block/iscsi: validate block size returned from target

It has been reported that at least tgtd returns a block size of 0
for LUN 0. To avoid running into divide by zero later on and protect
against other problematic block sizes validate the block size right
at connection time.

Cc: qemu-stable@nongnu.org
Reported-by: NAndrey Korolyov <andrey@xdel.ru>
Signed-off-by: NPeter Lieven <pl@kamp.de>
Message-Id: <1439552016-8557-1-git-send-email-pl@kamp.de>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

block/iscsi.c

@@ -1214,6 +1214,10 @@ static void iscsi_readcapacity_sync(IscsiLun *iscsilun, Error **errp)
 
     if (task == NULL || task->status != SCSI_STATUS_GOOD) {
         error_setg(errp, "iSCSI: failed to send readcapacity10 command.");
+    } else if (!iscsilun->block_size ||
+               iscsilun->block_size % BDRV_SECTOR_SIZE) {
+        error_setg(errp, "iSCSI: the target returned an invalid "
+                   "block size of %d.", iscsilun->block_size);
     }
     if (task) {
         scsi_free_scsi_task(task);