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From 1109b8bd05947ac09b6f24f46a9f784c39cac46e Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <>
Date: Tue, 26 Apr 2016 14:48:06 +0200
Subject: [PATCH 5/5] vga: make sure vga register setup for vbe stays intact

Call vbe_update_vgaregs() when the guest touches GFX, SEQ or CRT
registers, to make sure the vga registers will always have the
values needed by vbe mode.  This makes sure the sanity checks
applied by vbe_fixup_regs() are effective.

Without this guests can muck with shift_control, can turn on planar
vga modes or text mode emulation while VBE is active, making qemu
take code paths meant for CGA compatibility, but with the very
large display widths and heigts settable using VBE registers.

Which is good for one or another buffer overflow.  Not that
critical as they typically read overflows happening somewhere
in the display code.  So guests can DoS by crashing qemu with a
segfault, but it is probably not possible to break out of the VM.

Fixes: CVE-2016-3712
Reported-by: Zuozhi Fzz <>
Reported-by: P J P <>
Signed-off-by: Gerd Hoffmann <>
Signed-off-by: Stefano Stabellini <>
 tools/qemu-xen/hw/display/vga.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tools/qemu-xen/hw/display/vga.c b/tools/qemu-xen/hw/display/vga.c
index f265b97..a3feb2d 100644
--- a/tools/qemu-xen/hw/display/vga.c
+++ b/tools/qemu-xen/hw/display/vga.c
@@ -166,6 +166,8 @@ static uint32_t expand4[256];
 static uint16_t expand2[256];
 static uint8_t expand4to8[16];
+static void vbe_update_vgaregs(VGACommonState *s);
 static inline bool vbe_enabled(VGACommonState *s)
     return s->vbe_regs[VBE_DISPI_INDEX_ENABLE] & VBE_DISPI_ENABLED;
@@ -509,6 +511,7 @@ void vga_ioport_write(void *opaque, uint32_t addr, uint32_t val)
         printf("vga: write SR%x = 0x%02x\n", s->sr_index, val);
         s->sr[s->sr_index] = val & sr_mask[s->sr_index];
+        vbe_update_vgaregs(s);
         if (s->sr_index == VGA_SEQ_CLOCK_MODE) {
@@ -540,6 +543,7 @@ void vga_ioport_write(void *opaque, uint32_t addr, uint32_t val)
         printf("vga: write GR%x = 0x%02x\n", s->gr_index, val);
         s->gr[s->gr_index] = val & gr_mask[s->gr_index];
+        vbe_update_vgaregs(s);
     case VGA_CRT_IM:
@@ -558,10 +562,12 @@ void vga_ioport_write(void *opaque, uint32_t addr, uint32_t val)
             if (s->cr_index == VGA_CRTC_OVERFLOW) {
                 s->cr[VGA_CRTC_OVERFLOW] = (s->cr[VGA_CRTC_OVERFLOW] & ~0x10) |
                     (val & 0x10);
+                vbe_update_vgaregs(s);
         s->cr[s->cr_index] = val;
+        vbe_update_vgaregs(s);
         switch(s->cr_index) {
         case VGA_CRTC_H_TOTAL: