References: bsc#1026636 CVE-2017-2633

Subject: ui/vnc: fix potential memory corruption issues
From: Peter Lieven pl@kamp.de Mon Jun 30 10:57:51 2014 +0200
Date: Tue Jul 1 13:26:40 2014 +0200:
Git: bea60dd7679364493a0d7f5b54316c767cf894ef

this patch makes the VNC server work correctly if the
server surface and the guest surface have different sizes.

Basically the server surface is adjusted to not exceed VNC_MAX_WIDTH
x VNC_MAX_HEIGHT and additionally the width is rounded up to multiple of
VNC_DIRTY_PIXELS_PER_BIT.

If we have a resolution whose width is not dividable by VNC_DIRTY_PIXELS_PER_BIT
we now get a small black bar on the right of the screen.

If the surface is too big to fit the limits only the upper left area is shown.

On top of that this fixes 2 memory corruption issues:

The first was actually discovered during playing
around with a Windows 7 vServer. During resolution
change in Windows 7 it happens sometimes that Windows
changes to an intermediate resolution where
server_stride % cmp_bytes != 0 (in vnc_refresh_server_surface).
This happens only if width % VNC_DIRTY_PIXELS_PER_BIT != 0.

The second is a theoretical issue, but is maybe exploitable
by the guest. If for some reason the guest surface size is bigger
than VNC_MAX_WIDTH x VNC_MAX_HEIGHT we end up in severe corruption since
this limit is nowhere enforced.

This patch also includes the following commits:
b4c85ddcec24c60616aad9b3b7fc36ce19ba3ca4
919372251cbfa9e43b0264fec475dd1eca23784f
12b316d4c173bf07f421ef9dc98ba4b53916066e
2f487a3d40faff1772e14da6b921900915501f9a
6cd859aa8a7fb60fe6edb89e628cddfe25dfe186

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

Index: xen-4.4.4-testing/tools/qemu-xen-dir-remote/ui/vnc.c
===================================================================
--- xen-4.4.4-testing.orig/tools/qemu-xen-dir-remote/ui/vnc.c
+++ xen-4.4.4-testing/tools/qemu-xen-dir-remote/ui/vnc.c
@@ -427,32 +427,35 @@ static void framebuffer_update_request(V
 static void vnc_refresh(DisplayChangeListener *dcl);
 static int vnc_refresh_server_surface(VncDisplay *vd);
 
+static void vnc_set_area_dirty(DECLARE_BITMAP(dirty[VNC_MAX_HEIGHT],
+                               VNC_MAX_WIDTH / VNC_DIRTY_PIXELS_PER_BIT),
+                               int width, int height,
+                               int x, int y, int w, int h) {
+    /* this is needed this to ensure we updated all affected
+     * blocks if x % VNC_DIRTY_PIXELS_PER_BIT != 0 */
+    w += (x % VNC_DIRTY_PIXELS_PER_BIT);
+    x -= (x % VNC_DIRTY_PIXELS_PER_BIT);
+
+    x = MIN(x, width);
+    y = MIN(y, height);
+    w = MIN(x + w, width) - x;
+    h = MIN(y + h, height);
+
+    for (; y < h; y++) {
+        bitmap_set(dirty[y], x / VNC_DIRTY_PIXELS_PER_BIT,
+                   DIV_ROUND_UP(w, VNC_DIRTY_PIXELS_PER_BIT));
+    }
+}
+
 static void vnc_dpy_update(DisplayChangeListener *dcl,
                            int x, int y, int w, int h)
 {
-    int i;
     VncDisplay *vd = container_of(dcl, VncDisplay, dcl);
     struct VncSurface *s = &vd->guest;
-    int width = surface_width(vd->ds);
-    int height = surface_height(vd->ds);
-
-    h += y;
-
-    /* round x down to ensure the loop only spans one 16-pixel block per,
-       iteration.  otherwise, if (x % 16) != 0, the last iteration may span
-       two 16-pixel blocks but we only mark the first as dirty
-    */
-    w += (x % 16);
-    x -= (x % 16);
+    int width = pixman_image_get_width(vd->server);
+    int height = pixman_image_get_height(vd->server);
 
