Fix race between main thread and a vCPU thread (#102)

* afl-bridge: fix race between main thread and a vCPU thread

In some cases qemu_main_loop() can exit before libafl_sync_exit_cpu()
completes. This will case race between Rust code that restarts QEMU
and vCPU thread that updates last_exit_reason. What I observed is

libafl_exit_signal_vm_start() from a new iteration cleared
last_exit_reason.cpu before libafl_sync_exit_cpu() tried to
access *last_exit_reason.cpu. This caused NULL pointer dereference.

Fix this by not setting cpu->exit in prepare_qemu_exit() and updating
it only in rr_cpu_thread_fn() and MTTCG counterpart. This will ensure
that qemu_main_loop() waits for vCPU thread to actually stop before
returning control to the Rust code.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>

---------

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Co-authored-by: Romain Malmain <romain.malmain@pm.me>

accel/tcg/tcg-accel-ops-mttcg.c

@@ -56,6 +56,12 @@ static void mttcg_force_rcu(Notifier *notify, void *data)
     async_run_on_cpu(cpu, do_nothing, RUN_ON_CPU_NULL);
 }
 
+//// --- Begin LibAFL code ---
+
+#include "libafl/exit.h"
+
+//// --- End LibAFL code ---
+
 /*
  * In the multi-threaded case each vCPU has its own thread. The TLS
  * variable current_cpu can be used deep in the code to find the
@@ -104,6 +110,11 @@ static void *mttcg_cpu_thread_fn(void *arg)
                  * reset by another thread by the time we arrive here.
                  */
                 break;
+//// --- Begin LibAFL code ---
+            case EXCP_LIBAFL_EXIT:
+                cpu->stopped = true;
+                break;
+//// --- End LibAFL code ---
             case EXCP_ATOMIC:
                 bql_unlock();
                 cpu_exec_step_atomic(cpu);

accel/tcg/tcg-accel-ops-rr.c

@@ -169,6 +169,12 @@ static int rr_cpu_count(void)
     return cpu_count;
 }
 
+//// --- Begin LibAFL code ---
+
+#include "libafl/exit.h"
+
+//// --- End LibAFL code ---
+
 /*
  * In the single-threaded case each vCPU is simulated in turn. If
  * there is more than a single vCPU we create a simple timer to kick
@@ -273,6 +279,12 @@ static void *rr_cpu_thread_fn(void *arg)
                     bql_lock();
                     break;
                 }
+//// --- Begin LibAFL code ---
+                else if (r == EXCP_LIBAFL_EXIT) {
+                    cpu->stopped = true;
+                    break;
+                }
+//// --- End LibAFL code ---
             } else if (cpu->stop) {
                 if (cpu->unplug) {
                     cpu = CPU_NEXT(cpu);

libafl/exit.c

@@ -78,7 +78,6 @@ static void prepare_qemu_exit(CPUState* cpu, target_ulong next_pc)
 
 #ifndef CONFIG_USER_ONLY
     qemu_system_debug_request();
-    cpu->stopped = true; // TODO check if still needed
 #endif
 
     // in usermode, this may be called from the syscall hook, thus already out