[Xen-changelog] [qemu-xen stable-4.6] vga: make sure vga register setup for vbe stays intact (CVE-2016-3712).

[patchbot@xxxxxxx]

commit 14a60f98e0cd16e2636afb136129ed7f11cbfce0
Author:     Gerd Hoffmann <kraxel@xxxxxxxxxx>
AuthorDate: Tue Apr 26 14:48:06 2016 +0200
Commit:     Anthony PERARD <anthony.perard@xxxxxxxxxx>
CommitDate: Mon May 9 17:42:49 2016 +0100

    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.
    
    upstream-commit-id: fd3c136b3e1482cd0ec7285d6bc2a3e6a62c38d7
    
    Fixes: CVE-2016-3712
    Reported-by: Zuozhi Fzz <zuozhi.fzz@xxxxxxxxxxxxxxx>
    Reported-by: P J P <ppandit@xxxxxxxxxx>
    Signed-off-by: Gerd Hoffmann <kraxel@xxxxxxxxxx>
    Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 hw/display/vga.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/hw/display/vga.c b/hw/display/vga.c
index 3739758..e7be97e 100644
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -140,6 +140,8 @@ 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;
@@ -483,6 +485,7 @@ void vga_ioport_write(void *opaque, uint32_t addr, uint32_t 
val)
         printf("vga: write SR%x = 0x%02x\n", s->sr_index, val);
 #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);
         }
@@ -514,6 +517,7 @@ void vga_ioport_write(void *opaque, uint32_t addr, uint32_t 
val)
         printf("vga: write GR%x = 0x%02x\n", s->gr_index, val);
 #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:
@@ -532,10 +536,12 @@ 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);
             }
             return;
         }
         s->cr[s->cr_index] = val;
+        vbe_update_vgaregs(s);
 
         switch(s->cr_index) {
         case VGA_CRTC_H_TOTAL:
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#stable-4.6

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