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

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

On 7/13/20 4:00 PM, Gerd Hoffmann wrote:
> On Mon, Jul 13, 2020 at 02:51:05PM +0200, Philippe Mathieu-Daudé wrote:
>> 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;
> 
> No.
> 
> That doesn't clear VFIODisplay->region.surface, but it sets
> QemuConsole->surface to NULL no matter what got passed to
> dpy_gfx_replace_surface().
> 
> Guesswork based just on the patch chunk doesn't always work,
> sometimes you have to consult the source code to see what the
> patch actually does ;)

I'm certainly not a source of trust producing perfect code :)
I appreciate being reviewed on snippets, so I can learn from my
mistakes and improve. I looked at the code (ui/console.c was not
in the previous context) but I missed to look at the other call
sites for dpy_gfx_replace_surface() and how it is used.

Thanks,

Phil.

> 
> take care,
>   Gerd
>