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

Gerd Hoffmann posted 1 patch 5 years, 6 months ago
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git fetch https://github.com/patchew-project/qemu tags/patchew/20200713124520.23266-1-kraxel@redhat.com
Maintainers: Alex Williamson <alex.williamson@redhat.com>
hw/vfio/display.c | 1 +
1 file changed, 1 insertion(+)
[PATCH] vfio: fix use-after-free in display
Posted by Gerd Hoffmann 5 years, 6 months ago 
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;
     }
-- 
2.18.4
Re: [PATCH] vfio: fix use-after-free in display
Posted by Philippe Mathieu-Daudé 5 years, 6 months ago 
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;
 }
---
Re: [PATCH] vfio: fix use-after-free in display
Posted by Gerd Hoffmann 5 years, 6 months ago 
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 ;)

take care,
  Gerd
Re: [PATCH] vfio: fix use-after-free in display
Posted by Philippe Mathieu-Daudé 5 years, 6 months ago 
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
>
Re: [PATCH] vfio: fix use-after-free in display
Posted by Alex Williamson 5 years, 6 months ago 
On Mon, 13 Jul 2020 14:45:20 +0200
Gerd Hoffmann <kraxel@redhat.com> 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)
                               ^o

> 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;
>      }

Tricky, but I think I follow that dpy->region.surface is only ever
allocated to replace dpy->con->surface, so when ramfb_display_update()
then replaces and frees dpy->con->surface with dpy->ramfb->ds, that's
where the object point to by dpy->region.surface was freed.  Right?

If so, looks ok to me.  If you're constructing a pull request, I'll
give you an:

Acked-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>

If you need me to send a pull, let me know.  Thanks,

Alex
Re: [PATCH] vfio: fix use-after-free in display
Posted by Gerd Hoffmann 5 years, 6 months ago 
  Hi,

> Tricky, but I think I follow that dpy->region.surface is only ever
> allocated to replace dpy->con->surface, so when ramfb_display_update()
> then replaces and frees dpy->con->surface with dpy->ramfb->ds, that's
> where the object point to by dpy->region.surface was freed.  Right?

Correct.

> If so, looks ok to me.  If you're constructing a pull request, I'll
> give you an:
> 
> Acked-by: Alex Williamson <alex.williamson@redhat.com>
> Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
> 
> If you need me to send a pull, let me know.

I'll go create a pull req, I have one or two other patches pending
anyway.

take care,
  Gerd

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