From 57115e923e24105000d202121a8d4cdc3d91076e Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Thu, 10 Feb 2022 14:12:34 -0800
Subject: [PATCH] tailcfg: add start of SSH policy to be sent from control
 plane to nodes

Updates #3802

Change-Id: Iec58f35d445aaa267d0f7e7e2f30c049c1df4c0e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 control/controlclient/map.go |  5 +++
 tailcfg/tailcfg.go           | 82 ++++++++++++++++++++++++++++++++++++
 types/netmap/netmap.go       |  1 +
 3 files changed, 88 insertions(+)

diff --git a/control/controlclient/map.go b/control/controlclient/map.go
index b7ba87957..02308b364 100644
--- a/control/controlclient/map.go
+++ b/control/controlclient/map.go
@@ -39,6 +39,7 @@ type mapSession struct {
 	lastDERPMap            *tailcfg.DERPMap
 	lastUserProfile        map[tailcfg.UserID]tailcfg.UserProfile
 	lastParsedPacketFilter []filter.Match
+	lastSSHPolicy          *tailcfg.SSHPolicy
 	collectServices        bool
 	previousPeers          []*tailcfg.Node // for delta-purposes
 	lastDomain             string
@@ -97,6 +98,9 @@ func (ms *mapSession) netmapForResponse(resp *tailcfg.MapResponse) *netmap.Netwo
 	if c := resp.DNSConfig; c != nil {
 		ms.lastDNSConfig = c
 	}
+	if p := resp.SSHPolicy; p != nil {
+		ms.lastSSHPolicy = p
+	}
 
 	if v, ok := resp.CollectServices.Get(); ok {
 		ms.collectServices = v
@@ -117,6 +121,7 @@ func (ms *mapSession) netmapForResponse(resp *tailcfg.MapResponse) *netmap.Netwo
 		Domain:          ms.lastDomain,
 		DNS:             *ms.lastDNSConfig,
 		PacketFilter:    ms.lastParsedPacketFilter,
+		SSHPolicy:       ms.lastSSHPolicy,
 		CollectServices: ms.collectServices,
 		DERPMap:         ms.lastDERPMap,
 		Debug:           resp.Debug,
diff --git a/tailcfg/tailcfg.go b/tailcfg/tailcfg.go
index 07a1a1e38..da24d94c3 100644
--- a/tailcfg/tailcfg.go
+++ b/tailcfg/tailcfg.go
@@ -1157,6 +1157,10 @@ type MapResponse struct {
 	// sees.
 	Health []string `json:",omitempty"`
 
+	// SSHPolicy, if non-nil, updates the SSH policy for how incoming
+	// SSH connections should be handled.
+	SSHPolicy *SSHPolicy `json:",omitempty"`
+
 	// Debug is normally nil, except for when the control server
 	// is setting debug settings on a node.
 	Debug *Debug `json:",omitempty"`
@@ -1369,3 +1373,81 @@ type SetDNSRequest struct {
 	// Value is the value to add.
 	Value string
 }
+
+// SSHPolicy is the policy for how to handle incoming SSH connections
+// over Tailscale.
+type SSHPolicy struct {
+	// Rules are the rules to process for an incoming SSH
+	// connection. The first matching rule takes its action and
+	// stops processing further rules.
+	Rules []*SSHRule `json:"rules"`
+}
+
+// An SSH rule is a match predicate and associated action for an incoming SSH connection.
+type SSHRule struct {
+	// RuleExpires, if non-nil, is when this rule expires.
+	//
+	// For example, a (principal,sshuser) tuple might be granted
+	// prompt-free SSH access for N minutes, so this rule would be
+	// before a expiration-free rule for the same principal that
+	// required an auth prompt.  This permits the control plane to
+	// be out of the path for already-authorized SSH pairs.
+	//
+	// Once a rule matches, the lifetime of any accepting connection
+	// is subject to the SSHAction.SessionExpires time, if any.
+	RuleExpires *time.Time `json:"ruleExpires,omitempty"`
+
+	// Principals matches an incoming connection. If the connection
+	// matches anything in this list and also matches SSHUsers,
+	// then Action is applied.
+	Principals []*SSHPrincipal `json:"principals"`
+
+	// SSHUsers are the SSH users that this rule matches. It is a
+	// map from either ssh-user|"*" => local-user.  The map must
+	// contain a key for either ssh-user or, as a fallback, "*" to
+	// match anything. If it does, the map entry's value is the
+	// actual user that's logged in.
+	SSHUsers map[string]string `json:"sshUsers"`
+
+	// Action is the outcome to task.
+	// A nil or invalid action means to deny.
+	Action *SSHAction `json:"action"`
+}
+
+// SSHPrincipal is either a particular node or a user on any node.
+// At most one field should be non-zero specified.
+type SSHPrincipal struct {
+	Node      StableNodeID `json:"node,omitempty"`
+	NodeIP    string       `json:"nodeIP,omitempty"`
+	UserLogin string       `json:"userLogin,omitempty"` // email-ish: foo@example.com, bar@github
+
+	// TODO(bradfitz): add StableUserID, once that exists
+}
+
+// SSHAction is how to handle an incoming connection.
+// At most one field should be non-zero.
+type SSHAction struct {
+	// Message, if non-empty, is shown to the user before the
+	// action occurs.
+	Message string `json:"message,omitempty"`
+
+	// Reject, if true, terminates the connection. This action
+	// has higher priority that Accept, if given.
+	// The reason this is exists is primarily so a response
+	// from HoldAndDelegate has a way to stop the poll.
+	Reject bool `json:"reject,omitempty"`
+
+	// Accept, if true, accepts the connection immediately
+	// without further prompts.
+	Accept bool `json:"accept,omitempty"`
+
+	// SesssionExpires, if non-nil, is the time at which this
+	// session should forcefully terminate.
+	SesssionExpires *time.Time `json:"sessionExpires,omitempty"`
+
+	// HoldAndDelegate, if non-empty, is a URL that serves an outcome verdict.
+	// The connection will be accepted and will block until the
+	// provided long-polling URL serves a new SSHAction JSON
+	// value.
+	HoldAndDelegate string `json:"holdAndDelegate,omitempty"`
+}
diff --git a/types/netmap/netmap.go b/types/netmap/netmap.go
index 0507e3eb9..8871eeebb 100644
--- a/types/netmap/netmap.go
+++ b/types/netmap/netmap.go
@@ -39,6 +39,7 @@ type NetworkMap struct {
 	DNS           tailcfg.DNSConfig
 	Hostinfo      tailcfg.Hostinfo
 	PacketFilter  []filter.Match
+	SSHPolicy     *tailcfg.SSHPolicy // or nil, if not enabled/allowed
 
 	// CollectServices reports whether this node's Tailnet has
 	// requested that info about services be included in HostInfo.