Re: [Qemu-devel] [PATCH 0/1] display/bochs: fix pcie support (qemu security issue)

[Alex Williamson]

On Mon, 12 Aug 2019 14:39:53 +0100
Peter Maydell <address@hidden> wrote:

> On Mon, 12 Aug 2019 at 13:51, Philippe Mathieu-Daudé <address@hidden> wrote:
> >
> > On 8/12/19 2:45 PM, Paolo Bonzini wrote:  
> > > On 12/08/19 08:52, Gerd Hoffmann wrote:  
> > >> Just found while investigating
> > >>   https://bugzilla.redhat.com/show_bug.cgi?id=1707118
> > >>
> > >> Found PCIe extended config space filled with random crap due to
> > >> allocation being too small (conventional pci config space only).
> > >>  
> >
> > Can you amend this information to the commit description?
> >
> > <...
> >  
> > >> PCI(e) config space is guest writable.  Writes are limited by
> > >> write mask (which probably is also filled with random stuff),  
> > >
> > > Yes, it is also allocated with 256 bytes only.
> > >  
> > >> so the guest can only flip enabled bits.  But I suspect it
> > >> still might be exploitable, so rather serious because it might
> > >> be a host escape for the guest.  On the other hand the device
> > >> is probably not yet in widespread use.  
> >  
> > ...>  
> 
> I can add to the commit this paragraph of the cover letter,
> and I think also the 'mitigation' note might as well go in.
> 
> I've also put the cc:stable into the commit message.
> 
> Updated commit, ready to apply to master if we're OK with it:
> 
> https://git.linaro.org/people/peter.maydell/qemu-arm.git/commit/?h=staging&id=c075b5f318a8be628ab8edf93be33f5a93a4aacd

Quoting new commit log:

        This makes sure the pci config space allocation is big enough,
        so accessing the PCIe extended config space doesn't overflow
        the pci config space buffer.

        PCI(e) config space is guest writable.  Writes are limited
        bywrite mask (which probably is also filled with random stuff),
        so the guest can only flip enabled bits.  But I suspect it
        still might be exploitable, so rather serious because it might
        be a host escape for the guest.  On the other hand the device
        is probably not yet in widespread use.

        Mitigation: use "-device bochs-display" as conventional pci
        device only.

Is it clear to others that this mitigation remark seems to be
referencing an alternative configuration constraint to avoid the issue
rather than what's actually implemented in this patch?  IOW, if we
never place the bochs-display device into a PCIe hierarchy, then
extended config space is never accessible to the guest anyway, and
there is no issue.  I think this was meant to be an alternative to the
patch but the enforcement of that would happen above QEMU, probably why
it was mentioned in the cover letter rather than the original commit
log.  Thanks,

Alex