in the gRPC metadata. Requests will fail if this refers to an invalid or missing request.

Client-side streaming RPCs that involve more than one streamed message are not supported because it's not possible to evaluate the client's messages prior to the request.

Server-only flow

The server-only flow is the simplest scenario.

sequenceDiagram
    actor approver
    actor client
    client->>+server: /Mpa.Mpa/Store
    server->>server: PolicyCheck / log
    server->>-client: ID for request
    client->>approver: Give request ID
    approver->>+server: /Mpa.Mpa/Approve
    server->>-server: PolicyCheck / log
    client->>server: Make normal call with sansshell-mpa-request-id
    server->>server: PolicyCheck / log

Typical usage will involve /Mpa.Mpa/WaitForApproval so that sanssh can make the call as soon as it has approval.

sequenceDiagram
    actor approver
    actor client
    client->>+server: /Mpa.Mpa/Store
    server->>server: PolicyCheck / log
    server->>-client: ID for request
    client->>+server: /Mpa.Mpa/WaitForApproval
    server->>server: PolicyCheck / log
    client->>approver: Give request ID
    approver->>+server: /Mpa.Mpa/Approve
    server->>-server: PolicyCheck / log
    server->>-client: /Mpa.Mpa/WaitForApproval completes
    client->>server: Make normal call with sansshell-mpa-request-id
    server->>server: PolicyCheck / log

Server + Proxy flow

The proxy flow is a bit more complicated than the server flow because the proxy relies on the server for maintaining state.

sequenceDiagram
    actor approver
    actor client
    client->>+proxy: /Mpa.Mpa/Store
    proxy->>proxy: PolicyCheck / log
    proxy->>+server: /Mpa.Mpa/Store
    server->>server: PolicyCheck / log
    server->>-proxy: ID for request
    proxy->>-client: ID for request
    client->>+proxy: /Mpa.Mpa/WaitForApproval
    proxy->>proxy: PolicyCheck / log
    proxy->>+server: /Mpa.Mpa/WaitForApproval
    server->>server: PolicyCheck / log
    client->>approver: Give request ID
    approver->>+proxy: /Mpa.Mpa/Approve
    proxy->>proxy: PolicyCheck / log
    proxy->>-server: /Mpa.Mpa/Approve
    server->>server: PolicyCheck / log
    server->>-proxy: /Mpa.Mpa/WaitForApproval completes
    proxy->>-client: /Mpa.Mpa/WaitForApproval completes
    client->>+proxy: Make normal call with sansshell-mpa-request-id
    proxy->>+server: /Mpa.Mpa/Get
    server->>server: PolicyCheck / log
    server->>-proxy: Approver info
    proxy->>proxy: PolicyCheck / log
    proxy->>-server: Make normal call

Caveats

  • Due to the complexity of OPA policies, we don't support automatically recognizing that a request requires MPA.
    • If you want to give feedback that an action would succeed with MPA, check out DenialHints
  • We also don't support recognizing in advance whether MPA would let an action succeed.
  • You can easily write policies that allow people to approve actions even if their approval isn't useful
  • All state is stored in-memory in sansshell-server. Any restarts of the server will clear approvals.
  • Be wary of guarding too many actions behind MPA. If it gets used too regularly, humans will quickly get used to blindly approving commands without thinking through whether each command is necessary.