vga: fix display update region calculation (split screen)

vga display update mis-calculated the region for the dirty bitmap
snapshot in case split screen mode is used.  This can trigger an
assert in cpu_physical_memory_snapshot_get_dirty().

Impact:  DoS for privileged guest users.

(cherry picked from commit e65294157d4b69393b3f819c99f4f647452b48e3)

Fixes: CVE-2017-13673
Fixes: fec5e8c92becad223df9d972770522f64aafdb72
Cc: P J P <ppandit@redhat.com>
Reported-by: David Buchanan <d@vidbuchanan.co.uk>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Message-id: 20170828123307.15392-1-kraxel@redhat.com

diff --git a/hw/display/vga.c b/hw/display/vga.c
index 63421f9ee8563ce1f0a1f38de5957b29d43123c4..ab33668402ddb71ab4128cc15568446ce97d51d7 100644 (file)
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -1628,9 +1628,15 @@ static void vga_draw_graphic(VGACommonState *s, int full_update)
     y1 = 0;
 
     if (!full_update) {
+        ram_addr_t region_start = addr1;
+        ram_addr_t region_end = addr1 + line_offset * height;
         vga_sync_dirty_bitmap(s);
-        snap = memory_region_snapshot_and_clear_dirty(&s->vram, addr1,
-                                                      line_offset * height,
+        if (s->line_compare < height) {
+            /* split screen mode */
+            region_start = 0;
+        }
+        snap = memory_region_snapshot_and_clear_dirty(&s->vram, region_start,
+                                                      region_end - region_start,
                                                       DIRTY_MEMORY_VGA);
     }