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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Mon, 13 Jul 2015 10:39:44 +0200
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Subject: Re: [Qemu-devel] [PATCH 1/5] virtio-pci: ignore unaligned
 read/write in virtio_address_space_read()/write()
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>, qemu-devel@nongnu.org

On Mo, 2015-07-13 at 11:00 +0300, Michael S. Tsirkin wrote:
> On Mon, Jul 13, 2015 at 09:53:43AM +0200, Gerd Hoffmann wrote:
> > On Mo, 2015-07-13 at 10:36 +0300, Michael S. Tsirkin wrote:
> > > On Mon, Jul 13, 2015 at 01:46:47PM +0800, Jason Wang wrote:
> > > > We abort on unaligned read/write in
> > > > virtio_address_space_read()/write() but since len in under control of
> > > > guest so qemu will simply crash when booting a modern guest (guest is
> > > > try to read when len is zero).
> > > > read.
> > > 
> > > How can len be 0? Isn't this a guest bug? Or is this
> > > a theoretical issue?
> > 
> > Something dumping pci config space?
> > With pci access capability not being used before and therefore zeroed?
> > Then hitting the "data" field will trigger a zero-length read.
> 
> I suspect so, yes. All this worries me: what if length was not 0
> because the capability was previously used e.g. by bios?
> 
> > That assert actually triggers when booting a recent linux kernel with
> > disable-modern=off
> > 
> > cheers,
> >   Gerd
> > 
> 
> Which linux version?  Doesn't seem to trigger for me ... 

Fedora 22 guest with latest distro kernel (4.0.7)

cheers,
  Gerd