-    x = MIN(x, width);
-    y = MIN(y, height);
-    w = MIN(x + w, width) - x;
-    h = MIN(h, height);
-
-    for (; y < h; y++)
-        for (i = 0; i < w; i += 16)
-            set_bit((x + i) / 16, s->dirty[y]);
+    vnc_set_area_dirty(s->dirty, width, height, x, y, w, h);
 }
 
 void vnc_framebuffer_update(VncState *vs, int x, int y, int w, int h,
@@ -516,17 +519,15 @@ void buffer_advance(Buffer *buf, size_t
 
 static void vnc_desktop_resize(VncState *vs)
 {
-    DisplaySurface *ds = vs->vd->ds;
-
     if (vs->csock == -1 || !vnc_has_feature(vs, VNC_FEATURE_RESIZE)) {
         return;
     }
-    if (vs->client_width == surface_width(ds) &&
-        vs->client_height == surface_height(ds)) {
+    if (vs->client_width == pixman_image_get_width(vs->vd->server) &&
+        vs->client_height == pixman_image_get_height(vs->vd->server)) {
         return;
     }
-    vs->client_width = surface_width(ds);
-    vs->client_height = surface_height(ds);
+    vs->client_width = pixman_image_get_width(vs->vd->server);
+    vs->client_height = pixman_image_get_height(vs->vd->server);
     vnc_lock_output(vs);
     vnc_write_u8(vs, VNC_MSG_SERVER_FRAMEBUFFER_UPDATE);
     vnc_write_u8(vs, 0);
@@ -576,16 +577,18 @@ static void vnc_dpy_switch(DisplayChange
 {
     VncDisplay *vd = container_of(dcl, VncDisplay, dcl);
     VncState *vs;
+    int width, height;
 
     vnc_abort_display_jobs(vd);
 
     /* server surface */
     qemu_pixman_image_unref(vd->server);
     vd->ds = surface;
+    width = MIN(VNC_MAX_WIDTH, ROUND_UP(surface_width(vd->ds),
+                                        VNC_DIRTY_PIXELS_PER_BIT));
+    height = MIN(VNC_MAX_HEIGHT, surface_height(vd->ds));
     vd->server = pixman_image_create_bits(VNC_SERVER_FB_FORMAT,
-                                          surface_width(vd->ds),
-                                          surface_height(vd->ds),
-                                          NULL, 0);
+                                          width, height, NULL, 0);
 
     /* guest surface */
 #if 0 /* FIXME */
@@ -595,7 +598,9 @@ static void vnc_dpy_switch(DisplayChange
     qemu_pixman_image_unref(vd->guest.fb);
     vd->guest.fb = pixman_image_ref(surface->image);
     vd->guest.format = surface->format;
-    memset(vd->guest.dirty, 0xFF, sizeof(vd->guest.dirty));
+    memset(vd->guest.dirty, 0x00, sizeof(vd->guest.dirty));
+    vnc_set_area_dirty(vd->guest.dirty, width, height, 0, 0,
+                       width, height);
 
     QTAILQ_FOREACH(vs, &vd->clients, next) {
         vnc_colordepth(vs);
@@ -603,7 +608,9 @@ static void vnc_dpy_switch(DisplayChange
         if (vs->vd->cursor) {
             vnc_cursor_define(vs);
         }
-        memset(vs->dirty, 0xFF, sizeof(vs->dirty));
+        memset(vs->dirty, 0x00, sizeof(vs->dirty));
+        vnc_set_area_dirty(vs->dirty, width, height, 0, 0,
+                           width, height);
     }
 }
 
