vga: fix region calculation

Typically the scanline length and the line offset are identical. But case they are not our calculation for region_end is incorrect. Using line_offset is fine for all scanlines, except the last one where we have to use the actual scanline length.

---
 hw/display/vga.c | 2 ++
 1 file changed, 2 insertions(+)

index 28f298b342..72181330b8 100644
@@ -1483,6 +1483,8 @@ static void vga_draw_graphic(VGACommonState *s, int full_update)
 
     region_start = (s->start_addr * 4);
     region_end = region_start + (ram_addr_t)s->line_offset * height;
+    region_end += width * s->get_bpp(s) / 8; /* scanline length */
+    region_end -= s->line_offset;
     if (region_end > s->vbe_size) {
         /* wraps around (can happen with cirrus vbe modes) */
         region_start = 0;

2.9.3

+-- On Thu, 8 Mar 2018, Gerd Hoffmann wrote --+
| Typically the scanline length and the line offset are identical.
| But case 

But in case ...

| Cc: Ross Lagerwall <[email protected]>

s/Cc:/Reported-by: Ross Lagerwall ...

| Signed-off-by: Gerd Hoffmann <[email protected]>
| ---
|  hw/display/vga.c | 2 ++
|  1 file changed, 2 insertions(+)
| 
| diff --git a/hw/display/vga.c b/hw/display/vga.c
| index 28f298b342..72181330b8 100644
| --- a/hw/display/vga.c
| +++ b/hw/display/vga.c
| @@ -1483,6 +1483,8 @@ static void vga_draw_graphic(VGACommonState *s, int full_update)
|  
|      region_start = (s->start_addr * 4);
|      region_end = region_start + (ram_addr_t)s->line_offset * height;
| +    region_end += width * s->get_bpp(s) / 8; /* scanline length */
| +    region_end -= s->line_offset;
|      if (region_end > s->vbe_size) {
|          /* wraps around (can happen with cirrus vbe modes) */
|          region_start = 0;

It does help to fix a segmentation fault caused by OOB access.

Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
47AF CE69 3A90 54AA 9045 1053 DD13 3D32 FE5B 041F

Thanks!

Ross Lagerwall