tags: process tags on registration, simplify policy (#2931)

This PR investigates, adds tests and aims to correctly implement Tailscale's model for how Tags should be accepted, assigned and used to identify nodes in the Tailscale access and ownership model.

When evaluating in Headscale's policy, Tags are now only checked against a nodes "tags" list, which defines the source of truth for all tags for a given node. This simplifies the code for dealing with tags greatly, and should help us have less access bugs related to nodes belonging to tags or users.

A node can either be owned by a user, or a tag.

Next, to ensure the tags list on the node is correctly implemented, we first add tests for every registration scenario and combination of user, pre auth key and pre auth key with tags with the same registration expectation as observed by trying them all with the Tailscale control server. This should ensure that we implement the correct behaviour and that it does not change or break over time.

Lastly, the missing parts of the auth has been added, or changed in the cases where it was wrong. This has in large parts allowed us to delete and simplify a lot of code.
Now, tags can only be changed when a node authenticates or if set via the CLI/API. Tags can only be fully overwritten/replaced and any use of either auth or CLI will replace the current set if different.

A user owned device can be converted to a tagged device, but it cannot be changed back. A tagged device can never remove the last tag either, it has to have a minimum of one.

hscontrol/state/state.go

@@ -12,6 +12,7 @@ import (
 	"net/netip"
 	"os"
 	"slices"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -669,12 +670,27 @@ func (s *State) SetNodeTags(nodeID types.NodeID, tags []string) (types.NodeView,
 		return types.NodeView{}, change.EmptySet, fmt.Errorf("%w: %d", ErrNodeNotFound, nodeID)
 	}
 
-	// Validate tags against policy
-	validatedTags, err := s.validateAndNormalizeTags(existingNode.AsStruct(), tags)
-	if err != nil {
-		return types.NodeView{}, change.EmptySet, err
+	// Validate tags: must have correct format and exist in policy
+	validatedTags := make([]string, 0, len(tags))
+	invalidTags := make([]string, 0)
+
+	for _, tag := range tags {
+		if !strings.HasPrefix(tag, "tag:") || !s.polMan.TagExists(tag) {
+			invalidTags = append(invalidTags, tag)
+
+			continue
+		}
+
+		validatedTags = append(validatedTags, tag)
 	}
 
+	if len(invalidTags) > 0 {
+		return types.NodeView{}, change.EmptySet, fmt.Errorf("%w %v are invalid or not permitted", ErrRequestedTagsInvalidOrNotPermitted, invalidTags)
+	}
+
+	slices.Sort(validatedTags)
+	validatedTags = slices.Compact(validatedTags)
+
 	// Log the operation
 	logTagOperation(existingNode, validatedTags)
 
@@ -1128,6 +1144,41 @@ func (s *State) createAndSaveNewNode(params newNodeParams) (types.NodeView, erro
 		nodeToRegister.Tags = nil
 	}
 
+	// Reject advertise-tags for PreAuthKey registrations early, before any resource allocation.
+	// PreAuthKey nodes get their tags from the key itself, not from client requests.
+	if params.PreAuthKey != nil && params.Hostinfo != nil && len(params.Hostinfo.RequestTags) > 0 {
+		return types.NodeView{}, fmt.Errorf("%w %v are invalid or not permitted", ErrRequestedTagsInvalidOrNotPermitted, params.Hostinfo.RequestTags)
+	}
+
+	// Process RequestTags (from tailscale up --advertise-tags) ONLY for non-PreAuthKey registrations.
+	// Validate early before IP allocation to avoid resource leaks on failure.
+	if params.PreAuthKey == nil && params.Hostinfo != nil && len(params.Hostinfo.RequestTags) > 0 {
+		var approvedTags, rejectedTags []string
+
+		for _, tag := range params.Hostinfo.RequestTags {
+			if s.polMan.NodeCanHaveTag(nodeToRegister.View(), tag) {
+				approvedTags = append(approvedTags, tag)
+			} else {
+				rejectedTags = append(rejectedTags, tag)
+			}
+		}
+
+		// Reject registration if any requested tags are unauthorized
+		if len(rejectedTags) > 0 {
+			return types.NodeView{}, fmt.Errorf("%w %v are invalid or not permitted", ErrRequestedTagsInvalidOrNotPermitted, rejectedTags)
+		}
+
+		if len(approvedTags) > 0 {
+			nodeToRegister.Tags = approvedTags
+			slices.Sort(nodeToRegister.Tags)
+			nodeToRegister.Tags = slices.Compact(nodeToRegister.Tags)
+			log.Info().
+				Str("node.name", nodeToRegister.Hostname).
+				Strs("tags", nodeToRegister.Tags).
+				Msg("approved advertise-tags during registration")
+		}
+	}
+
 	// Validate before saving
 	err := validateNodeOwnership(&nodeToRegister)
 	if err != nil {