target/arm: Deliver BKPT/BRK exceptions to correct exception level

Most Arm architectural debug exceptions (eg watchpoints) are ignored
if the configured "debug exception level" is below the current
exception level (so for example EL1 can't arrange to get debug exceptions
for EL2 execution). Exceptions generated by the BRK or BPKT instructions
are a special case -- they must always cause an exception, so if
we're executing above the debug exception level then we
must take them to the current exception level.

This fixes a bug where executing BRK at EL2 could result in an
exception being taken at EL1 (which is strictly forbidden by the
architecture).

Fixes: https://bugs.launchpad.net/qemu/+bug/1838277Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190730132522.27086-1-peter.maydell@linaro.org

target/arm/op_helper.c

@@ -370,6 +370,9 @@ void HELPER(exception_with_syndrome)(CPUARMState *env, uint32_t excp,
  */
 void HELPER(exception_bkpt_insn)(CPUARMState *env, uint32_t syndrome)
 {
+    int debug_el = arm_debug_target_el(env);
+    int cur_el = arm_current_el(env);
+
     /* FSR will only be used if the debug target EL is AArch32. */
     env->exception.fsr = arm_debug_exception_fsr(env);
     /* FAR is UNKNOWN: clear vaddress to avoid potentially exposing
@@ -377,7 +380,18 @@ void HELPER(exception_bkpt_insn)(CPUARMState *env, uint32_t syndrome)
      * exception/security level.
      */
     env->exception.vaddress = 0;
-    raise_exception(env, EXCP_BKPT, syndrome, arm_debug_target_el(env));
+    /*
+     * Other kinds of architectural debug exception are ignored if
+     * they target an exception level below the current one (in QEMU
+     * this is checked by arm_generate_debug_exceptions()). Breakpoint
+     * instructions are special because they always generate an exception
+     * to somewhere: if they can't go to the configured debug exception
+     * level they are taken to the current exception level.
+     */
+    if (debug_el < cur_el) {
+        debug_el = cur_el;
+    }
+    raise_exception(env, EXCP_BKPT, syndrome, debug_el);
 }
 
 uint32_t HELPER(cpsr_read)(CPUARMState *env)