cpu-exec: fix icount out-of-bounds access

When icount is active, tb_add_jump is surprisingly called with an
out of bounds basic block index.  I have no idea how that can work,
but it does not seem like a good idea.  Clear *last_tb for all
TB_EXIT_ICOUNT_EXPIRED cases, even when all you have to do is
refill icount_extra.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 43d70ddf)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

cpu-exec.c

@@ -542,7 +542,7 @@ static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
 
     trace_exec_tb(tb, tb->pc);
     ret = cpu_tb_exec(cpu, tb);
-    *last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
+    tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
     *tb_exit = ret & TB_EXIT_MASK;
     switch (*tb_exit) {
     case TB_EXIT_REQUESTED:
@@ -566,6 +566,7 @@ static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
         abort();
 #else
         int insns_left = cpu->icount_decr.u32;
+        *last_tb = NULL;
         if (cpu->icount_extra && insns_left >= 0) {
             /* Refill decrementer and continue execution.  */
             cpu->icount_extra += insns_left;
@@ -575,17 +576,17 @@ static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
         } else {
             if (insns_left > 0) {
                 /* Execute remaining instructions.  */
-                cpu_exec_nocache(cpu, insns_left, *last_tb, false);
+                cpu_exec_nocache(cpu, insns_left, tb, false);
                 align_clocks(sc, cpu);
             }
             cpu->exception_index = EXCP_INTERRUPT;
-            *last_tb = NULL;
             cpu_loop_exit(cpu);
         }
         break;
 #endif
     }
     default:
+        *last_tb = tb;
         break;
     }
 }

include/exec/exec-all.h

@@ -320,6 +320,7 @@ static inline void tb_set_jmp_target(TranslationBlock *tb,
 static inline void tb_add_jump(TranslationBlock *tb, int n,
                                TranslationBlock *tb_next)
 {
+    assert(n < ARRAY_SIZE(tb->jmp_list_next));
     if (tb->jmp_list_next[n]) {
         /* Another thread has already done this while we were
          * outside of the lock; nothing to do in this case */