session/variable: forbid changing @@global.require_secure_transport t…

…o 'on' with SEM enabled (#47677) (#51207)

close #47665

pkg/sessionctx/sessionstates/BUILD.bazel

@@ -31,7 +31,7 @@ go_test(
     ],
     embed = [":sessionstates"],
     flaky = True,
-    shard_count = 15,
+    shard_count = 16,
     deps = [
         "//pkg/config",
         "//pkg/errno",

pkg/sessionctx/sessionstates/session_states_test.go

@@ -16,6 +16,7 @@ package sessionstates_test
 
 import (
 	"context"
+	"crypto/tls"
 	"encoding/binary"
 	"fmt"
 	"strconv"
@@ -336,6 +337,18 @@ func TestInvisibleVars(t *testing.T) {
 	}
 }
 
+func TestIssue47665(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.Session().GetSessionVars().TLSConnectionState = &tls.ConnectionState{} // unrelated mock for the test.
+	originSEM := config.GetGlobalConfig().Security.EnableSEM
+	config.GetGlobalConfig().Security.EnableSEM = true
+	tk.MustGetErrMsg("set @@global.require_secure_transport = on", "require_secure_transport can not be set to ON with SEM(security enhanced mode) enabled")
+	config.GetGlobalConfig().Security.EnableSEM = originSEM
+	tk.MustExec("set @@global.require_secure_transport = on")
+	tk.MustExec("set @@global.require_secure_transport = off") // recover to default value
+}
+
 func TestSessionCtx(t *testing.T) {
 	store := testkit.CreateMockStore(t)
 	tk := testkit.NewTestKit(t, store)

pkg/sessionctx/variable/sysvar.go

@@ -1118,6 +1118,14 @@ var defaultSysVars = []*SysVar{
 			return nil
 		}, Validation: func(vars *SessionVars, normalizedValue string, originalValue string, scope ScopeFlag) (string, error) {
 			if vars.StmtCtx.StmtType == "Set" && TiDBOptOn(normalizedValue) {
+				// On tidbcloud dedicated cluster with the default configuration, if an user modify
+				// @@global.require_secure_transport=on, he can not login the cluster anymore!
+				// A workaround for this is making require_secure_transport read-only for that case.
+				// SEM(security enhanced mode) is enabled by default with only that settings.
+				cfg := config.GetGlobalConfig()
+				if cfg.Security.EnableSEM {
+					return "", errors.New("require_secure_transport can not be set to ON with SEM(security enhanced mode) enabled")
+				}
 				// Refuse to set RequireSecureTransport to ON if the connection
 				// issuing the change is not secure. This helps reduce the chance of users being locked out.
 				if vars.TLSConnectionState == nil {