[QEMU HCI-8.0 PATCH 10/15] ui/vnc: fix out-of-bounds write in lossy refresh dirty marking

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

From: Marc-André Lureau <marcandre.lureau@redhat.com>

vnc_refresh_lossy_rect() marks a full VNC_STAT_RECT (64) rows of the
dirty bitmap when refreshing a lossy tile. When the display height is
not a multiple of VNC_STAT_RECT, the last tile row is a partial tile and
the loop writes past the end of vs->dirty[VNC_MAX_HEIGHT].

For example, with a 2160-pixel-high display (VNC_MAX_HEIGHT), the last
stat tile starts at y=2112. The unconditional 64-row loop writes rows
2112..2175, overflowing 16 rows (640 bytes) past the dirty bitmap into
subsequent VncState fields.

Fix by passing the effective display height into
vnc_refresh_lossy_rect() and clamping the inner loop.

Fixes: CVE-2026-61475
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3935
Reported-by: "Vulnerability Report" <vr@darknavy.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Signed-off-by: Marc-Andre Lureau <marcandre.lureau@redhat.com>
(cherry picked from commit e650e4fe0fb35b7a8ec9fc04e00346c02640bd58)
---
 ui/vnc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/ui/vnc.c b/ui/vnc.c
index d76a777cc94..40b4e7077da 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -3021,18 +3021,18 @@ void vnc_sent_lossy_rect(VncWorker *worker, int x, int y, int w, int h)
     }
 }
 
-static int vnc_refresh_lossy_rect(VncDisplay *vd, int x, int y)
+static int vnc_refresh_lossy_rect(VncDisplay *vd, int x, int y,
+                                  int height)
 {
     VncState *vs;
     int sty = y / VNC_STAT_RECT;
     int stx = x / VNC_STAT_RECT;
     int has_dirty = 0;
-    int height = MIN(pixman_image_get_height(vd->guest.fb),
-                     pixman_image_get_height(vd->server));
     int rows;
 
     y = QEMU_ALIGN_DOWN(y, VNC_STAT_RECT);
     x = QEMU_ALIGN_DOWN(x, VNC_STAT_RECT);
+    rows = MIN(VNC_STAT_RECT, height - y);
 
     rows = MIN(VNC_STAT_RECT, height - y);
     if (rows <= 0) {
@@ -3104,7 +3104,7 @@ static int vnc_update_stats(VncDisplay *vd,  struct timeval * tv)
 
             if (timercmp(&res, &VNC_REFRESH_LOSSY, >)) {
                 rect->freq = 0;
-                has_dirty += vnc_refresh_lossy_rect(vd, x, y);
+                has_dirty += vnc_refresh_lossy_rect(vd, x, y, height);
                 memset(rect->times, 0, sizeof (rect->times));
                 continue ;
             }
-- 
2.47.1