plugin/acl: Fix autopath from bypassing acl checks (#8290)

This fix prevents CoreDNS `autopath` from bypassing `acl` checks
when it rewrites an allowed query into a name within an ACL-protected zone.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

test/acl_autopath_test.go

@@ -0,0 +1,70 @@
+package test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestACLAuthorizesAutopathExpandedName(t *testing.T) {
+	resolvConf := filepath.Join(t.TempDir(), "resolv.conf")
+	if err := os.WriteFile(
+		resolvConf,
+		[]byte("search public.example protected.example\n"),
+		0644,
+	); err != nil {
+		t.Fatalf("Could not write resolv.conf: %s", err)
+	}
+
+	corefile := `.:0 {
+		acl protected.example. {
+			block
+		}
+		autopath ` + resolvConf + `
+		hosts {
+			192.0.2.123 secret.protected.example
+		}
+	}`
+
+	server, udp, _, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer server.Stop()
+
+	tests := []struct {
+		name  string
+		qname string
+	}{
+		{
+			name:  "direct protected query",
+			qname: "secret.protected.example.",
+		},
+		{
+			name:  "autopath expanded protected query",
+			qname: "secret.public.example.",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			m := new(dns.Msg)
+			m.SetQuestion(tc.qname, dns.TypeA)
+
+			r, err := dns.Exchange(m, udp)
+			if err != nil {
+				t.Fatalf("Query %q failed: %s", tc.qname, err)
+			}
+			if r.Rcode != dns.RcodeRefused {
+				t.Fatalf(
+					"Query %q: got rcode %s, want REFUSED; answers: %v",
+					tc.qname,
+					dns.RcodeToString[r.Rcode],
+					r.Answer,
+				)
+			}
+		})
+	}
+}