@@ -769,11 +776,12 @@ static void vnc_dpy_copy(DisplayChangeLi
         y = dst_y + h - 1;
         inc = -1;
     }
-    w_lim = w - (16 - (dst_x % 16));
-    if (w_lim < 0)
+    w_lim = w - (VNC_DIRTY_PIXELS_PER_BIT - (dst_x % VNC_DIRTY_PIXELS_PER_BIT));
+    if (w_lim < 0) {
         w_lim = w;
-    else
-        w_lim = w - (w_lim % 16);
+    } else {
+        w_lim = w - (w_lim % VNC_DIRTY_PIXELS_PER_BIT);
+    }
     for (i = 0; i < h; i++) {
         for (x = 0; x <= w_lim;
                 x += s, src_row += cmp_bytes, dst_row += cmp_bytes) {
@@ -781,10 +789,11 @@ static void vnc_dpy_copy(DisplayChangeLi
                 if ((s = w - w_lim) == 0)
                     break;
             } else if (!x) {
-                s = (16 - (dst_x % 16));
+                s = (VNC_DIRTY_PIXELS_PER_BIT -
+                    (dst_x % VNC_DIRTY_PIXELS_PER_BIT));
                 s = MIN(s, w_lim);
             } else {
-                s = 16;
+                s = VNC_DIRTY_PIXELS_PER_BIT;
             }
             cmp_bytes = s * VNC_SERVER_FB_BYTES;
             if (memcmp(src_row, dst_row, cmp_bytes) == 0)
@@ -792,7 +801,8 @@ static void vnc_dpy_copy(DisplayChangeLi
             memmove(dst_row, src_row, cmp_bytes);
             QTAILQ_FOREACH(vs, &vd->clients, next) {
                 if (!vnc_has_feature(vs, VNC_FEATURE_COPYRECT)) {
-                    set_bit(((x + dst_x) / 16), vs->dirty[y]);
+                    set_bit(((x + dst_x) / VNC_DIRTY_PIXELS_PER_BIT),
+                            vs->dirty[y]);
                 }
             }
         }
@@ -886,10 +896,9 @@ static int vnc_update_client(VncState *v
         VncDisplay *vd = vs->vd;
         VncJob *job;
         int y;
-        int width, height;
+        int height, width;
         int n = 0;
 
-
         if (vs->output.offset && !vs->audio_cap && !vs->force_update)
             /* kernel send buffers are full -> drop frames to throttle */
             return 0;
@@ -905,32 +914,30 @@ static int vnc_update_client(VncState *v
          */
         job = vnc_job_new(vs);
 
-        width = MIN(pixman_image_get_width(vd->server), vs->client_width);
-        height = MIN(pixman_image_get_height(vd->server), vs->client_height);
+        height = pixman_image_get_height(vd->server);
+        width = pixman_image_get_width(vd->server);
 
-        for (y = 0; y < height; y++) {
-            int x;
-            int last_x = -1;
-            for (x = 0; x < width / 16; x++) {
-                if (test_and_clear_bit(x, vs->dirty[y])) {
-                    if (last_x == -1) {
-                        last_x = x;
-                    }
-                } else {
-                    if (last_x != -1) {
-                        int h = find_and_clear_dirty_height(vs, y, last_x, x,
-                                                            height);
-
-                        n += vnc_job_add_rect(job, last_x * 16, y,
-                                              (x - last_x) * 16, h);
-                    }
-                    last_x = -1;
-                }
-            }
-            if (last_x != -1) {
-                int h = find_and_clear_dirty_height(vs, y, last_x, x, height);
-                n += vnc_job_add_rect(job, last_x * 16, y,
-                                      (x - last_x) * 16, h);
+        y = 0;
+        for (;;) {
+            int x, h;
+            unsigned long x2;
+            unsigned long offset = find_next_bit((unsigned long *) &vs->dirty,
+                                                 height * VNC_DIRTY_BPL(vs),
+                                                 y * VNC_DIRTY_BPL(vs));
+            if (offset == height * VNC_DIRTY_BPL(vs)) {
+                /* no more dirty bits */
+                break;
+            }
+            y = offset / VNC_DIRTY_BPL(vs);
+            x = offset % VNC_DIRTY_BPL(vs);
+            x2 = find_next_zero_bit((unsigned long *) &vs->dirty[y],
+                                    VNC_DIRTY_BPL(vs), x);
+            bitmap_clear(vs->dirty[y], x, x2 - x);
+            h = find_and_clear_dirty_height(vs, y, x, x2, height);
+            x2 = MIN(x2, width / VNC_DIRTY_PIXELS_PER_BIT);
+            if (x2 > x) {
+                n += vnc_job_add_rect(job, x * VNC_DIRTY_PIXELS_PER_BIT, y,
+                                      (x2 - x) * VNC_DIRTY_PIXELS_PER_BIT, h);
             }
         }
 
