Re: [Qemu-devel] [PATCH 1/5] virtio-pci: ignore unaligned read/write in virtio_address_space_read()/write()

[Michael S. Tsirkin]

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 ... 

-- 
MST