Re: [Qemu-devel] [RFC][PATCH 06/45] msix: Prevent bogus mask updates on MMIO accesses

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 17, 2011 at 09:11:29PM +0200, Jan Kiszka wrote:
> On 2011-10-17 14:50, Michael S. Tsirkin wrote:
> > On Mon, Oct 17, 2011 at 02:07:10PM +0200, Jan Kiszka wrote:
> >> On 2011-10-17 13:57, Michael S. Tsirkin wrote:
> >>> On Mon, Oct 17, 2011 at 01:23:46PM +0200, Jan Kiszka wrote:
> >>>> On 2011-10-17 13:10, Michael S. Tsirkin wrote:
> >>>>> On Mon, Oct 17, 2011 at 11:27:40AM +0200, Jan Kiszka wrote:
> >>>>>> Only accesses to the MSI-X table must trigger a call to
> >>>>>> msix_handle_mask_update or a notifier invocation.
> >>>>>>
> >>>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> >>>>>
> >>>>> Why would msix_mmio_write be called on an access
> >>>>> outside the table?
> >>>>
> >>>> Because it handles both the table and the PBA.
> >>>
> >>> Hmm. Interesting. Is there a bug in how we handle PBA
> >>> updates then? If yes I'd like a separate patch for that
> >>> to apply to the stable tree.
> >>
> >> I first thought it was a serious bug, but it just triggers if the guest
> >> write to PBA (which is very uncommon) and that actually triggers any
> >> spurious out-of-bounds vector injection. Highly unlikely.
> > 
> > Yes guests don't really use PBA ATM. But is there something
> > bad a malicious guest can do? For example, what if
> > msix_clr_pending gets invoked with this huge vector value?
> > 
> > It does seem serious ...
> 
> I checked it before and I think it is harmless. The largest vector that
> can be miscalculated is 255. But bit 255 in the PBA is still safe inside
> our MMIO page.
> 
> Jan
> 

you are right. we got lucky.