[Spice-devel] [PATCH] Avoid NULL-dereference if canvas_get_image errors out

[Frediano Ziglio]

> 
> On Fri, Jun 29, 2018 at 05:21:22PM +0200, Christophe de Dinechin wrote:
> > From: Christophe de Dinechin <dinechin at redhat.com>
> > 
> > In some error cases, canvas_get_image may return NULL.
> > When this happens, calls like pixman_image_get_width(s)
> > will crash. Add additional error paths to deal with
> > these cases.
> 
> I assume you don't have a reproducer for this, this is just something
> you noticed while looking at that code?
> 
> > 
> > Signed-off-by: Christophe de Dinechin <dinechin at redhat.com>
> > ---
> >  common/canvas_base.c | 38 +++++++++++++++++++++++++++++++++++---
> >  1 file changed, 35 insertions(+), 3 deletions(-)
> > 
> > diff --git a/common/canvas_base.c b/common/canvas_base.c
> > index 6bf6e5d..bbaaf96 100644
> > --- a/common/canvas_base.c
> > +++ b/common/canvas_base.c
> > @@ -3072,6 +3072,7 @@ static void canvas_draw_stroke(SpiceCanvas
> > *spice_canvas, SpiceRect *bbox,
> >              gc.tile = canvas_get_image(canvas,
> >                                         stroke->brush.u.pattern.pat,
> >                                         FALSE);
> > +            spice_return_if_fail(gc.tile != NULL);
> 
> I'd favour g_return_if_fail(), I'd really like to kill these various
> spice_* calls mirroring glib API. And spice_return_if_fail() will
> abort() I think.
> 
> >          }
> >          gc.tile_offset_x = stroke->brush.u.pattern.pos.x;
> >          gc.tile_offset_y = stroke->brush.u.pattern.pos.y;
> > @@ -3158,6 +3159,10 @@ static void canvas_draw_rop3(SpiceCanvas
> > *spice_canvas, SpiceRect *bbox,
> >      heigth = bbox->bottom - bbox->top;
> > 
> >      d = canvas_get_image_from_self(spice_canvas, bbox->left, bbox->top,
> >      width, heigth, FALSE);
> > +    if (d == NULL) {
> > +        spice_critical("no rop3 destination");
> 
> spice_critical calls abort() too
> 
> > +        goto d_error;
> > +    }
> >      surface_canvas = canvas_get_surface(canvas, rop3->src_bitmap);
> >      if (surface_canvas) {
> >          s = surface_canvas->ops->get_image(surface_canvas, FALSE);
> > @@ -3176,10 +3181,14 @@ static void canvas_draw_rop3(SpiceCanvas
> > *spice_canvas, SpiceRect *bbox,
> >          src_pos.x = rop3->src_area.left;
> >          src_pos.y = rop3->src_area.top;
> >      }
> > +    if (s == NULL) {
> > +        spice_critical("no rop3 source");
> > +        goto s_error;
> > +    }
> >      if (pixman_image_get_width(s) - src_pos.x < width ||
> >          pixman_image_get_height(s) - src_pos.y < heigth) {
> >          spice_critical("bad src bitmap size");
> > -        return;
> > +        goto s_error;
> >      }
> >      if (rop3->brush.type == SPICE_BRUSH_TYPE_PATTERN) {
> >          SpiceCanvas *_surface_canvas;
> > @@ -3191,6 +3200,12 @@ static void canvas_draw_rop3(SpiceCanvas
> > *spice_canvas, SpiceRect *bbox,
> >          } else {
> >              p = canvas_get_image(canvas, rop3->brush.u.pattern.pat,
> >              FALSE);
> >          }
> > +        if (p == NULL) {
> > +            spice_critical("no rop3 pattern");
> > +            /* degrade to color only */
> > +            do_rop3_with_color(rop3->rop3, d, s, &src_pos,
> > rop3->brush.u.color);
> 
> Could you test this? If not, I feel a bit uncomfortable adding such a
> fallback.
> 

The fact that you tried to use rop3->brush.u.pattern means that
probably rop3->brush.u.color is invalid (u is an union).

Are we trying to replace secure crashes with some other security
issues? I really don't like not tested code.

> > +            goto p_error;
> > +        }
> >          SpicePoint pat_pos;
> > 
> >          pat_pos.x = (bbox->left - rop3->brush.u.pattern.pos.x) %
> >          pixman_image_get_width(p);
> > @@ -3200,14 +3215,16 @@ static void canvas_draw_rop3(SpiceCanvas
> > *spice_canvas, SpiceRect *bbox,
> >      } else {
> >          do_rop3_with_color(rop3->rop3, d, s, &src_pos,
> >          rop3->brush.u.color);
> >      }
> > +p_error:
> 
> Do we need 3 labels with very similar names? I'd think something like
> this could work?
> 
> error:
>     if (s != NULL) {
>       pixman_image_unref(s);
> 
>       spice_canvas->ops->blit_image(spice_canvas, &dest_region, d,
>                                     bbox->left,
>                                     bbox->top);

why faking a draw if we don't know? Won't be better to do a

    // normal path
    do final stuff ...
    return SUCCESS;

error:
    if (x != NULL) { clean x; }
    if (whatever) { other clean }
    return FAILURE;
}

??

I don't like kernel-like error prone (this syntax caused multiple security
issues and leaks in the past) multiple label cleanup code.

>     }
> 
>     if (d != NULL) {
>       pixman_image_unref(d);
>     }
> 
>     pixman_region32_fini(&dest_region);
> 
> Christophe
> 

Frediano