From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH] virtio: Don't access device data after unregistration.
Date: Mon, 3 Sep 2012 23:18:47 +0300
Message-ID: <20120903201847.GC6181@redhat.com>
References: <1346680242-5717-1-git-send-email-sjur.brandeland@stericsson.com>
	<20120903141445.GA5054@redhat.com>
	<CAJK669aRsbOOmjmT+K8OHYvenOhkYWuSOx6nBM7hxC_C4mmw-g@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
Return-path: <virtualization-bounces@lists.linux-foundation.org>
Content-Disposition: inline
In-Reply-To: <CAJK669aRsbOOmjmT+K8OHYvenOhkYWuSOx6nBM7hxC_C4mmw-g@mail.gmail.com>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/virtualization/>
List-Post: <mailto:virtualization@lists.linux-foundation.org>
List-Help: <mailto:virtualization-request@lists.linux-foundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=subscribe>
Sender: virtualization-bounces@lists.linux-foundation.org
Errors-To: virtualization-bounces@lists.linux-foundation.org
To: Sjur =?iso-8859-1?Q?Br=E6ndeland?= <sjurbren@gmail.com>
Cc: linux-kernel@vger.kernel.org, "Guzman Lugo,
	Fernadndo" <fernando.lugo@ti.com>, virtualization@lists.linux-foundation.org
List-Id: virtualization@lists.linuxfoundation.org

On Mon, Sep 03, 2012 at 04:50:10PM +0200, Sjur Br=E6ndeland wrote:
> Hi Michael,
> =

> >> Fix panic in virtio.c when CONFIG_DEBUG_SLAB is set.
> >
> > What's the root cause of the panic?
> =

> I believe the cause of the panic is calling
> ida_simple_remove(&virtio_index_ida, dev->index);
> when the dev structure is "poisoned" after kfree.
> It might be the "BUG_ON((int)id < 0)" that bites...
> =

> >> Use device_del() and put_device() instead of
> >> device_unregister(), and access device data before
> >> calling put_device().
> =

> > Why does this help? Does device_unregister free the
> > device so dev->index access crashes?
> =

> Yes, if device ref-count is one when calling unregister
> the device is freed.

Interesting. Where exactly? Note that:

struct rproc_vdev {
        struct list_head node;
        struct rproc *rproc;
        struct virtio_device vdev;
        struct rproc_vring vring[RVDEV_NUM_VRINGS];
        unsigned long dfeatures;
        unsigned long gfeatures;
};              =


kfree(&proc_vdev->vdev) is unlikely to be the right thing to do.

> > If yes virtio_pci_remove will crash too
> > as it accesses the device after the
> > call to unregister_virtio_device so the
> > fix won't be effective.
> =

> I discovered this using the remoteproc framework.
> It might be that device is unregistered with ref-count greater
> than one normally, in that case this bug will not show up.
> =

> Regards,
> Sjur

It might be remoteproc has an unrelated bug?

-- =

MST