File CVE-2016-3710-qemuu-0005-vga-make-sure-vga-register-setup-for-vbe-stays-intac.patch of Package xen.openSUSE_13.2_Update

References: bsc#978164 CVE-2016-3710 XSA-179

From f7926c73685c2bc7124265f567bafb502864c5dd Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Tue, 26 Apr 2016 14:48:06 +0200
Subject: [PATCH 5/5] vga: make sure vga register setup for vbe stays intact
 (CVE-2016-3712).

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 <zuozhi.fzz@alibaba-inc.com>
Reported-by: P J P <ppandit@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
---
 hw/display/vga.c | 6 ++++++
 1 file changed, 6 insertions(+)

Index: xen-4.4.4-testing/tools/qemu-xen-dir-remote/hw/display/vga.c
===================================================================
--- xen-4.4.4-testing.orig/tools/qemu-xen-dir-remote/hw/display/vga.c
+++ xen-4.4.4-testing/tools/qemu-xen-dir-remote/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;
@@ -513,6 +515,7 @@ void vga_ioport_write(void *opaque, uint
         printf("vga: write SR%x = 0x%02x\n", s->sr_index, val);
 #endif
         s->sr[s->sr_index] = val & sr_mask[s->sr_index];
+        vbe_update_vgaregs(s);
         if (s->sr_index == VGA_SEQ_CLOCK_MODE) {
             s->update_retrace_info(s);
         }
@@ -544,6 +547,7 @@ void vga_ioport_write(void *opaque, uint
         printf("vga: write GR%x = 0x%02x\n", s->gr_index, val);
 #endif
         s->gr[s->gr_index] = val & gr_mask[s->gr_index];
+        vbe_update_vgaregs(s);
         vga_update_memory_access(s);
         break;
     case VGA_CRT_IM:
@@ -562,10 +566,12 @@ void vga_ioport_write(void *opaque, uint
             if (s->cr_index == VGA_CRTC_OVERFLOW) {
                 s->cr[VGA_CRTC_OVERFLOW] = (s->cr[VGA_CRTC_OVERFLOW] & ~0x10) |
                     (val & 0x10);
+                vbe_update_vgaregs(s);
             }
             return;
         }
         s->cr[s->cr_index] = val;
+        vbe_update_vgaregs(s);
 
         switch(s->cr_index) {
         case VGA_CRTC_H_TOTAL: