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,

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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...

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

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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.

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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