@@ -1491,8 +1498,8 @@ static void check_pointer_type_change(No
         vnc_write_u8(vs, 0);
         vnc_write_u16(vs, 1);
         vnc_framebuffer_update(vs, absolute, 0,
-                               surface_width(vs->vd->ds),
-                               surface_height(vs->vd->ds),
+                               pixman_image_get_width(vs->vd->server),
+                               pixman_image_get_height(vs->vd->server),
                                VNC_ENCODING_POINTER_TYPE_CHANGE);
         vnc_unlock_output(vs);
         vnc_flush(vs);
@@ -1504,8 +1511,8 @@ static void pointer_event(VncState *vs,
 {
     int buttons = 0;
     int dz = 0;
-    int width = surface_width(vs->vd->ds);
-    int height = surface_height(vs->vd->ds);
+    int width = pixman_image_get_width(vs->vd->server);
+    int height = pixman_image_get_height(vs->vd->server);
 
     if (button_mask & 0x01)
         buttons |= MOUSE_EVENT_LBUTTON;
@@ -1857,29 +1864,18 @@ static void ext_key_event(VncState *vs,
 }
 
 static void framebuffer_update_request(VncState *vs, int incremental,
-                                       int x_position, int y_position,
-                                       int w, int h)
+                                       int x, int y, int w, int h)
 {
-    int i;
-    const size_t width = surface_width(vs->vd->ds) / 16;
-    const size_t height = surface_height(vs->vd->ds);
-
-    if (y_position > height) {
-        y_position = height;
-    }
-    if (y_position + h >= height) {
-        h = height - y_position;
-    }
+    int width = pixman_image_get_width(vs->vd->server);
+    int height = pixman_image_get_height(vs->vd->server);
 
     vs->need_update = 1;
-    if (!incremental) {
-        vs->force_update = 1;
-        for (i = 0; i < h; i++) {
-            bitmap_set(vs->dirty[y_position + i], 0, width);
-            bitmap_clear(vs->dirty[y_position + i], width,
-                         VNC_DIRTY_BITS - width);
-        }
+
+    if (incremental) {
+        return;
     }
+
+    vnc_set_area_dirty(vs->dirty, width, height, x, y, w, h);
 }
 
 static void send_ext_key_event_ack(VncState *vs)
@@ -1889,8 +1885,8 @@ static void send_ext_key_event_ack(VncSt
     vnc_write_u8(vs, 0);
     vnc_write_u16(vs, 1);
     vnc_framebuffer_update(vs, 0, 0,
-                           surface_width(vs->vd->ds),
-                           surface_height(vs->vd->ds),
+                           pixman_image_get_width(vs->vd->server),
+                           pixman_image_get_height(vs->vd->server),
                            VNC_ENCODING_EXT_KEY_EVENT);
     vnc_unlock_output(vs);
     vnc_flush(vs);
@@ -1903,8 +1899,8 @@ static void send_ext_audio_ack(VncState
     vnc_write_u8(vs, 0);
     vnc_write_u16(vs, 1);
     vnc_framebuffer_update(vs, 0, 0,
-                           surface_width(vs->vd->ds),
-                           surface_height(vs->vd->ds),
+                           pixman_image_get_width(vs->vd->server),
+                           pixman_image_get_height(vs->vd->server),
                            VNC_ENCODING_AUDIO);
     vnc_unlock_output(vs);
     vnc_flush(vs);
@@ -2092,8 +2088,8 @@ static void vnc_colordepth(VncState *vs)
         vnc_write_u8(vs, 0);
         vnc_write_u16(vs, 1); /* number of rects */
         vnc_framebuffer_update(vs, 0, 0,
-                               surface_width(vs->vd->ds),
-                               surface_height(vs->vd->ds),
+                               pixman_image_get_width(vs->vd->server),
+                               pixman_image_get_height(vs->vd->server),
                                VNC_ENCODING_WMVi);
         pixel_format_message(vs);
         vnc_unlock_output(vs);
@@ -2315,8 +2311,8 @@ static int protocol_client_init(VncState
     }
     vnc_set_share_mode(vs, mode);
 
-    vs->client_width = surface_width(vs->vd->ds);
-    vs->client_height = surface_height(vs->vd->ds);
+    vs->client_width = pixman_image_get_width(vs->vd->server);
+    vs->client_height = pixman_image_get_height(vs->vd->server);
     vnc_write_u16(vs, vs->client_width);
     vnc_write_u16(vs, vs->client_height);
 
