Re: [Qemu-devel] [CVE-2014-3615 PATCH v2 3/3] spice: make sure we don't overflow ssd->buf

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> I can't track this back far enough. I'd feel safer if you checked that
> the multiplication can't overflow even in uint64_t.

Effectively it comes from the emulated graphics hardware (anything in
hw/display/*).  The gfx emulation must make sure that the framebuffer
fits into the video memory, which in turn pretty much implies that we
can't overflow uint64_t.  I think even uint32_t can't overflow with the
gfx hardware we are emulating today.

> (5) Instead, you really need to make sure that the very first
> multiplication fits in a signed int:

Makes sense.  I think it is easier to just multiply in 64bit, then check
the result is small enougth (new patch attached).

> >  /* display listener callbacks */
> > @@ -495,7 +503,7 @@ static void interface_get_init_info(QXLInstance *sin, QXLDevInitInfo *info)
> >      info->num_memslots = NUM_MEMSLOTS;
> >      info->num_memslots_groups = NUM_MEMSLOTS_GROUPS;
> >      info->internal_groupslot_id = 0;
> > -    info->qxl_ram_size = ssd->bufsize;
> > +    info->qxl_ram_size = 16 * 1024 * 1024;
> >      info->n_surfaces = ssd->num_surfaces;
> >  }

spice-server doesn't do anything with it, other than passing to
spice-client.  Not fully sure what spice-client uses this for, maybe as
some kind of hint for resource management.  Maybe not at all.

It surely doesn't matter at all for ssd->buf size.

cheers,
  Gerd