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 Mon, Feb 25, 2019 at 10:26:51AM +0100, Greg Kurz wrote:
> On Mon, 25 Feb 2019 10:37:11 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > 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.
> > 
> 
> Indeed but the two ifs can be merged anyway:
> 
>             if (!section.mr ||
>                 !memory_region_is_ram(section.mr) ||
>                 memory_region_is_rom(section.mr) ||
>                 memory_region_is_romd(section.mr)) {
>                 trace_virtio_balloon_bad_addr(pa);
>                 memory_region_unref(section.mr);
>                 continue;
>             }

Oh, I see what you mean.  Hrm, I still kind of prefer visually
separating the validity check from tests which depend on that validity.

> 
> > > 
> > > 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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