Re: [Qemu-devel] [Qemu-ppc] [PATCH 2/5] virtio-balloon: Corrections to address verification

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Feb 22, 2019 at 10:08:22AM +0100, Greg Kurz wrote:
> On Thu, 14 Feb 2019 15:39:13 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > The virtio-balloon device's verification of the address given to it by the
> > guest has a number of faults:
> >     * The addresses here are guest physical addresses, which should be
> >       'hwaddr' rather than 'ram_addr_t' (the distinction is admittedly
> >       pretty subtle and confusing)
> >     * We don't check for section.mr being NULL, which is the main way that
> >       memory_region_find() reports basic failures.  We really need to check
> >       that before looking at any other section fields, because
> >       memory_region_find() doesn't initialize them on the failure path
> >     * We're passing a length of '1' to memory_region_find(), but really the
> >       guest is requesting that we put the entire page into the balloon,
> >       so it makes more sense to call it with BALLOON_PAGE_SIZE
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > Reviewed-by: David Hildenbrand <david@redhat.com>
> > Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  hw/virtio/virtio-balloon.c | 17 ++++++++++-------
> >  1 file changed, 10 insertions(+), 7 deletions(-)
> > 
> > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> > index 43af521884..eb357824d8 100644
> > --- a/hw/virtio/virtio-balloon.c
> > +++ b/hw/virtio/virtio-balloon.c
> > @@ -221,17 +221,20 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
> >          }
> >  
> >          while (iov_to_buf(elem->out_sg, elem->out_num, offset, &pfn, 4) == 4) {
> > -            ram_addr_t pa;
> > -            ram_addr_t addr;
> > +            hwaddr pa;
> > +            hwaddr addr;
> >              int p = virtio_ldl_p(vdev, &pfn);
> >  
> > -            pa = (ram_addr_t) p << VIRTIO_BALLOON_PFN_SHIFT;
> > +            pa = (hwaddr) p << VIRTIO_BALLOON_PFN_SHIFT;
> >              offset += 4;
> >  
> > -            /* FIXME: remove get_system_memory(), but how? */
> > -            section = memory_region_find(get_system_memory(), pa, 1);
> > -            if (!int128_nz(section.size) ||
> > -                !memory_region_is_ram(section.mr) ||
> > +            section = memory_region_find(get_system_memory(), pa,
> > +                                         BALLOON_PAGE_SIZE);
> > +            if (!section.mr) {
> > +                trace_virtio_balloon_bad_addr(pa);
> > +                continue;
> > +            }
> 
> memory_region_unref(section.mr) with section.mr == NULL is safe and
> resolves to a nop. Not sure you need a separate if to handle this
> case.

memory_region_is_ram() and friends are not, however - they will
dereference their argument unconditionally.

> 
> Apart from that,
> 
> Reviewed-by: Greg Kurz <groug@kaod.org>
> 
> > +            if (!memory_region_is_ram(section.mr) ||
> >                  memory_region_is_rom(section.mr) ||
> >                  memory_region_is_romd(section.mr)) {
> >                  trace_virtio_balloon_bad_addr(pa);
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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