Fix CVE-2017-15289:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-15289

Patch copied from upstream source repository:

https://git.qemu.org/?p=qemu.git;a=commitdiff;h=eb38e1bc3740725ca29a535351de94107ec58d51 

From eb38e1bc3740725ca29a535351de94107ec58d51 Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Wed, 11 Oct 2017 10:43:14 +0200
Subject: [PATCH] cirrus: fix oob access in mode4and5 write functions

Move dst calculation into the loop, so we apply the mask on each
interation and will not overflow vga memory.

Cc: Prasad J Pandit <pjp@fedoraproject.org>
Reported-by: Niu Guoxiang <niuguoxiang@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20171011084314.21752-1-kraxel@redhat.com
---
 hw/display/cirrus_vga.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c
index b4d579857a..bc32bf1e39 100644
--- a/hw/display/cirrus_vga.c
+++ b/hw/display/cirrus_vga.c
@@ -2038,15 +2038,14 @@ static void cirrus_mem_writeb_mode4and5_8bpp(CirrusVGAState * s,
     unsigned val = mem_value;
     uint8_t *dst;
 
-    dst = s->vga.vram_ptr + (offset &= s->cirrus_addr_mask);
     for (x = 0; x < 8; x++) {
+        dst = s->vga.vram_ptr + ((offset + x) & s->cirrus_addr_mask);
 	if (val & 0x80) {
 	    *dst = s->cirrus_shadow_gr1;
 	} else if (mode == 5) {
 	    *dst = s->cirrus_shadow_gr0;
 	}
 	val <<= 1;
-	dst++;
     }
     memory_region_set_dirty(&s->vga.vram, offset, 8);
 }
@@ -2060,8 +2059,8 @@ static void cirrus_mem_writeb_mode4and5_16bpp(CirrusVGAState * s,
     unsigned val = mem_value;
     uint8_t *dst;
 
-    dst = s->vga.vram_ptr + (offset &= s->cirrus_addr_mask);
     for (x = 0; x < 8; x++) {
+        dst = s->vga.vram_ptr + ((offset + 2 * x) & s->cirrus_addr_mask & ~1);
 	if (val & 0x80) {
 	    *dst = s->cirrus_shadow_gr1;
 	    *(dst + 1) = s->vga.gr[0x11];
@@ -2070,7 +2069,6 @@ static void cirrus_mem_writeb_mode4and5_16bpp(CirrusVGAState * s,
 	    *(dst + 1) = s->vga.gr[0x10];
 	}
 	val <<= 1;
-	dst += 2;
     }
     memory_region_set_dirty(&s->vga.vram, offset, 16);
 }
-- 
2.15.0

