From 26ef545b3502f61ca722a7a3373507e88ef64110 Mon Sep 17 00:00:00 2001
From: Olivier Fourdan <ofourdan@redhat.com>
Date: Mon, 13 Mar 2023 11:08:47 +0100
Subject: [PATCH xserver] composite: Fix use-after-free of the COW
 
ZDI-CAN-19866/CVE-2023-1393
 
If a client explicitly destroys the compositor overlay window (aka COW),
we would leave a dangling pointer to that window in the CompScreen
structure, which will trigger a use-after-free later.
 
Make sure to clear the CompScreen pointer to the COW when the latter gets
destroyed explicitly by the client.
 
This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative
 
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
---
 composite/compwindow.c | 5 +++++
 1 file changed, 5 insertions(+)
 
diff --git a/composite/compwindow.c b/composite/compwindow.c
index 4e2494b86..b30da589e 100644
--- a/composite/compwindow.c
+++ b/composite/compwindow.c
@@ -620,6 +620,11 @@ compDestroyWindow(WindowPtr pWin)
     ret = (*pScreen->DestroyWindow) (pWin);
     cs->DestroyWindow = pScreen->DestroyWindow;
     pScreen->DestroyWindow = compDestroyWindow;
+
+    /* Did we just destroy the overlay window? */
+    if (pWin == cs->pOverlayWin)
+        cs->pOverlayWin = NULL;
+
 /*    compCheckTree (pWin->drawable.pScreen); can't check -- tree isn't good*/
     return ret;
 }
-- 
2.40.0