[Qemu-devel] [PATCH] vga: fix region calculation

[Qemu-devel] [PATCH] vga: fix region calculation

[Gerd Hoffmann]

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.

Fixes: CVE-2018-<to-be-filled>
Cc: P J P <ppandit@redhat.com>
Cc: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 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;
-- 
2.9.3

Re: [Qemu-devel] [PATCH] vga: fix region calculation

[P J P]

+-- 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 <ross.lagerwall@citrix.com>

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

| Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
| ---
|  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.

Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>

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

Re: [Qemu-devel] [PATCH] vga: fix region calculation

[Ross Lagerwall]

On 03/08/2018 07:31 AM, Gerd Hoffmann wrote:
> 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.
> 
> Fixes: CVE-2018-<to-be-filled>
> Cc: P J P <ppandit@redhat.com>
> Cc: Ross Lagerwall <ross.lagerwall@citrix.com>
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---

Tested-by: Ross Lagerwall <ross.lagerwall@citrix.com>

Thanks!
-- 
Ross Lagerwall