xen/blkfront: avoid NULL blkfront_info dereference on device removal

vliaskov · axboe · commit f92898e7f32e · 2018-10-25T11:17:39.000-06:00
If a block device is hot-added when we are out of grants,
gnttab_grant_foreign_access fails with -ENOSPC (log message "28
granting access to ring page") in this code path:

  talk_to_blkback -&gt;
	setup_blkring -&gt;
		xenbus_grant_ring -&gt;
			gnttab_grant_foreign_access

and the failing path in talk_to_blkback sets the driver_data to NULL:

 destroy_blkring:
        blkif_free(info, 0);

        mutex_lock(&amp;blkfront_mutex);
        free_info(info);
        mutex_unlock(&amp;blkfront_mutex);

        dev_set_drvdata(&amp;dev-&gt;dev, NULL);

This results in a NULL pointer BUG when blkfront_remove and blkif_free
try to access the failing device's NULL struct blkfront_info.

Cc: stable@vger.kernel.org # 4.5 and later
Signed-off-by: Vasilis Liaskovitis &lt;vliaskovitis@suse.com&gt;
Reviewed-by: Roger Pau Monné &lt;roger.pau@citrix.com&gt;
Signed-off-by: Konrad Rzeszutek Wilk &lt;konrad.wilk@oracle.com&gt;
Signed-off-by: Jens Axboe &lt;axboe@kernel.dk&gt;
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
@@ -2493,6 +2493,9 @@ static int blkfront_remove(struct xenbus_device *xbdev)
 
 	dev_dbg(&xbdev->dev, "%s removed", xbdev->nodename);
 
+	if (!info)
+		return 0;
+
 	blkif_free(info, 0);
 
 	mutex_lock(&info->mutex);
