target/arm: Catch illegal-exception-return from EL3 with bad NSE/NS

The architecture requires (R_TYTWB) that an attempt to return from EL3
when SCR_EL3.{NSE,NS} are {1,0} is an illegal exception return. (This
enforces that the CPU can't ever be executing below EL3 with the
NSE,NS bits indicating an invalid security state.)

We were missing this check; add it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230807150618.101357-1-peter.maydell@linaro.org

target/arm/tcg/helper-a64.c

@@ -780,6 +780,15 @@ void HELPER(exception_return)(CPUARMState *env, uint64_t new_pc)
         spsr &= ~PSTATE_SS;
     }
 
+    /*
+     * FEAT_RME forbids return from EL3 with an invalid security state.
+     * We don't need an explicit check for FEAT_RME here because we enforce
+     * in scr_write() that you can't set the NSE bit without it.
+     */
+    if (cur_el == 3 && (env->cp15.scr_el3 & (SCR_NS | SCR_NSE)) == SCR_NSE) {
+        goto illegal_return;
+    }
+
     new_el = el_from_spsr(spsr);
     if (new_el == -1) {
         goto illegal_return;