[Qemu-stable] [PATCH v3] cirrus: fix oob access issue (CVE-2017-2615)

[Gerd Hoffmann]

From: Li Qiang <address@hidden>

When doing bitblt copy in backward mode, we should minus the
blt width first just like the adding in the forward mode. This
can avoid the oob access of the front of vga's vram.

Signed-off-by: Li Qiang <address@hidden>
Message-id: address@hidden

{ kraxel: with backward blits (negative pitch) addr is the topmost
          address, so check it as-is against vram size ]

Cc: address@hidden
Cc: P J P <address@hidden>
Cc: Laszlo Ersek <address@hidden>
Cc: Paolo Bonzini <address@hidden>
Cc: Wolfgang Bumiller <address@hidden>
Fixes: d3532a0db02296e687711b8cdc7791924efccea0 (CVE-2014-8106)
Signed-off-by: Gerd Hoffmann <address@hidden>
---
 hw/display/cirrus_vga.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c
index 95873ae..3961712 100644
--- a/hw/display/cirrus_vga.c
+++ b/hw/display/cirrus_vga.c
@@ -277,10 +277,9 @@ static bool blit_region_is_unsafe(struct CirrusVGAState *s,
     }
     if (pitch < 0) {
         int64_t min = addr
-            + ((int64_t)s->cirrus_blt_height-1) * pitch;
-        int32_t max = addr
-            + s->cirrus_blt_width;
-        if (min < 0 || max > s->vga.vram_size) {
+            + ((int64_t)s->cirrus_blt_height - 1) * pitch
+            - s->cirrus_blt_width;
+        if (min < -1 || addr >= s->vga.vram_size) {
             return true;
         }
     } else {
-- 
1.8.3.1