@@ -2580,7 +2576,9 @@ static int vnc_refresh_lossy_rect(VncDis
 
         vs->lossy_rect[sty][stx] = 0;
         for (j = 0; j < VNC_STAT_RECT; ++j) {
-            bitmap_set(vs->dirty[y + j], x / 16, VNC_STAT_RECT / 16);
+            bitmap_set(vs->dirty[y + j],
+                       x / VNC_DIRTY_PIXELS_PER_BIT,
+                       VNC_STAT_RECT / VNC_DIRTY_PIXELS_PER_BIT);
         }
         has_dirty++;
     }
@@ -2681,12 +2679,12 @@ static void vnc_rect_updated(VncDisplay
 
 static int vnc_refresh_server_surface(VncDisplay *vd)
 {
-    int width = pixman_image_get_width(vd->guest.fb);
-    int height = pixman_image_get_height(vd->guest.fb);
-    int y;
-    uint8_t *guest_row;
-    uint8_t *server_row;
-    int cmp_bytes;
+    int width = MIN(pixman_image_get_width(vd->guest.fb),
+                    pixman_image_get_width(vd->server));
+    int height = MIN(pixman_image_get_height(vd->guest.fb),
+                     pixman_image_get_height(vd->server));
+    int cmp_bytes, server_stride, min_stride, guest_stride, y = 0;
+    uint8_t *guest_row0 = NULL, *server_row0;
     VncState *vs;
     int has_dirty = 0;
     pixman_image_t *tmpbuf = NULL;
@@ -2703,47 +2701,66 @@ static int vnc_refresh_server_surface(Vn
      * Check and copy modified bits from guest to server surface.
      * Update server dirty map.
      */
-    cmp_bytes = 64;
-    if (cmp_bytes > vnc_server_fb_stride(vd)) {
-        cmp_bytes = vnc_server_fb_stride(vd);
-    }
+    server_row0 = (uint8_t *)pixman_image_get_data(vd->server);
+    server_stride = guest_stride = pixman_image_get_stride(vd->server);
+    cmp_bytes = MIN(VNC_DIRTY_PIXELS_PER_BIT * VNC_SERVER_FB_BYTES,
+                    server_stride);
     if (vd->guest.format != VNC_SERVER_FB_FORMAT) {
         int width = pixman_image_get_width(vd->server);
         tmpbuf = qemu_pixman_linebuf_create(VNC_SERVER_FB_FORMAT, width);
+    } else {
+        guest_row0 = (uint8_t *)pixman_image_get_data(vd->guest.fb);
+        guest_stride = pixman_image_get_stride(vd->guest.fb);
     }
-    guest_row = (uint8_t *)pixman_image_get_data(vd->guest.fb);
-    server_row = (uint8_t *)pixman_image_get_data(vd->server);
-    for (y = 0; y < height; y++) {
-        if (!bitmap_empty(vd->guest.dirty[y], VNC_DIRTY_BITS)) {
-            int x;
-            uint8_t *guest_ptr;
-            uint8_t *server_ptr;
-
-            if (vd->guest.format != VNC_SERVER_FB_FORMAT) {
-                qemu_pixman_linebuf_fill(tmpbuf, vd->guest.fb, width, 0, y);
-                guest_ptr = (uint8_t *)pixman_image_get_data(tmpbuf);
-            } else {
-                guest_ptr = guest_row;
-            }
-            server_ptr = server_row;
+    min_stride = MIN(server_stride, guest_stride);
 
