[Qemu-devel] [PATCH RESEND 2/2] vnc: Fix heap corruption

[Wen Congyang <wency@cn.fujitsu.com>]

This bug is reported by Stefan Weil:
========
Commit bc2429b9174ac2d3c56b7fd35884b0d89ec7fb02 introduced
a severe bug (heap corruption).

bitmap_clear was called with a wrong argument
which caused out-of-bound writes to width_mask.

This bug was detected with QEMU running on windows.
It also occurs with wine:

*** stack smashing detected ***:  terminated
wine: Unhandled illegal instruction at address 0x6115c7 (thread 0009), starting debugger...

The bug is not windows specific!
========

The third argument of bitmap_clear() is number of bits to be cleared, but we pass
the end bits to be cleared to bitmap_clear().

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Reported-by: Stefan Weil <weil@mail.berlios.de>

---
 ui/vnc.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/ui/vnc.c b/ui/vnc.c
index e3761b0..e7d0b5b 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -2390,6 +2390,7 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
     unsigned long width_mask[VNC_DIRTY_WORDS];
     VncState *vs;
     int has_dirty = 0;
+    const size_t width = ds_get_width(vd->ds) / 16;
 
     struct timeval tv = { 0, 0 };
 
@@ -2403,9 +2404,8 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
      * Check and copy modified bits from guest to server surface.
      * Update server dirty map.
      */
-    bitmap_set(width_mask, 0, (ds_get_width(vd->ds) / 16));
-    bitmap_clear(width_mask, (ds_get_width(vd->ds) / 16),
-                 VNC_DIRTY_WORDS * BITS_PER_LONG);
+    bitmap_set(width_mask, 0, width);
+    bitmap_clear(width_mask, width, VNC_DIRTY_WORDS * BITS_PER_LONG - width);
     cmp_bytes = 16 * ds_get_bytes_per_pixel(vd->ds);
     guest_row  = vd->guest.ds->data;
     server_row = vd->server->data;
-- 
1.7.1