From 3b2c6ef26bdde6363ca750008cef962076e2bf0f Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Fri, 26 Jan 2018 08:12:43 +0100
Subject: [PATCH 6/8] vga: check the validation of memory addr when draw text

RH-Author: Gerd Hoffmann <kraxel@redhat.com>
Message-id: <20180126081243.19785-2-kraxel@redhat.com>
Patchwork-id: 78712
O-Subject: [RHV7.5 qemu-kvm-rhev PATCH 1/1] vga: check the validation of memory addr when draw text
Bugzilla: 1534682
RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
RH-Acked-by: Laszlo Ersek <lersek@redhat.com>
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>

From: linzhecheng <linzhecheng@huawei.com>

Start a vm with qemu-kvm -enable-kvm -vnc :66 -smp 1 -m 1024 -hda
redhat_5.11.qcow2  -device pcnet -vga cirrus,
then use VNC client to connect to VM, and excute the code below in guest
OS will lead to qemu crash:

int main()
 {
    iopl(3);
    srand(time(NULL));
    int a,b;
    while(1){
	a = rand()%0x100;
	b = 0x3c0 + (rand()%0x20);
        outb(a,b);
    }
    return 0;
}

The above code is writing the registers of VGA randomly.
We can write VGA CRT controller registers index 0x0C or 0x0D
(which is the start address register) to modify the
the display memory address of the upper left pixel
or character of the screen. The address may be out of the
range of vga ram. So we should check the validation of memory address
when reading or writing it to avoid segfault.

Signed-off-by: linzhecheng <linzhecheng@huawei.com>
Message-id: 20180111132724.13744-1-linzhecheng@huawei.com
Fixes: CVE-2018-5683
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 191f59dc17396bb5a8da50f8c59b6e0a430711a4)
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 hw/display/vga.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/hw/display/vga.c b/hw/display/vga.c
index 06ca3da..b1cdf36 100644
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -1280,6 +1280,9 @@ static void vga_draw_text(VGACommonState *s, int full_update)
         cx_min = width;
         cx_max = -1;
         for(cx = 0; cx < width; cx++) {
+            if (src + sizeof(uint16_t) > s->vram_ptr + s->vram_size) {
+                break;
+            }
             ch_attr = *(uint16_t *)src;
             if (full_update || ch_attr != *ch_attr_ptr || src == cursor_ptr) {
                 if (cx < cx_min)
-- 
1.8.3.1