qcow2: Fix dangling pointer after reopen for 'file'

Without an external data file, s->data_file is a second pointer with the
same value as bs->file. When changing bs->file to a different BdrvChild
and freeing the old BdrvChild, s->data_file must also be updated,
otherwise it points to freed memory and causes crashes.

This problem was caught by iotests case 245.

Fixes: df2b7086f169239ebad5d150efa29c9bb6d4f820
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210708114709.206487-2-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -1926,6 +1926,7 @@ static void qcow2_refresh_limits(BlockDriverState *bs, Error **errp)
 static int qcow2_reopen_prepare(BDRVReopenState *state,
                                 BlockReopenQueue *queue, Error **errp)
 {
+    BDRVQcow2State *s = state->bs->opaque;
     Qcow2ReopenState *r;
     int ret;
 
@@ -1956,6 +1957,16 @@ static int qcow2_reopen_prepare(BDRVReopenState *state,
         }
     }
 
+    /*
+     * Without an external data file, s->data_file points to the same BdrvChild
+     * as bs->file. It needs to be resynced after reopen because bs->file may
+     * be changed. We can't use it in the meantime.
+     */
+    if (!has_data_file(state->bs)) {
+        assert(s->data_file == state->bs->file);
+        s->data_file = NULL;
+    }
+
     return 0;
 
 fail:
@@ -1966,7 +1977,16 @@ static int qcow2_reopen_prepare(BDRVReopenState *state,
 
 static void qcow2_reopen_commit(BDRVReopenState *state)
 {
+    BDRVQcow2State *s = state->bs->opaque;
+
     qcow2_update_options_commit(state->bs, state->opaque);
+    if (!s->data_file) {
+        /*
+         * If we don't have an external data file, s->data_file was cleared by
+         * qcow2_reopen_prepare() and needs to be updated.
+         */
+        s->data_file = state->bs->file;
+    }
     g_free(state->opaque);
 }
 
@@ -1990,6 +2010,15 @@ static void qcow2_reopen_commit_post(BDRVReopenState *state)
 
 static void qcow2_reopen_abort(BDRVReopenState *state)
 {
+    BDRVQcow2State *s = state->bs->opaque;
+
+    if (!s->data_file) {
+        /*
+         * If we don't have an external data file, s->data_file was cleared by
+         * qcow2_reopen_prepare() and needs to be restored.
+         */
+        s->data_file = state->bs->file;
+    }
     qcow2_update_options_abort(state->bs, state->opaque);
     g_free(state->opaque);
 }