ipn/ipnlocal: add capability for debugging peers over peerapi

The default is still users can debug their own nodes. But like
cd916b728b did, this adds support for admins to grant additional
capabilities with the new tailcfg.CapabilityDebugPeer cap.

Updates #4217

Change-Id: Ifce3d9a1f8e8845797970a4f97b393194663d35f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/peerapi.go

@@ -622,23 +622,21 @@ func (f *incomingFile) PartialFile() ipn.PartialFile {
 
 // canPutFile reports whether h can put a file ("Taildrop") to this node.
 func (h *peerAPIHandler) canPutFile() bool {
-	if h.isSelf {
-		return true
-	}
-	if h.peerNode == nil {
-		// Shouldn't happen, but in case.
-		return false
+	return h.isSelf || h.peerHasCap(tailcfg.CapabilityFileSharingSend)
 }
-	for _, addr := range h.peerNode.Addresses {
-		if !addr.IsSingleIP() {
-			continue
+
+// canDebug reports whether h can debug this node (goroutines, metrics,
+// magicsock internal state, etc).
+func (h *peerAPIHandler) canDebug() bool {
+	return h.isSelf || h.peerHasCap(tailcfg.CapabilityDebugPeer)
 }
-		for _, cap := range h.ps.b.PeerCaps(addr.IP()) {
-			if cap == tailcfg.CapabilityFileSharingSend {
+
+func (h *peerAPIHandler) peerHasCap(wantCap string) bool {
+	for _, hasCap := range h.ps.b.PeerCaps(h.remoteAddr.IP()) {
+		if hasCap == wantCap {
 			return true
 		}
 	}
-	}
 	return false
 }
 
@@ -763,8 +761,8 @@ func approxSize(n int64) string {
 }
 
 func (h *peerAPIHandler) handleServeGoroutines(w http.ResponseWriter, r *http.Request) {
-	if !h.isSelf {
-		http.Error(w, "not owner", http.StatusForbidden)
+	if !h.canDebug() {
+		http.Error(w, "denied; no debug access", http.StatusForbidden)
 		return
 	}
 	var buf []byte
@@ -779,8 +777,8 @@ func (h *peerAPIHandler) handleServeGoroutines(w http.ResponseWriter, r *http.Re
 }
 
 func (h *peerAPIHandler) handleServeEnv(w http.ResponseWriter, r *http.Request) {
-	if !h.isSelf {
-		http.Error(w, "not owner", http.StatusForbidden)
+	if !h.canDebug() {
+		http.Error(w, "denied; no debug access", http.StatusForbidden)
 		return
 	}
 	var data struct {
@@ -799,8 +797,8 @@ func (h *peerAPIHandler) handleServeEnv(w http.ResponseWriter, r *http.Request)
 }
 
 func (h *peerAPIHandler) handleServeMagicsock(w http.ResponseWriter, r *http.Request) {
-	if !h.isSelf {
-		http.Error(w, "not owner", http.StatusForbidden)
+	if !h.canDebug() {
+		http.Error(w, "denied; no debug access", http.StatusForbidden)
 		return
 	}
 	eng := h.ps.b.e
@@ -814,8 +812,8 @@ func (h *peerAPIHandler) handleServeMagicsock(w http.ResponseWriter, r *http.Req
 }
 
 func (h *peerAPIHandler) handleServeMetrics(w http.ResponseWriter, r *http.Request) {
-	if !h.isSelf {
-		http.Error(w, "not owner", http.StatusForbidden)
+	if !h.canDebug() {
+		http.Error(w, "denied; no debug access", http.StatusForbidden)
 		return
 	}
 	w.Header().Set("Content-Type", "text/plain")

tailcfg/tailcfg.go

@@ -1587,6 +1587,9 @@ const (
 	// CapabilityFileSharingSend grants the ability to receive files from a
 	// node that's owned by a different user.
 	CapabilityFileSharingSend = "https://tailscale.com/cap/file-send"
+	// CapabilityDebugPeer grants the ability for a peer to read this node's
+	// goroutines, metrics, magicsock internal state, etc.
+	CapabilityDebugPeer = "https://tailscale.com/cap/debug-peer"
 )
 
 // SetDNSRequest is a request to add a DNS record.