Re: [PATCH 4/4] hw/display/artist.c: fix out of bounds check

[Richard Henderson]

On 7/27/20 2:46 PM, Helge Deller wrote:
> -        for (i = 0; i < pix_count; i++) {
> +        for (i = 0; i < pix_count && offset + i < buf->size; i++) {
>              artist_rop8(s, p + offset + pix_count - 1 - i,
>                          (data & 1) ? (s->plane_mask >> 24) : 0);
>              data >>= 1;

This doesn't look right.

You're writing to "offset + pix_count - 1 - i" and yet you're checking bounds
vs "offset + i".

This could be fixed by computing the complete offset into a local variable and
then have an inner if to avoid the write, as you do for the second loop.

But it would be better to precompute the correct loop bounds.


r~


> @@ -398,7 +390,9 @@ static void vram_bit_write(ARTISTState *s, int posx, int 
> posy, bool incr_x,
>          for (i = 3; i >= 0; i--) {
>              if (!(s->image_bitmap_op & 0x20000000) ||
>                  s->vram_bitmask & (1 << (28 + i))) {
> -                artist_rop8(s, p + offset + 3 - i, data8[ROP8OFF(i)]);
> +                if (offset + 3 - i < buf->size) {
> +                    artist_rop8(s, p + offset + 3 - i, data8[ROP8OFF(i)]);
> +                }
>              }
>          }
>          memory_region_set_dirty(&buf->mr, offset, 3);
> @@ -420,7 +414,7 @@ static void vram_bit_write(ARTISTState *s, int posx, int 
> posy, bool incr_x,
>              break;
>          }
> 
> -        for (i = 0; i < pix_count; i++) {
> +        for (i = 0; i < pix_count && offset + i < buf->size; i++) {
>              mask = 1 << (pix_count - 1 - i);
> 
>              if (!(s->image_bitmap_op & 0x20000000) ||