Re: [PATCH] vfio: fix use-after-free in display

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 7/13/20 2:45 PM, Gerd Hoffmann wrote:
> Calling ramfb_display_update() might replace the DisplaySurface with the
> boot display, which in turn will free the currently active
> DisplaySurface.
> 
> So clear our DisplaySurface pinter (dpy->region.surface pointer) to (a)
> avoid use-after-free and (b) force replacing the boot display with the
> real display when switching back.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  hw/vfio/display.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/hw/vfio/display.c b/hw/vfio/display.c
> index a57a22674d62..342054193b3c 100644
> --- a/hw/vfio/display.c
> +++ b/hw/vfio/display.c
> @@ -405,6 +405,7 @@ static void vfio_display_region_update(void *opaque)
>      if (!plane.drm_format || !plane.size) {
>          if (dpy->ramfb) {
>              ramfb_display_update(dpy->con, dpy->ramfb);
> +            dpy->region.surface = NULL;
>          }
>          return;
>      }
> 

More generic fix:

-- >8 --
--- a/ui/console.c
+++ b/ui/console.c
@@ -1580,10 +1580,10 @@ void dpy_gfx_replace_surface(QemuConsole *con,
                              DisplaySurface *surface)
 {
     DisplayState *s = con->ds;
-    DisplaySurface *old_surface = con->surface;
+    QemuConsole *old_con = con;
     DisplayChangeListener *dcl;

-    assert(old_surface != surface || surface == NULL);
+    assert(con->surface != surface || surface == NULL);

     con->surface = surface;
     QLIST_FOREACH(dcl, &s->listeners, next) {
@@ -1594,7 +1594,8 @@ void dpy_gfx_replace_surface(QemuConsole *con,
             dcl->ops->dpy_gfx_switch(dcl, surface);
         }
     }
-    qemu_free_displaysurface(old_surface);
+    qemu_free_displaysurface(old_con->surface);
+    old_con->surface = NULL;
 }
---