[block-xen-blkback] question about pontential null pointer dereference

From: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
To: "Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Subject: [block-xen-blkback] question about pontential null pointer dereference
Date: Wed, 10 May 2017 11:49:53 -0500	[thread overview]
Message-ID: <20170510114953.Horde.7C1VjUQI-YiZdSL2tKfFVHL@gator4166.hostgator.com> (raw)

Hello everybody,

While looking into Coverity ID 1350942 I ran into the following piece  
of code at drivers/block/xen-blkback/xenbus.c:490:

490static int xen_blkbk_remove(struct xenbus_device *dev)
491{
492        struct backend_info *be = dev_get_drvdata(&dev->dev);
493
494        pr_debug("%s %p %d\n", __func__, dev, dev->otherend_id);
495
496        if (be->major || be->minor)
497                xenvbd_sysfs_delif(dev);
498
499        if (be->backend_watch.node) {
500                unregister_xenbus_watch(&be->backend_watch);
501                kfree(be->backend_watch.node);
502                be->backend_watch.node = NULL;
503        }
504
505        dev_set_drvdata(&dev->dev, NULL);
506
507        if (be->blkif)
508                xen_blkif_disconnect(be->blkif);
509
510        /* Put the reference we set in xen_blkif_alloc(). */
511        xen_blkif_put(be->blkif);
512        kfree(be->mode);
513        kfree(be);
514        return 0;
515}

The issue here is that line 507 implies that be->blkif might be NULL.  
If this is the case, there is a NULL pointer dereference when  
executing line 511 once macro xen_blkif_put() dereference be->blkif

Is there any chance for be->blkif to be NULL at line 511?

I'm trying to figure out if this is a false positive or something that  
actually needs to be fixed.

I'd really appreciate any comment on this.

Thank you!
--
Gustavo A. R. Silva