block-backend: Retain permissions after migration

After migration, the permissions the guest device wants to impose on its
BlockBackend are stored in blk->perm and blk->shared_perm.  In
blk_root_activate(), we take our permissions, but keep all shared
permissions open by calling `blk_set_perm(blk->perm, BLK_PERM_ALL)`.

Only afterwards (immediately or later, depending on the runstate) do we
restrict the shared permissions by calling
`blk_set_perm(blk->perm, blk->shared_perm)`.  Unfortunately, our first
call with shared_perm=BLK_PERM_ALL has overwritten blk->shared_perm to
be BLK_PERM_ALL, so this is a no-op and the set of shared permissions is
not restricted.

Fix this bug by saving the set of shared permissions before invoking
blk_set_perm() with BLK_PERM_ALL and restoring it afterwards.

Fixes: 5f7772c
       ("block-backend: Defer shared_perm tightening migration
       completion")
Reported-by: Peng Liang <liangpeng10@huawei.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211125135317.186576-2-hreitz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Peng Liang <liangpeng10@huawei.com>

block/block-backend.c

@@ -190,19 +190,30 @@ static void blk_root_activate(BdrvChild *child, Error **errp)
 {
     BlockBackend *blk = child->opaque;
     Error *local_err = NULL;
+    uint64_t saved_shared_perm;
 
     if (!blk->disable_perm) {
         return;
     }
 
     blk->disable_perm = false;
 
+    /*
+     * blk->shared_perm contains the permissions we want to share once
+     * migration is really completely done.  For now, we need to share
+     * all; but we also need to retain blk->shared_perm, which is
+     * overwritten by a successful blk_set_perm() call.  Save it and
+     * restore it below.
+     */
+    saved_shared_perm = blk->shared_perm;
+
     blk_set_perm(blk, blk->perm, BLK_PERM_ALL, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         blk->disable_perm = true;
         return;
     }
+    blk->shared_perm = saved_shared_perm;
 
     if (runstate_check(RUN_STATE_INMIGRATE)) {
         /* Activation can happen when migration process is still active, for