hw/intc/arm_gic: Allow to use QTest without crashing

Alexander reported an issue in gic_get_current_cpu() using the
fuzzer. Yet another "deref current_cpu with QTest" bug, reproducible
doing:

  $ echo readb 0xf03ff000 | qemu-system-arm -M npcm750-evb,accel=qtest -qtest stdio
  [I 1611849440.651452] OPENED
  [R +0.242498] readb 0xf03ff000
  hw/intc/arm_gic.c:63:29: runtime error: member access within null pointer of type 'CPUState' (aka 'struct CPUState')
  SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior hw/intc/arm_gic.c:63:29 in
  AddressSanitizer:DEADLYSIGNAL
  =================================================================
  ==3719691==ERROR: AddressSanitizer: SEGV on unknown address 0x0000000082a0 (pc 0x5618790ac882 bp 0x7ffca946f4f0 sp 0x7ffca946f4a0 T0)
  ==3719691==The signal is caused by a READ memory access.
      #0 0x5618790ac882 in gic_get_current_cpu hw/intc/arm_gic.c:63:29
      #1 0x5618790a8901 in gic_dist_readb hw/intc/arm_gic.c:955:11
      #2 0x5618790a7489 in gic_dist_read hw/intc/arm_gic.c:1158:17
      #3 0x56187adc573b in memory_region_read_with_attrs_accessor softmmu/memory.c:464:9
      #4 0x56187ad7903a in access_with_adjusted_size softmmu/memory.c:552:18
      #5 0x56187ad766d6 in memory_region_dispatch_read1 softmmu/memory.c:1426:16
      #6 0x56187ad758a8 in memory_region_dispatch_read softmmu/memory.c:1449:9
      #7 0x56187b09e84c in flatview_read_continue softmmu/physmem.c:2822:23
      #8 0x56187b0a0115 in flatview_read softmmu/physmem.c:2862:12
      #9 0x56187b09fc9e in address_space_read_full softmmu/physmem.c:2875:18
      #10 0x56187aa88633 in address_space_read include/exec/memory.h:2489:18
      #11 0x56187aa88633 in qtest_process_command softmmu/qtest.c:558:13
      #12 0x56187aa81881 in qtest_process_inbuf softmmu/qtest.c:797:9
      #13 0x56187aa80e02 in qtest_read softmmu/qtest.c:809:5

current_cpu is NULL because QTest accelerator does not use CPU.

Fix by skipping the check and returning the first CPU index when
QTest accelerator is used, similarly to commit c781a2c
("hw/i386/vmport: Allow QTest use without crashing").

Reported-by: Alexander Bulekov <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Darren Kenny <[email protected]>
Reviewed-by: Alexander Bulekov <[email protected]>
Message-id: [email protected]
Signed-off-by: Peter Maydell <[email protected]>

hw/intc/arm_gic.c

@@ -28,6 +28,7 @@
 #include "qemu/module.h"
 #include "trace.h"
 #include "sysemu/kvm.h"
+#include "sysemu/qtest.h"
 
 /* #define DEBUG_GIC */
 
@@ -57,7 +58,7 @@ static const uint8_t gic_id_gicv2[] = {
 
 static inline int gic_get_current_cpu(GICState *s)
 {
-    if (s->num_cpu > 1) {
+    if (!qtest_enabled() && s->num_cpu > 1) {
         return current_cpu->cpu_index;
     }
     return 0;