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

[Christophe de Dinechin]

> On 3 Jul 2018, at 12:11, Frediano Ziglio <fziglio at redhat.com> wrote:
> 
>> 
>> 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.

I was trying to recreate the fall-through scenario which I believe existed before. But I agree with you, failure and exit is cleaner.

> 
>>> +            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;
> }

Again, my rationale was to try to remove closer to the orignal logic of the code.

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

I personally don’t see an error exit with many unexercised if / branches as less “error prone”. So if I can branch to a well-exercised and maintained main branch, I’d prefer that.

But there is an agreement that the solution is ugly (including by me), So I’ll rewrite it without gotos.

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