Re: [Qemu-devel] [PATCH] pci: add checking whether the device is realized before unlinking the capability

[SeokYeon Hwang <syeon.hwang@samsung.com>]

> On Wed, 2014-06-25 at 15:03 +0300, Michael S. Tsirkin wrote:
> > On Wed, Jun 25, 2014 at 06:59:08PM +0900, SeokYeon Hwang wrote:
> > > In case of the unrealized "pdev", memory can be illegally accessed and
> corrupted.
> > > Refer to device_unparent() in the commit
> 5c21ce77d7e5643089ceec556c0408445d017f32.
> Hi,
> Thank you for submitting the patch.
> 
> Can you please send to the list how to reproduce the issue?
> Is this a regression? Before the above commit did it work?

Hi,

When VirtIO device is hot-unplugged, Qemu unrealizes the device first then child_bus by device_unparent().
(After the commit 5c21ce77d7e5643089ceec556c0408445d017f32.)

For example, if I suppose to hot-unplug "virtio block" device, unparent "virtio-blk-pci" first, then "virtio-blk".

virtio_bus_device_unplugged() function in virtio-bus.c calls "msix_uninit()" with the parameter of its parent PCI device.
The problem is that it accesses already freed member of pdev, such as "pdev->config".
This illegal access causes the heap corruption, and it kills Qemu process suddenly.

You can detect this illegal access by using "electric fence" or "valgrind".

> 
> > >
> > > Change-Id: Iacb195a092c86d4c677ad0404582af104b2251ae
> > > Signed-off-by: SeokYeon Hwang <syeon.hwang@samsung.com>
> >
> > Another case of qemu mailing list dropping patches :( I will bounce it
> > now so people can see the original message.
> >
> >
> > > ---
> > >  hw/pci/pci.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/hw/pci/pci.c b/hw/pci/pci.c index 49eca95..bb7f0c5
> > > 100644
> > > --- a/hw/pci/pci.c
> > > +++ b/hw/pci/pci.c
> > > @@ -2056,7 +2056,12 @@ int pci_add_capability(PCIDevice *pdev,
> > > uint8_t cap_id,
> > >  /* Unlink capability from the pci config space. */  void
> > > pci_del_capability(PCIDevice *pdev, uint8_t cap_id, uint8_t size)  {
> > > -    uint8_t prev, offset = pci_find_capability_list(pdev, cap_id,
> &prev);
> > > +    uint8_t prev, offset;
> > > +    /* Check whether the device is realized or not */
> > > +    if (!pdev->qdev.realized) {
> The 'qdev' field and 'realize' property are private, you should use one of
> QOM's 'property_get' methods.
> 
> I am also concerned about adding the realize check here, it seems too
> "deep" in implementation, seeing this checks everywhere doesn't seem a
> good idea.
> I think we should first understand the root cause and try making this
> change in a higher level.
> 
> I hope I helped,
> Marcel

Ok, I see. I think you're right.
If needed, I will send an another patch using "object_property_get_bool()".

Thanks.

> 
> 
> > > +        return;
> > > +    }
> > > +    offset = pci_find_capability_list(pdev, cap_id, &prev);
> > >      if (!offset)
> > >          return;
> > >      pdev->config[prev] = pdev->config[offset + PCI_CAP_LIST_NEXT];
> > > --
> > > 1.9.1
> >
>