ipn/ipnlocal: q-encode (RFC 2047) Tailscale serve header values

Updates #11603

RELNOTE=Tailscale serve headers are now RFC 2047 Q-encoded

Change-Id: I1314b65ecf5d39a5a601676346ec2c334fdef042
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/serve.go

@@ -12,6 +12,7 @@
 	"errors"
 	"fmt"
 	"io"
+	"mime"
 	"net"
 	"net/http"
 	"net/http/httputil"
@@ -25,6 +26,7 @@
 	"sync"
 	"sync/atomic"
 	"time"
+	"unicode/utf8"
 
 	"golang.org/x/net/http2"
 	"tailscale.com/ipn"
@@ -717,12 +719,27 @@ func (b *LocalBackend) addTailscaleIdentityHeaders(r *httputil.ProxyRequest) {
 		// Only currently set for nodes with user identities.
 		return
 	}
-	r.Out.Header.Set("Tailscale-User-Login", user.LoginName)
-	r.Out.Header.Set("Tailscale-User-Name", user.DisplayName)
+	r.Out.Header.Set("Tailscale-User-Login", encTailscaleHeaderValue(user.LoginName))
+	r.Out.Header.Set("Tailscale-User-Name", encTailscaleHeaderValue(user.DisplayName))
 	r.Out.Header.Set("Tailscale-User-Profile-Pic", user.ProfilePicURL)
 	r.Out.Header.Set("Tailscale-Headers-Info", "https://tailscale.com/s/serve-headers")
 }
 
+// encTailscaleHeaderValue cleans or encodes as necessary v, to be suitable in
+// an HTTP header value. See
+// https://github.com/tailscale/tailscale/issues/11603.
+//
+// If v is not a valid UTF-8 string, it returns an empty string.
+// If v is a valid ASCII string, it returns v unmodified.
+// If v is a valid UTF-8 string with non-ASCII characters, it returns a
+// RFC 2047 Q-encoded string.
+func encTailscaleHeaderValue(v string) string {
+	if !utf8.ValidString(v) {
+		return ""
+	}
+	return mime.QEncoding.Encode("utf-8", v)
+}
+
 // serveWebHandler is an http.HandlerFunc that maps incoming requests to the
 // correct *http.
 func (b *LocalBackend) serveWebHandler(w http.ResponseWriter, r *http.Request) {

ipn/ipnlocal/serve_test.go

@@ -823,3 +823,21 @@ func Test_isGRPCContentType(t *testing.T) {
 		}
 	}
 }
+
+func TestEncTailscaleHeaderValue(t *testing.T) {
+	tests := []struct {
+		in   string
+		want string
+	}{
+		{"", ""},
+		{"Alice Smith", "Alice Smith"},
+		{"Bad\xffUTF-8", ""},
+		{"Krūmiņa", "=?utf-8?q?Kr=C5=ABmi=C5=86a?="},
+	}
+	for _, tt := range tests {
+		got := encTailscaleHeaderValue(tt.in)
+		if got != tt.want {
+			t.Errorf("encTailscaleHeaderValue(%q) = %q, want %q", tt.in, got, tt.want)
+		}
+	}
+}