exec.c: Don't reallocate IOMMUNotifiers that are in use

The tcg_register_iommu_notifier() code has a GArray of
TCGIOMMUNotifier structs which it has registered by passing
memory_region_register_iommu_notifier() a pointer to the embedded
IOMMUNotifier field. Unfortunately, if we need to enlarge the
array via g_array_set_size() this can cause a realloc(), which
invalidates the pointer that memory_region_register_iommu_notifier()
put into the MemoryRegion's iommu_notify list. This can result
in segfaults.

Switch the GArray to holding pointers to the TCGIOMMUNotifier
structs, so that we can individually allocate and free them.

Cc: qemu-stable@nongnu.org
Fixes: 1f871c5 ("exec.c: Handle IOMMUs in address_space_translate_for_iotlb()")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190128174241.5860-1-peter.maydell@linaro.org
(cherry picked from commit 5601be3)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

exec.c

@@ -664,15 +664,16 @@ static void tcg_register_iommu_notifier(CPUState *cpu,
     int i;
 
     for (i = 0; i < cpu->iommu_notifiers->len; i++) {
-        notifier = &g_array_index(cpu->iommu_notifiers, TCGIOMMUNotifier, i);
+        notifier = g_array_index(cpu->iommu_notifiers, TCGIOMMUNotifier *, i);
         if (notifier->mr == mr && notifier->iommu_idx == iommu_idx) {
             break;
         }
     }
     if (i == cpu->iommu_notifiers->len) {
         /* Not found, add a new entry at the end of the array */
         cpu->iommu_notifiers = g_array_set_size(cpu->iommu_notifiers, i + 1);
-        notifier = &g_array_index(cpu->iommu_notifiers, TCGIOMMUNotifier, i);
+        notifier = g_new0(TCGIOMMUNotifier, 1);
+        g_array_index(cpu->iommu_notifiers, TCGIOMMUNotifier *, i) = notifier;
 
         notifier->mr = mr;
         notifier->iommu_idx = iommu_idx;
@@ -704,8 +705,9 @@ static void tcg_iommu_free_notifier_list(CPUState *cpu)
     TCGIOMMUNotifier *notifier;
 
     for (i = 0; i < cpu->iommu_notifiers->len; i++) {
-        notifier = &g_array_index(cpu->iommu_notifiers, TCGIOMMUNotifier, i);
+        notifier = g_array_index(cpu->iommu_notifiers, TCGIOMMUNotifier *, i);
         memory_region_unregister_iommu_notifier(notifier->mr, &notifier->n);
+        g_free(notifier);
     }
     g_array_free(cpu->iommu_notifiers, true);
 }
@@ -975,7 +977,7 @@ void cpu_exec_realizefn(CPUState *cpu, Error **errp)
         vmstate_register(NULL, cpu->cpu_index, cc->vmsd, cpu);
     }
 
-    cpu->iommu_notifiers = g_array_new(false, true, sizeof(TCGIOMMUNotifier));
+    cpu->iommu_notifiers = g_array_new(false, true, sizeof(TCGIOMMUNotifier *));
 #endif
 }