client/web: fix CSRF handler order in web UI (#15143)

Fix the order of the CSRF handlers (HTTP plaintext context setting,
_then_ enforcement) in the construction of the web UI server. This
resolves false-positive "invalid Origin" 403 exceptions when attempting
to update settings in the web UI.

Add unit test to exercise the CSRF protection failure and success cases
for our web UI configuration.

Updates #14822
Updates #14872

Signed-off-by: Patrick O'Doherty <patrick@tailscale.com>

client/web/web_test.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"net/http/cookiejar"
 	"net/http/httptest"
 	"net/netip"
 	"net/url"
@@ -20,6 +21,7 @@ import (
 	"time"
 
 	"github.com/google/go-cmp/cmp"
+	"github.com/gorilla/csrf"
 	"tailscale.com/client/local"
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/ipn"
@@ -1477,3 +1479,83 @@ func mockWaitAuthURL(_ context.Context, id string, src tailcfg.NodeID) (*tailcfg
 		return nil, errors.New("unknown id")
 	}
 }
+
+func TestCSRFProtect(t *testing.T) {
+	s := &Server{}
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("GET /test/csrf-token", func(w http.ResponseWriter, r *http.Request) {
+		token := csrf.Token(r)
+		_, err := io.WriteString(w, token)
+		if err != nil {
+			t.Fatal(err)
+		}
+	})
+	mux.HandleFunc("POST /test/csrf-protected", func(w http.ResponseWriter, r *http.Request) {
+		_, err := io.WriteString(w, "ok")
+		if err != nil {
+			t.Fatal(err)
+		}
+	})
+	h := s.withCSRF(mux)
+	ser := httptest.NewServer(h)
+	defer ser.Close()
+
+	jar, err := cookiejar.New(nil)
+	if err != nil {
+		t.Fatalf("unable to construct cookie jar: %v", err)
+	}
+
+	client := ser.Client()
+	client.Jar = jar
+
+	// make GET request to populate cookie jar
+	resp, err := client.Get(ser.URL + "/test/csrf-token")
+	if err != nil {
+		t.Fatalf("unable to make request: %v", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("unexpected status: %v", resp.Status)
+	}
+	tokenBytes, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatalf("unable to read body: %v", err)
+	}
+
+	csrfToken := strings.TrimSpace(string(tokenBytes))
+	if csrfToken == "" {
+		t.Fatal("empty csrf token")
+	}
+
+	// make a POST request without the CSRF header; ensure it fails
+	resp, err = client.Post(ser.URL+"/test/csrf-protected", "text/plain", nil)
+	if err != nil {
+		t.Fatalf("unable to make request: %v", err)
+	}
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("unexpected status: %v", resp.Status)
+	}
+
+	// make a POST request with the CSRF header; ensure it succeeds
+	req, err := http.NewRequest("POST", ser.URL+"/test/csrf-protected", nil)
+	if err != nil {
+		t.Fatalf("error building request: %v", err)
+	}
+	req.Header.Set("X-CSRF-Token", csrfToken)
+	resp, err = client.Do(req)
+	if err != nil {
+		t.Fatalf("unable to make request: %v", err)
+	}
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("unexpected status: %v", resp.Status)
+	}
+	defer resp.Body.Close()
+	out, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatalf("unable to read body: %v", err)
+	}
+	if string(out) != "ok" {
+		t.Fatalf("unexpected body: %q", out)
+	}
+}