-            for (x = 0; x + 15 < width;
-                    x += 16, guest_ptr += cmp_bytes, server_ptr += cmp_bytes) {
-                if (!test_and_clear_bit((x / 16), vd->guest.dirty[y]))
-                    continue;
-                if (memcmp(server_ptr, guest_ptr, cmp_bytes) == 0)
-                    continue;
-                memcpy(server_ptr, guest_ptr, cmp_bytes);
-                if (!vd->non_adaptive)
-                    vnc_rect_updated(vd, x, y, &tv);
-                QTAILQ_FOREACH(vs, &vd->clients, next) {
-                    set_bit((x / 16), vs->dirty[y]);
-                }
-                has_dirty++;
+    for (;;) {
+        int x;
+        uint8_t *guest_ptr, *server_ptr;
+        unsigned long offset = find_next_bit((unsigned long *) &vd->guest.dirty,
+                                             height * VNC_DIRTY_BPL(&vd->guest),
+                                             y * VNC_DIRTY_BPL(&vd->guest));
+        if (offset == height * VNC_DIRTY_BPL(&vd->guest)) {
+            /* no more dirty bits */
+            break;
+        }
+        y = offset / VNC_DIRTY_BPL(&vd->guest);
+        x = offset % VNC_DIRTY_BPL(&vd->guest);
+
+        server_ptr = server_row0 + y * server_stride + x * cmp_bytes;
+
+        if (vd->guest.format != VNC_SERVER_FB_FORMAT) {
+            qemu_pixman_linebuf_fill(tmpbuf, vd->guest.fb, width, 0, y);
+            guest_ptr = (uint8_t *)pixman_image_get_data(tmpbuf);
+        } else {
+            guest_ptr = guest_row0 + y * guest_stride;
+        }
+        guest_ptr += x * cmp_bytes;
+
+        for (; x < DIV_ROUND_UP(width, VNC_DIRTY_PIXELS_PER_BIT);
+             x++, guest_ptr += cmp_bytes, server_ptr += cmp_bytes) {
+            int _cmp_bytes = cmp_bytes;
+            if (!test_and_clear_bit(x, vd->guest.dirty[y])) {
+                continue;
             }
+            if ((x + 1) * cmp_bytes > min_stride) {
+                _cmp_bytes = min_stride - x * cmp_bytes;
+            }
+            if (memcmp(server_ptr, guest_ptr, _cmp_bytes) == 0) {
+                continue;
+            }
+            memcpy(server_ptr, guest_ptr, _cmp_bytes);
+            if (!vd->non_adaptive) {
+                vnc_rect_updated(vd, x * VNC_DIRTY_PIXELS_PER_BIT,
+                                 y, &tv);
+            }
+            QTAILQ_FOREACH(vs, &vd->clients, next) {
+                set_bit(x, vs->dirty[y]);
+            }
+            has_dirty++;
         }
-        guest_row  += pixman_image_get_stride(vd->guest.fb);
-        server_row += pixman_image_get_stride(vd->server);
+
+        y++;
     }
     qemu_pixman_image_unref(tmpbuf);
     return has_dirty;
Index: xen-4.4.4-testing/tools/qemu-xen-dir-remote/ui/vnc.h
===================================================================
--- xen-4.4.4-testing.orig/tools/qemu-xen-dir-remote/ui/vnc.h
+++ xen-4.4.4-testing/tools/qemu-xen-dir-remote/ui/vnc.h
@@ -77,12 +77,21 @@ typedef void VncSendHextileTile(VncState
                                 void *last_fg,
                                 int *has_bg, int *has_fg);
 
-/* VNC_MAX_WIDTH must be a multiple of 16. */
-#define VNC_MAX_WIDTH 2560
+/* VNC_DIRTY_PIXELS_PER_BIT is the number of dirty pixels represented
+ * by one bit in the dirty bitmap, should be a power of 2 */
+#define VNC_DIRTY_PIXELS_PER_BIT 16
+
+/* VNC_MAX_WIDTH must be a multiple of VNC_DIRTY_PIXELS_PER_BIT. */
+
+#define VNC_MAX_WIDTH ROUND_UP(2560, VNC_DIRTY_PIXELS_PER_BIT)
 #define VNC_MAX_HEIGHT 2048
 
 /* VNC_DIRTY_BITS is the number of bits in the dirty bitmap. */
-#define VNC_DIRTY_BITS (VNC_MAX_WIDTH / 16)
+#define VNC_DIRTY_BITS (VNC_MAX_WIDTH / VNC_DIRTY_PIXELS_PER_BIT)
+
+/* VNC_DIRTY_BPL (BPL = bits per line) might be greater than
+ * VNC_DIRTY_BITS due to alignment */
+#define VNC_DIRTY_BPL(x) (sizeof((x)->dirty) / VNC_MAX_HEIGHT * BITS_PER_BYTE)
 
 #define VNC_STAT_RECT  64
 #define VNC_STAT_COLS (VNC_MAX_WIDTH / VNC_STAT_RECT)
@@ -118,7 +127,8 @@ typedef struct VncRectStat VncRectStat;
 struct VncSurface
 {
     struct timeval last_freq_check;
-    DECLARE_BITMAP(dirty[VNC_MAX_HEIGHT], VNC_MAX_WIDTH / 16);
+    DECLARE_BITMAP(dirty[VNC_MAX_HEIGHT],
+                   VNC_MAX_WIDTH / VNC_DIRTY_PIXELS_PER_BIT);
     VncRectStat stats[VNC_STAT_ROWS][VNC_STAT_COLS];
     pixman_image_t *fb;
     pixman_format_code_t format;
