Re: [Qemu-devel] [PATCH] cirrus: fix oob access issue

[Laszlo Ersek <lersek@redhat.com>]

On 01/24/17 11:48, Gerd Hoffmann wrote:
>>> diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c
>>> index 379910d..7ddd289 100644
>>> --- a/hw/display/cirrus_vga.c
>>> +++ b/hw/display/cirrus_vga.c
>>> @@ -277,7 +277,8 @@ static bool blit_region_is_unsafe(struct CirrusVGAState *s,
>>>      }
>>>      if (pitch < 0) {
>>>          int64_t min = addr
>>> -            + ((int64_t)s->cirrus_blt_height-1) * pitch;
>>> +            + ((int64_t)s->cirrus_blt_height-1) * pitch
>>> +            - s->cirrus_blt_width;
>>>          int32_t max = addr
>>>              + s->cirrus_blt_width;
>>>          if (min < 0 || max > s->vga.vram_size) {
>>>
>>
>> I believe this is incorrect. In this case (AFAIR), "addr" points to the
>> left-most pixel (= lowest address) of the bottom line (= highest
>> address).
> 
> If I read the code correctly it is backwards *both* x and y axis, so
> addr is the right-most pixel of the bottom line.

What is "max" then? If "addr" is the right-most pixel of the bottom
line, then "max" has the highest address just past the rectangle, and
then adding anything non-negative to it makes no sense.

Adding a negative value to it (i.e., if s->cirrus_blt_width were
negative) might make sense, for getting the leftmost pixel on the bottom
line, but then that "max" value shouldn't be used for the final boundary
check (against RAM size).

... Really as I remember it from the downstream review, the pitch is
negative (bottom-up), but the horizontal direction remains left to right.

0-----------------------------------------------------------------> x
|
|   addr + (height - 1) * pitch, pitch < 0, height > 0
|   |
|   v
|   *--------------------* <-- addr + (height - 1) * pitch + width
|   |                    |
|   |                    |
|   |                    |
|   |                    |
|   |                    |
|   |                    |
|   *--------------------*
|   ^                    ^
|   |                    |
|   addr                 max = addr + width, width > 0
|
v
y


The rightmost column represented here is exclusive, the rest is inclusive.

Thanks
Laszlo