[Qemu-devel] [PATCH v2] cirrus_vga: fix off-by-one in blit_region_is_unsafe

[Qemu-devel] [PATCH v2] cirrus_vga: fix off-by-one in blit_region_is_unsafe

[Paolo Bonzini]

The "max" value is being compared with >=, but addr + width points to
the first byte that will _not_ be copied.  Laszlo suggested using a
"greater than" comparison, instead of subtracting one like it is
already done above for the height, so that max remains always positive.

The mistake is "safe"---it will reject some blits, but will never cause
out-of-bounds writes.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/display/cirrus_vga.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c
index b6ce1c8..57b91a7 100644
--- a/hw/display/cirrus_vga.c
+++ b/hw/display/cirrus_vga.c
@@ -276,14 +276,14 @@ static bool blit_region_is_unsafe(struct CirrusVGAState *s,
             + ((int64_t)s->cirrus_blt_height-1) * pitch;
         int32_t max = addr
             + s->cirrus_blt_width;
-        if (min < 0 || max >= s->vga.vram_size) {
+        if (min < 0 || max > s->vga.vram_size) {
             return true;
         }
     } else {
         int64_t max = addr
             + ((int64_t)s->cirrus_blt_height-1) * pitch
             + s->cirrus_blt_width;
-        if (max >= s->vga.vram_size) {
+        if (max > s->vga.vram_size) {
             return true;
         }
     }
-- 
2.5.0

Re: [Qemu-devel] [PATCH v2] cirrus_vga: fix off-by-one in blit_region_is_unsafe

[Laszlo Ersek]

On 02/10/16 17:17, Paolo Bonzini wrote:
> The "max" value is being compared with >=, but addr + width points to
> the first byte that will _not_ be copied.  Laszlo suggested using a
> "greater than" comparison, instead of subtracting one like it is
> already done above for the height, so that max remains always positive.
> 
> The mistake is "safe"---it will reject some blits, but will never cause
> out-of-bounds writes.
> 
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  hw/display/cirrus_vga.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c
> index b6ce1c8..57b91a7 100644
> --- a/hw/display/cirrus_vga.c
> +++ b/hw/display/cirrus_vga.c
> @@ -276,14 +276,14 @@ static bool blit_region_is_unsafe(struct CirrusVGAState *s,
>              + ((int64_t)s->cirrus_blt_height-1) * pitch;
>          int32_t max = addr
>              + s->cirrus_blt_width;
> -        if (min < 0 || max >= s->vga.vram_size) {
> +        if (min < 0 || max > s->vga.vram_size) {
>              return true;
>          }
>      } else {
>          int64_t max = addr
>              + ((int64_t)s->cirrus_blt_height-1) * pitch
>              + s->cirrus_blt_width;
> -        if (max >= s->vga.vram_size) {
> +        if (max > s->vga.vram_size) {
>              return true;
>          }
>      }
> 

Thank you for your patience. :)

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Re: [Qemu-devel] [PATCH v2] cirrus_vga: fix off-by-one in blit_region_is_unsafe

[Gerd Hoffmann]

On Mi, 2016-02-10 at 17:17 +0100, Paolo Bonzini wrote:
> The "max" value is being compared with >=, but addr + width points to
> the first byte that will _not_ be copied.  Laszlo suggested using a
> "greater than" comparison, instead of subtracting one like it is
> already done above for the height, so that max remains always
> positive.
> 
> The mistake is "safe"---it will reject some blits, but will never
> cause
> out-of-bounds writes.

added to vga queue.

thanks,
  Gerd