From e543257370cce5153bbcf0085a116e6aa4a6d91b Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Wed, 22 Feb 2017 12:36:25 +0100
Subject: [PATCH 07/24] vnc: fix memory corruption (CVE-2015-5225)
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RH-Author: Gerd Hoffmann <kraxel@redhat.com>
Message-id: <1487766986-6329-8-git-send-email-kraxel@redhat.com>
Patchwork-id: 73978
O-Subject: [RHEL-7.4 qemu-kvm PATCH 7/8] vnc: fix memory corruption (CVE-2015-5225)
Bugzilla: 1377977
RH-Acked-by: Thomas Huth <thuth@redhat.com>
RH-Acked-by: Marc-André Lureau <mlureau@redhat.com>
RH-Acked-by: Laurent Vivier <lvivier@redhat.com>

The _cmp_bytes variable added by commit "bea60dd ui/vnc: fix potential
memory corruption issues" can become negative.  Result is (possibly
exploitable) memory corruption.  Reason for that is it uses the stride
instead of bytes per scanline to apply limits.

For the server surface is is actually fine.  vnc creates that itself,
there is never any padding and thus scanline length always equals stride.

For the guest surface scanline length and stride are typically identical
too, but it doesn't has to be that way.  So add and use a new variable
(guest_ll) for the guest scanline length.  Also rename min_stride to
line_bytes to make more clear what it actually is.  Finally sprinkle
in an assert() to make sure we never use a negative _cmp_bytes again.

Reported-by: 范祚至(库特) <zuozhi.fzz@alibaba-inc.com>
Reviewed-by: P J P <ppandit@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit eb8934b0418b3b1d125edddc4fc334a54334a49b)
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 ui/vnc.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/ui/vnc.c b/ui/vnc.c
index 80b7792..d0ada7e 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -2676,7 +2676,7 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
                     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;
+    int cmp_bytes, server_stride, line_bytes, guest_ll, guest_stride, y = 0;
     uint8_t *guest_row0 = NULL, *server_row0;
     VncState *vs;
     int has_dirty = 0;
@@ -2695,17 +2695,21 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
      * Update server dirty map.
      */
     server_row0 = (uint8_t *)pixman_image_get_data(vd->server);
-    server_stride = guest_stride = pixman_image_get_stride(vd->server);
+    server_stride = guest_stride = guest_ll =
+        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 {
+        int guest_bpp =
+            PIXMAN_FORMAT_BPP(pixman_image_get_format(vd->guest.fb));
         guest_row0 = (uint8_t *)pixman_image_get_data(vd->guest.fb);
         guest_stride = pixman_image_get_stride(vd->guest.fb);
+        guest_ll = pixman_image_get_width(vd->guest.fb) * ((guest_bpp + 7) / 8);
     }
-    min_stride = MIN(server_stride, guest_stride);
+    line_bytes = MIN(server_stride, guest_ll);
 
     for (;;) {
         int x;
@@ -2736,9 +2740,10 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
             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 ((x + 1) * cmp_bytes > line_bytes) {
+                _cmp_bytes = line_bytes - x * cmp_bytes;
             }
+            assert(_cmp_bytes >= 0);
             if (memcmp(server_ptr, guest_ptr, _cmp_bytes) == 0) {
                 continue;
             }
-- 
1.8.3.1