[PATCH 3/9] vfio/migration: Fix NULL pointer dereference bug

[Avihai Horon <avihaih@nvidia.com>]

As part of its error flow, vfio_vmstate_change() accesses
MigrationState->to_dst_file without any checks. This can cause a NULL
pointer dereference if the error flow is taken and
MigrationState->to_dst_file is not set.

For example, this can happen if VM is started or stopped not during
migration and vfio_vmstate_change() error flow is taken, as
MigrationState->to_dst_file is not set at that time.

Fix it by checking that MigrationState->to_dst_file is set before using
it.

Fixes: 02a7e71b1e5b ("vfio: Add VM state change handler to know state of VM")
Signed-off-by: Avihai Horon <avihaih@nvidia.com>
---
 hw/vfio/migration.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
index 835608cd23..21e8f9d4d4 100644
--- a/hw/vfio/migration.c
+++ b/hw/vfio/migration.c
@@ -744,7 +744,9 @@ static void vfio_vmstate_change(void *opaque, bool running, RunState state)
          */
         error_report("%s: Failed to set device state 0x%x", vbasedev->name,
                      (migration->device_state & mask) | value);
-        qemu_file_set_error(migrate_get_current()->to_dst_file, ret);
+        if (migrate_get_current()->to_dst_file) {
+            qemu_file_set_error(migrate_get_current()->to_dst_file, ret);
+        }
     }
     vbasedev->migration->vm_running = running;
     trace_vfio_vmstate_change(vbasedev->name, running, RunState_str(state),
-- 
2.21.3