[QEMU HCI-8.0 PATCH 04/15] ui/vnc: fix OOB write in lossy rect worker code

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

From: Daniel P. Berrangé <berrange@redhat.com>

Incorrect calculation of the boundary condition when tracking lossy
rectangles in the worker thread will result in an OOB write which
can corrupt further worker state, and/or trigger any guard pages
that may lie beyond the VncWorker struct. This can be triggered
through careful choice of the display resolution in the guest
OS by an unprivileged user.

Fixes: CVE-2026-48002
Reported-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20260521103353.1645561-4-berrange@redhat.com>
[Marc-André - added assert() suggest by philmd@linaro.org]
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
(cherry picked from commit 46ee49034d26d04d95ba8f3183d4fbfa9d2b89b4)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(cherry picked from commit ed1400016319f8cffb5d6ba8809a8d774976ff65)
---
 ui/vnc.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/ui/vnc.c b/ui/vnc.c
index 8ca77b2971f..7373c83b692 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -3005,13 +3005,15 @@ void vnc_sent_lossy_rect(VncWorker *worker, int x, int y, int w, int h)
 {
     int i, j;
 
-    w = (x + w) / VNC_STAT_RECT;
-    h = (y + h) / VNC_STAT_RECT;
+    w = DIV_ROUND_UP((x + w), VNC_STAT_RECT);
+    h = DIV_ROUND_UP((y + h), VNC_STAT_RECT);
+    assert(h <= VNC_STAT_ROWS);
+    assert(w <= VNC_STAT_COLS);
     x /= VNC_STAT_RECT;
     y /= VNC_STAT_RECT;
 
-    for (j = y; j <= h; j++) {
-        for (i = x; i <= w; i++) {
+    for (j = y; j < h; j++) {
+        for (i = x; i < w; i++) {
             worker->lossy_rect[j][i] = 1;
         }
     }
-- 
2.47.1