linux-kernel - Re: [PATCH] virtio: Don't access device data after unregistration.

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Date:	Mon, 3 Sep 2012 23:18:47 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sjur Brændeland <sjurbren@...il.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	"Guzman Lugo, Fernadndo" <fernando.lugo@...com>,
	virtualization@...ts.linux-foundation.org,
	Ohad Ben-Cohen <ohad@...ery.com>
Subject: Re: [PATCH] virtio: Don't access device data after unregistration.

On Mon, Sep 03, 2012 at 04:50:10PM +0200, Sjur Brændeland 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
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